• unlimited access with print and download
    $ 37 00
  • read full document, no print or download, expires after 72 hours
    $ 4 99
More info
Unlimited access including download and printing, plus availability for reading and annotating in your in your Udini library.
  • Access to this article in your Udini library for 72 hours from purchase.
  • The article will not be available for download or print.
  • Upgrade to the full version of this document at a reduced price.
  • Your trial access payment is credited when purchasing the full version.
Buy
Continue searching

Imaging doses in radiation therapy from kilovoltage cone-beam computed tomography

Dissertation
Author: Daniel Ellis Hyer
Abstract:
Advances in radiation treatment delivery, such as intensity modulated radiation therapy (IMRT), have made it possible to deliver large doses of radiation with a high degree of conformity. While highly conformal treatments offers the advantage of sparing surrounding normal tissue, this benefit can only be realized if the patient is accurately positioned during each treatment fraction. The need to accurately position the patient has led to the development and use of gantry mounted kilovoltage cone-beam computed tomography (kV-CBCT) systems. These systems are used to acquire high resolution volumetric images of the patient which are then digitally registered with the planning CT dataset to confirm alignment of the patient on the treatment table. While kV-CBCT is a very useful tool for aligning the patient prior to treatment, daily use in a high fraction therapy regimen results in a substantial radiation dose. In order to quantify the radiation dose associated with CBCT imaging, an anthropomorphic phantom representing a 50th percentile adult male and a fiber-optic coupled (FOC) dosimetry system were both constructed as part of this dissertation. These tools were then used to directly measure organ doses incurred during clinical protocols for the head, chest, and pelvis. For completeness, the dose delivered from both the X-ray Volumetric Imager (XVI, Elekta Oncology Systems, Crawley, UK) and the On-Board Imager (OBI, Varian Medical Systems, Palo Alto, CA) were investigated. While this study provided a direct measure of organ doses for estimating risk to the patient, a practical method for estimating organ doses that could be performed with phantoms and dosimeters currently available at most clinics was also desired. To accomplish this goal, a 100 mm pencil ion chamber was used to measure the "cone beam dose index" (CBDI) inside standard CT dose index (CTDI) acrylic phantoms. A weighted CBDI (CBDIw), similar to the weighted CT dose index (CTDIw), was then calculated to represent the average dose in the acrylic phantom. By comparing this value to the measured organ doses, organ dose conversion coefficients were developed. These conversion coefficients allow specific organ doses to be estimated quickly and easily using readily available clinical equipment.

TABLE OF CONTENTS

page

ACKNOWLEDGMENTS ...............................................................................................................4   LIST OF TABLES ...........................................................................................................................8   LIST OF FIGURES .........................................................................................................................9   ABSTRACT ...................................................................................................................................11 CHAPTER 1 INTRODUCTION ..................................................................................................................13   Cancer .....................................................................................................................................13   Radiation Therapy ..................................................................................................................13   Need for Imaging in Radiation Therapy .................................................................................16   Megavoltage Cone Beam Computed Tomography (MV-CBCT) ...................................17   CT-on-Rails .....................................................................................................................17   Kilovoltage Cone Beam Computed Tomography (kV-CBCT) .......................................18   Current State of Radiation Dosimetry in kV-CBCT ...............................................................20   Objectives of this Research ....................................................................................................28   2 CONSTRUCTION OF A 50 TH PERCENTILE ADULT MALE ANTHROPOMORPHIC PHANTOM ....................................................................................36   Introduction .............................................................................................................................36   Methods and Materials ...........................................................................................................38   Materials ..........................................................................................................................38   Soft tissue-equivalent substitute (STES) ..................................................................39   Lung tissue-equivalent substitute (LTES) ................................................................40   Bone tissue-equivalent substitute (BTES) ................................................................40   Phantom Construction Methodology ...............................................................................41   Creating soft tissue molds ........................................................................................42   Introduction of soft tissue .........................................................................................43   Introduction of bone tissue .......................................................................................44   Phantom assembly ....................................................................................................45   Introduction of lung tissue ........................................................................................45   Results .....................................................................................................................................46   Materials ..........................................................................................................................46   Soft tissue-equivalent substitute ...............................................................................46   Lung tissue-equivalent substitute .............................................................................46   Bone tissue-equivalent substitute .............................................................................46   Completed Phantom ........................................................................................................46   Discussion ...............................................................................................................................47   Conclusions .............................................................................................................................48   5

3 CONSTRUCTION AND CHARACTERIZATION OF A WATER-EQUIVALENT FIBER OPTIC COUPLED DOSIMETER FOR USE AT DIAGNOSTIC ENERGIES ........54   Introduction .............................................................................................................................54   Methods and Materials ...........................................................................................................56   FOC Dosimetry System ...................................................................................................56   PMT Control Program .....................................................................................................58   Exposure Measurements ..................................................................................................59   Energy Dependence .........................................................................................................59   Linearity ..........................................................................................................................60   Reproducibility ................................................................................................................60   Dosimeter Response versus Bend Radius .......................................................................60   Angular Dependence .......................................................................................................61   Results .....................................................................................................................................61   Energy Dependence .........................................................................................................61   Linearity ..........................................................................................................................62   Reproducibility ................................................................................................................62   Dosimeter Response versus Bend Radius .......................................................................62   Angular Dependence .......................................................................................................63   Discussion ...............................................................................................................................63   Conclusions .............................................................................................................................65   4 CHARACTERIZATION OF THE FIBER-OPTIC COUPLED DOSIMETER AT MEGAVOLTAGE ENERGIES .............................................................................................74   Introduction .............................................................................................................................74   Methods and Materials ...........................................................................................................76   FOC Dosimetry System ...................................................................................................76   Dose Measurements .........................................................................................................77   Linearity ..........................................................................................................................77   Reproducibility ................................................................................................................78   Dose Rate Dependence ....................................................................................................78   Field Size Dependence ....................................................................................................78   Results .....................................................................................................................................79   Linearity ..........................................................................................................................79   Reproducibility ................................................................................................................79   Dose Rate Dependence ....................................................................................................79   Field Size Dependence ....................................................................................................79   Discussion ...............................................................................................................................80   Conclusion ..............................................................................................................................84   5 AN ORGAN AND EFFECTIVE DOSE STUDY OF XVI ® AND OBI ® CONE-BEAM CT SYSTEMS ........................................................................................................................89   Introduction .............................................................................................................................89   Methods and Materials ...........................................................................................................90   CBCT Systems Evaluated ...............................................................................................90   6

Anthropomorphic Phantom .............................................................................................92   Dosimetry System ...........................................................................................................93   Calculation of Organ Doses .............................................................................................95   Calculation of Effective Dose .........................................................................................97   Image Quality ..................................................................................................................98   Results .....................................................................................................................................99   Head ...............................................................................................................................100   Chest ..............................................................................................................................100   Pelvis .............................................................................................................................101   Discussion .............................................................................................................................101   Conclusions ...........................................................................................................................105   6 ESTIMATION OF ORGAN DOSES FROM KILOVOLTAGE CONE-BEAM CT IMAGING USED DURING RADIOTHERAPY PATIENT POSITION VERIFICATION ..................................................................................................................117   Introduction ...........................................................................................................................117   Methods and Materials .........................................................................................................118   CBCT Systems Evaluated .............................................................................................118   Phantom Setup and Dosimetry ......................................................................................119   Cone Beam Dose Index (CBDI) ....................................................................................119   ImPACT Dose Calculation ............................................................................................123   Results ...................................................................................................................................124   Discussion .............................................................................................................................124   Conclusions ...........................................................................................................................128   7 CONCLUSION .....................................................................................................................134   Results of this Work .............................................................................................................134   Opportunities for Future Work and Development ................................................................135   Anthropomorphic Phantom Development .....................................................................135   Fiber-optic Coupled (FOC) Dosimetry System .............................................................136   CBCT Dosimetry ...........................................................................................................137   Final Thoughts ......................................................................................................................138   LIST OF REFERENCES .............................................................................................................139   BIOGRAPHICAL SKETCH .......................................................................................................149  

7

LIST OF TABLES Table page

1-1 Comparison of CBDI W and RANDO phantom measurements ..........................................31   1-2 Imaging doses from Elekta XVI and Varian OBI ..............................................................31   1-3 Varian OBI organ doses measured in standard dose mode ................................................32   2-1 ICRP 103 organs of interest for the calculation of effective dose and their associated weighting factors ................................................................................................................49   3-1 Reproducibility of measurements with FOC dosimeter at kV energies ............................67   4-1 Reproducibility of measurements with FOC dosimeter at MV energies ...........................85   4-2 Counts from signal and reference fibers as well as output factors normalized to a 10x10 cm field size from a 200 MU irradiation ................................................................85   4-3 Counts from each optical fiber with no scintillator during a 50 MU irradiation ...............85   4-4 Counts from signal and reference fibers from a 50 MU irradiation after passing through a 400 nm high pass filter ......................................................................................86   4-5 Counts from signal and reference fibers from a 50 MU irradiation free in air. .................86   5-1 Nominal technical settings and measured HVLs for each imaging protocol investigated ......................................................................................................................107   5-2 Organs investigated and number of measurement locations............................................108   5-3 Weight fractions of red bone marrow, A i , and endosteum, E i , for various locations of interest of the 50th percentile adult male hybrid phantom. .............................................109   5-4 Organ and effective doses from the Elekta XVI CBCT system ......................................110   5-5 Organ and effective doses from the Varian OBI CBCT system ......................................111   5-6 Results of image quality tests for manufacturer installed protocols ................................112   6-1 Empirical ImPACT factors and normalized CBDI values for use with Equation 6-4. ....130   6-2 Measured CBDI values (in mGy air ) ± 1σ .........................................................................130   6-3 Measured organ doses, in mGy tissue , taken from Chapter 5, along with organ dose estimates made using the ImPACT CT patient dose calculator, also in mGy tissue ...........131   6-4 Organ dose conversion coefficients for each protocol .....................................................132   8

LIST OF FIGURES Figure page

1-1 Elekta Synergy linac with XVI CBCT system ..................................................................33   1-2 Varian 23iX linac with OBI CBCT system .......................................................................33   1-3 Head and body CTDI phantoms ........................................................................................34   1-4 Longitudinal dose profiles across a 26 cm imaging field in a CTDI body phantom .........34   1-5 TLD measured doses (cGy) within RANDO pelvic phantom using OBI system .............35   2-1 Completed 50 th percentile hybrid computational phantom ................................................50   2-2 Axial image with segmented organs from the hybrid computational phantom .................51   2-3 A finished bitmap image ready to be imported into the milling software .........................51   2-4 Finished soft tissue mold, ready to be poured with STES material ...................................52   2-5 Completed axial slice with a) STES, b) LTES, and c) BTES ............................................52   2-6 Torso of completed 50 th percentile physical phantom .......................................................53   3-1 Completed FOC dosimeter. Scintillating element at left and SMA connector at right .....68   3-2 Close up of scintillating element end of dosimeter ............................................................68   3-3 New dosimeter design with reference fiber incorporated into FOC dosimeter assembly .............................................................................................................................69   3-4 FOC dosimetry system schematic ......................................................................................69   3-5 Screenshot of PMT control program GUI .........................................................................70   3-6 Energy dependence of FOC dosimeter ..............................................................................71   3-7 Energy dependence of FOC dosimeter as a function of depth in soft tissue-equivalent material ..............................................................................................................................71   3-8 Dose linearity of FOC dosimeter. ......................................................................................72   3-9 Response of FOC dosimeter versus the bend radius of the optical fiber ...........................72   3-10 Angular dependence of FOC dosimeter to an axial irradiation .........................................73   3-11 Angular dependence of FOC dosimeter to a normal-to-axial irradiation ..........................73   9

4-1 Radiation induced light in a silica optical fiber. ................................................................87   4-2 Dose linearity of FOC dosimeter .......................................................................................87   4-3 Response of FOC dosimeter versus dose rate in MU/min .................................................88   5-1 Scanning configuration for each imaging protocol used .................................................113   5-2 Schematic detailing the components of the dosimetry system ........................................114   5-3 Axial slice of the physical phantom with an FOC dosimeter installed in the right kidney for dose measurements .........................................................................................115   5-4 Example of reconstructed images of CTP592 resolution phantom .................................115   5-5 Example of reconstructed images of CTP401 low contrast phantom ..............................116   6-1 Dose profiles from XVI chest protocol as measured at the center and periphery of a CTDI body phantom ........................................................................................................133   10

Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy IMAGING DOSES IN RADIATION THERAPY FROM KILOVOLTAGE CONE-BEAM COMPUTED TOMOGRAPHY By Daniel Ellis Hyer

May 2010

Chair: David E. Hintenlang Major: Nuclear Engineering Sciences

Advances in radiation treatment delivery, such as intensity modulated radiation therapy (IMRT), have made it possible to deliver large doses of radiation with a high degree of conformity. While highly conformal treatments offers the advantage of sparing surrounding normal tissue, this benefit can only be realized if the patient is accurately positioned during each treatment fraction. The need to accurately position the patient has led to the development and use of gantry mounted kilovoltage cone-beam computed tomography (kV-CBCT) systems. These systems are used to acquire high resolution volumetric images of the patient which are then digitally registered with the planning CT dataset to confirm alignment of the patient on the treatment table. While kV-CBCT is a very useful tool for aligning the patient prior to treatment, daily use in a high fraction therapy regimen results in a substantial radiation dose. In order to quantify the radiation dose associated with CBCT imaging, an anthropomorphic phantom representing a 50th percentile adult male and a fiber-optic coupled (FOC) dosimetry system were both constructed as part of this dissertation. These tools were then used to directly measure organ doses incurred during clinical protocols for the head, chest, and pelvis. For completeness, the dose delivered from both the X-ray Volumetric Imager (XVI, Elekta Oncology Systems, Crawley, UK) and the 11

12 On-Board Imager (OBI, Varian Medical Systems, Palo Alto, CA) were investigated. While this study provided a direct measure of organ doses for estimating risk to the patient, a practical method for estimating organ doses that could be performed with phantoms and dosimeters currently available at most clinics was also desired. To accomplish this goal, a 100 mm pencil ion chamber was used to measure the “cone beam dose index” (CBDI) inside standard CT dose index (CTDI) acrylic phantoms. A weighted CBDI (CBDIw), similar to the weighted CT dose index (CTDIw), was then calculated to represent the average dose in the acrylic phantom. By comparing this value to the measured organ doses, organ dose conversion coefficients were developed. These conversion coefficients allow specific organ doses to be estimated quickly and easily using readily available clinical equipment.

CHAPTER 1 INTRODUCTION Cancer Cancer refers to the group of diseases characterized by uncontrolled proliferation and spread of abnormal cells. 1 This uncontrolled proliferation commonly produces a mass of tissue called a malignant tumor. A malignant tumor may be contrasted with a non-cancerous benign tumor in that a malignancy is not self-limited in its growth, is capable of invading adjacent tissues, and may be capable of spreading to distant tissues (metastasizing), while a benign tumor has none of these properties. 2 If not controlled, the spread of cancerous cells to surrounding tissue or other parts of the body can result in death. In the United States alone, it is expected that nearly 1.5 million people will be diagnosed with cancer in 2009, resulting in approximately 560,000 deaths. 3 This ranks cancer as the second most common cause of death in the United States, exceeded only by heart disease. Once diagnosed, cancer is typically treated by surgery, chemotherapy, hormone therapy, gene therapy, radiation therapy, or a combination of these therapies depending on the specific type, location, and stage of cancer. Radiation therapy, which is of interest for the purpose of this work, will be explored in the following sections. Radiation Therapy In radiation therapy (also called radiotherapy), high-energy ionizing radiation is used to damage the genetic material (DNA) of cancer cells and stop them from growing and dividing. This ultimately helps control the spread of cancer and shrinks the size of tumors. The most common form of radiation therapy is external beam treatment, which typically uses a linear accelerator to produce a beam of x-rays that can be directed at the treatment site. The energy of the x-rays used is on the order of millions of electron volts (MeV), therefore the linear accelerator is said to produce a megavoltage (MV) beam. The MV beam of radiation has several 13

favorable characteristics which make it valuable for use in treating tumors. First, MV photons have sufficient energy to painlessly reach deep inside the patient. Second, skin tissue is spared with MV radiation compared to lower energy kilovoltage (kV) radiation used in diagnostic procedures. This is because secondary electrons resulting from interactions with MV photons at the surface of the patient have sufficient energy to travel away from the surface of the patient and deposit their energy at a finite depth in the patient rather than locally at the skin. Radiation therapy can also be administered internally, by placing a small radioactive source within a body cavity in close proximity to the area being treated (intracavitary brachytherapy) or by implanting small radioactive seeds directly into the tissue to be treated (interstitial brachytherapy). Some forms of radiation therapy also include directly injecting a radioactive material into the bloodstream that has been attached to a compound which is preferentially absorbed by the organ to be treated. The remainder of this section focuses on the treatment of cancer using external beam radiotherapy, laying a foundation for future discussion about the details of accurately positioning the patient on the treatment table. After diagnosis of the disease, a decision is typically made by the oncologist regarding which treatment modality(ies) will be used. If external beam radiotherapy is chosen, the first step is to develop a treatment plan. Modern treatment plans are based on computed tomography (CT) simulations, which are volumetric studies of the patient in the treatment position. 4 Before performing the CT simulation, the geometry and extent of the tumor to be treated is determined from previous diagnostic scans used to diagnose the disease. Once determined, the patient is brought in and placed on the CT table where a reference treatment isocenter is selected with the aid of a scout film. At this time, the treatment isocenter is marked on the patient using skin tattoos to aid in future alignment of the patient on the treatment table. Small metal markers are 14

also temporarily placed at these reference isocenter positions to serve as reference markers that are visible on the subsequent CT images. A CT scan of the region to be treated is then acquired and transferred to a computer workstation equipped with treatment planning software. At this workstation, organs near the treatment volume which could be at risk for radiation damage, referred to as organs at risk, as well as the gross visible extent of the tumor, referred to as the gross tumor volume (GTV), are localized and contoured. A margin around the GTV is then created based on the type, location, and aggressiveness of the tumor. This additional margin is known as the clinical target volume (CTV) and accounts for microscopic extensions of the disease which cannot be seen on the images but must be treated with radiation the same as the visible tumor. The CTV may include surrounding lymph nodes that have tested positive as well. An additional margin on the order of 1 cm is then drawn around the CTV, known as the planning target volume (PTV). This additional margin accounts for patient set-up uncertainties, machine variations in delivering the radiation, and intra-treatment variations that cause the treatment volume to move such as breathing. 5-7 As a consequence of creating the PTV, there may be some overlap between surrounding organs at risk and the treatment volume; ultimately restricting the dose that can be delivered to the target without causing normal tissue complications. 8, 9 However, if patient setup variations are reduced, then smaller margins can be used to create the PTV, resulting in a smaller treatment volume and potentially lowering the dose to surrounding organs at risk. 10 In an attempt to more accurately and reproducibly align the patient at each treatment session, there has been a recent trend towards the use of traditional imaging modalities in the treatment room, which is the topic of the next section. The final step in treatment planning is to determine the beam geometry and optimize the dose to the PTV using treatment planning software. The finished treatment plan is then exported 15

to a treatment console where it can be delivered to the patient. Treatment is typically delivered in discrete sessions, referred to as fractions, on an outpatient basis 5 days a week for several weeks. Fractions are used to exploit the differences in the ability of healthy cells to repair themselves following radiation damage while cancerous cells are not as efficient at repairing damage and subsequently die. 11

Need for Imaging in Radiation Therapy Recent advances in radiation treatment delivery such as 3D conformal radiation therapy (3DCRT) and intensity modulated radiation therapy (IMRT) have made it possible to deliver large doses of radiation to target volumes with a high degree of conformity. This is particularly true for IMRT, where very tight margins around the target volume are possible along with very steep dose gradients outside of the target volume. While a highly conformal treatment offers the advantage of sparing surrounding normal tissue, this benefit can only be realized if the patient is accurately positioned during each treatment fraction. 12-16 Traditionally, this is done in two steps: 1. The patient is aligned to the treatment isocenter using skin tattoos made during the CT simulation and 2. A pair of orthogonal portal images of the patient are taken using the MV treatment beam and an electronic portal imaging device (EPID). 17, 18 These portal images are then compared to digitally reconstructed radiographs (DRRs) from the CT simulation to verify the patient and tumor’s position at the time of treatment at a dose of 4-16 cGy per image pair. 19

Unfortunately, the 2-D nature of projection images and the inherently low contrast of MV imaging limit the accuracy of this technique. Because soft tissue targets typically cannot be seen in the MV portal images, bony anatomy is often used as a surrogate landmark for target localization. 20-22 These shortcomings have led to the development of several other imaging modalities to verify patient setup based on soft tissue structures. These alternate imaging modalities are described in the following sections. 16

Megavoltage Cone Beam Computed Tomography (MV-CBCT) As discussed above, verification of patient setup prior to treatment typically relies on the comparison of 2-D portal images to DRRs. However, this process does not make full use of the volumetric information available from treatment planning. Because both CT simulation and treatment planning generate and utilize 3-D datasets, the logical next step is to validate the patient’s position in 3-D. Megavoltage cone-beam computed tomography (MV-CBCT) utilizes the megavoltage beam and the same EPID employed for portal imaging to acquire a series of low-dose 2D projections, with the patient dose reported to be in the range of 1-10 cGy. 23 These 2D projections are then used to reconstruct 24 a 3D volumetric data set that can be registered with the planning CT to determine if the patient is properly aligned with the machine isocenter. It should also be noted that this procedure is repeated at each treatment fraction, leading to an accumulation of dose on the order of a Gy or more to tissue in the imaging field. A recent clinical study has shown that MV-CBCT images provide adequate quality for the purpose of patient positioning based on implanted metal seeds, bony landmarks, and air cavities. 25 However, the limiting factor of MV images will always be the lack of soft tissue contrast and the ability to register with the planning CT based on soft tissue landmarks. 26

CT-on-Rails The desire to achieve target localization with soft tissue contrast comparable to planning CT images has led to the development of “CT-on-rails” systems. 27, 28 These systems incorporate a diagnostic CT system into the treatment room that shares the same patient table with the linear accelerator. In a typical configuration, the patient table is rotated 180 o between imaging and treatment. Once the patient is positioned on the table, the CT gantry is mobilized and slides on rails, passing over the patient while scanning (rather than the table moving like in typical diagnostic CT scans). The end result is a diagnostic quality CT image that can be registered with 17

the planning CT. The patient table is then rotated 180 o towards the treatment gantry and the appropriate shifts are made to the patient table, as calculated from the image registration, to ensure that the patient is accurately aligned with the treatment isocenter. Imaging doses to the patient are on the order of 1-3 cGy, on par with a typical diagnostic CT exam. 23 While the CT- on-rails system provides arguably the best treatment room image quality possible, it is not without its own faults. The complexity, space requirements, and cost of such a system are the most obvious criticisms and have limited its popularity with very few units in use today. 29

Kilovoltage Cone Beam Computed Tomography (kV-CBCT) Kilovoltage cone-beam computed tomography (kV-CBCT) combines the superior soft tissue contrast of a CT-on-rails system with the convenience of a MV-CBCT system. These systems consist of a kV generator and flat-panel detector mounted to the linac gantry using retractable arms and share a common axis of rotation with the treatment beam. This technology has gained popularity in recent years and two such systems are currently commercially available: the X-ray Volumetric Imager (XVI ® , Elekta Oncology Systems, Crawley, UK), seen in Figure 1- 1, and the On-Board Imager (OBI ® , Varian Medical Systems, Palo Alto, CA), seen in Figure 1-2. Image acquisition is similar to MV-CBCT, with a number of 2D projections taken as beam rotates around the patient which are later reconstructed into a 3D volumetric data set. As a patient setup tool, a kV-CBCT scan is taken after the patient is aligned to the treatment isocenter using the skin tattoos and then digitally registered with the planning CT to verify the patient’s position. The use of kV-CBCT yields much better image contrast over traditional MV portal images or MV-CBCT, permitting better localization of soft-tissue structures for improved patient setup accuracy. One recent publication has identified a positioning accuracy improvement, when compared to conventional MV portal images, of up to 3 mm for lung cancer patients with the use of kV-CBCT. 30 The improved patient setup accuracy at each treatment fraction allows complex 18

radiation treatments to be delivered safely and precisely, ultimately leading to improved tumor control and reduced treatment-related toxicity. 31

While the dose from a single kV-CBCT is reported to be 1-10 cGy, 23 the repeated daily use for patient setup in a high fraction therapy regimen (~40 fractions) has been projected to lead to a total dose of up to 4 Gy 32 to surrounding tissue. Unfortunately, unlike traditional MV portal images or MV-CBCT, the additional dose from daily kV-CBCT image guidance cannot be taken into account using current treatment planning software. 33 This is because the imaging dose from kV-CBCT is not radiobiologically equivalent to the MV dose from the treatment beam. 34 A value in the range of 2-4 has been suggested for the relative biological effectiveness (RBE) of kV versus MV radiation with chromosomal damage as the endpoint. 35 Another issue to consider is the volume of tissue irradiated by each modality. The dose from a kV-CBCT will be distributed through a much larger volume than a conventional MV portal image due to the increased field of view (FOV). The FOV for CBCT can be up to 52.4 cm in diameter and 26 cm long, 36 while MV portal images are typically limited to 20x20 cm projections. 37 This large FOV results in critical structures and normal tissue well outside of the treatment volume receiving an imaging dose at every fraction. This effect is further magnified by the fact that some organs near the treatment volume typically approach institution or protocol specific dose limits from the treatment beam alone. Therefore, when the extra dose from CBCT imaging is considered, institution or protocol specific organ dose limits, which are put in place to provide a balance between tumor control and normal tissue damage, may be exceeded. This is especially of concern for serial organs such as the spinal cord where the organ can be permanently damaged by an overdose of radiation. Another concern regarding the daily use of kV-CBCT imaging for patient positioning is the increased risk of inducing a secondary cancer. According to Publication 60 from the 19

International Commission of Radiological Protection (ICRP), the probability of inducing a fatal cancer from a single radiographic exposure is 5x10 -5 per mSv. 38 This value is based on a linear no-threshold model of radiation risk and is derived primarily from studies of atomic bomb survivors. A recent study reported that the effective dose from a single chest or pelvis kV-CBCT was approximately 23 mSv. 39 Using this value along with the risk of inducing a fatal cancer published in ICRP 60, a 35 fraction therapy regimen with daily kV-CBCT imaging has been suggested to induce an additional cancer risk of up to 4%. 39

For the reasons discussed above, it is important to quantify organ doses from kV-CBCT imaging in radiation therapy. In the following chapters of this dissertation, the tools necessary to perform this task are described and results are presented. However, before presenting this work, it is important to understand the current state of radiation dosimetry in kV-CBCT through an extensive literature review, presented in the next section. For simplicity, when the term CBCT is used in the remainder of this dissertation, it will refer to kV-CBCT unless otherwise noted. Current State of Radiation Dosimetry in kV-CBCT This section begins by presenting a brief history of fan-beam CT dosimetry in order to provide a background for the discussion of the current state of CBCT dosimetry. The standard for determining radiation dose in fan-beam CT is the multiple scan average dose (MSAD). 40 The MSAD represents the average dose along the longitudinal scan axis, as measured in a standardized cylindrical acrylic phantom, which includes the primary beam contribution as well as the dose attributable to scattered radiation emanating from all adjacent slices. The MSAD can be measured directly by taking a large series of axial CT scans of a phantom with a detector remaining in the center of the scan length and summing the dose contribution from each slice. In the early days of CT, the direct measurement of MSAD was very time consuming as multiple axial scans resulted in considerable x-ray tube heating which required additional time to be taken 20

between scans for cooling. This led to the development of the computed tomography dose index (CTDI), a nominally equivalent method of estimating MSAD that could be performed with a single axial scan. 41 The equivalence of MSAD and CTDI requires that all contributions from the tails of the radiation dose profile be included in the CTDI dose measurement. The formal definition of CTDI is shown in Equation 1-1, where L is the nominal slice thickness, D(z) is the longitudinal dose profile, and Z is the integration range.

( ) 2 2 1 CTDI Z Z D z dz L − = ∫ (1-1) For simplicity, the integral of the dose profile is typically measured by taking a single axial slice at the center of a 100 mm long ionization chamber, and this measurement is known as CTDI 100 . By definition, CTDI 100 represents the MSAD at the center of a 100 mm scan and underestimates the MSAD for longer scan lengths. This is because the contributions from the tails of the radiation dose profile beyond the 100 mm integration range are not included. In order to reflect the variation of dose deposition at depth, CTDI 100 is measured at both central and peripheral locations within a specialized phantom. The phantoms used for CTDI measurements were standardized by the Food and Drug Administration (FDA) and consist of polymethylmethacrylate (PMMA) cylinders that are 15 cm in length and either 16 or 32 cm in diameter, representing an adult head and body, respectively. 42 In the clinic, these phantoms are typically referred to as head and body CTDI phantoms. The phantoms have five holes along the longitudinal axis in which to place an ionization chamber: one in the center and four other peripheral locations 1 cm from the surface at 0 o , 90 o , 180 o , and 270 o . For clarification, both the head and body CTDI phantoms are shown in Figure 1-3. In an attempt to represent the average dose within the scan plane, the central as well as the average of the peripheral measurements are combined into the weighted CTDI (CTDI w ), as shown below in Equation 1-2. 21

Full document contains 150 pages
Abstract: Advances in radiation treatment delivery, such as intensity modulated radiation therapy (IMRT), have made it possible to deliver large doses of radiation with a high degree of conformity. While highly conformal treatments offers the advantage of sparing surrounding normal tissue, this benefit can only be realized if the patient is accurately positioned during each treatment fraction. The need to accurately position the patient has led to the development and use of gantry mounted kilovoltage cone-beam computed tomography (kV-CBCT) systems. These systems are used to acquire high resolution volumetric images of the patient which are then digitally registered with the planning CT dataset to confirm alignment of the patient on the treatment table. While kV-CBCT is a very useful tool for aligning the patient prior to treatment, daily use in a high fraction therapy regimen results in a substantial radiation dose. In order to quantify the radiation dose associated with CBCT imaging, an anthropomorphic phantom representing a 50th percentile adult male and a fiber-optic coupled (FOC) dosimetry system were both constructed as part of this dissertation. These tools were then used to directly measure organ doses incurred during clinical protocols for the head, chest, and pelvis. For completeness, the dose delivered from both the X-ray Volumetric Imager (XVI, Elekta Oncology Systems, Crawley, UK) and the On-Board Imager (OBI, Varian Medical Systems, Palo Alto, CA) were investigated. While this study provided a direct measure of organ doses for estimating risk to the patient, a practical method for estimating organ doses that could be performed with phantoms and dosimeters currently available at most clinics was also desired. To accomplish this goal, a 100 mm pencil ion chamber was used to measure the "cone beam dose index" (CBDI) inside standard CT dose index (CTDI) acrylic phantoms. A weighted CBDI (CBDIw), similar to the weighted CT dose index (CTDIw), was then calculated to represent the average dose in the acrylic phantom. By comparing this value to the measured organ doses, organ dose conversion coefficients were developed. These conversion coefficients allow specific organ doses to be estimated quickly and easily using readily available clinical equipment.