George X. Ding a) Department of Radiation Oncology, Vanderbilt University School of Medicine, Nashville, TN 37232, USA

Transcription

1 Image guidance doses delivered during radiotherapy: Quantification, management, and reduction: Report of the AAPM Therapy Physics Committee Task Group 180 George X. Ding a) Department of Radiation Oncology, Vanderbilt University School of Medicine, Nashville, TN 37232, USA Parham Alaei University of Minnesota, Minneapolis, MN 55455, USA Bruce Curran Virginia Commonwealth University, Richmond, VA 23284, USA Ryan Flynn University of Iowa, Iowa City, IA 52242, USA Michael Gossman Tri-State Regional Cancer Center, Ashland, KY 41101, USA T. Rock Mackie University of Wisconsin, Madison, WI 53715, USA Moyed Miften University of Colorado, Aurora, CO 80045, USA Richard Morin Mayo Clinic, Jacksonville, FL 32224, USA X. George Xu Rensselaer Polytechnic Institute, Troy, NY 12180, USA Timothy C. Zhu University of Pennsylvania, Philadelphia, PA 19104, USA (Received 18 July 2017; revised 10 January 2018; accepted for publication 10 January 2018; published 24 March 2018) Background: With radiotherapy having entered the era of image guidance, or image-guided radiation therapy (IGRT), imaging procedures are routinely performed for patient positioning and target localization. The imaging dose delivered may result in excessive dose to sensitive organs and potentially increase the chance of secondary cancers and, therefore, needs to be managed. Aims: This task group was charged with: a) providing an overview on imaging dose, including megavoltage electronic portal imaging (MV EPI), kilovoltage digital radiography (kv DR), Tomotherapy MV-CT, megavoltage cone-beam CT (MV-CBCT) and kilovoltage cone-beam CT (kv- CBCT), and b) providing general guidelines for commissioning dose calculation methods and managing imaging dose to patients. Materials & Methods: We briefly review the dose to radiotherapy (RT) patients resulting from different image guidance procedures and list typical organ doses resulting from MV and kv image acquisition procedures. Results: We provide recommendations for managing the imaging dose, including different methods for its calculation, and techniques for reducing it. The recommended threshold beyond which imaging dose should be considered in the treatment planning process is 5% of the therapeutic target dose. Discussion: Although the imaging dose resulting from current kv acquisition procedures is generally below this threshold, the ALARA principle should always be applied in practice. Medical physicists should make radiation oncologists aware of the imaging doses delivered to patients under their care. Conclusion: Balancing ALARA with the requirement for effective target localization requires that imaging dose be managed based on the consideration of weighing risks and benefits to the patient American Association of Physicists in Medicine [ Key words: IGRT, image dose management, image dose reduction techniques, image guidance doses, recommended dose threshold e84 Med. Phys. 45 (5), May /2018/45(5)/e84/ American Association of Physicists in Medicine e84

2 e85 Ding et al.: Image Guidance Doses to Radiotherapy Patients e85 TABLE OF CONTENTS 1. INTRODUCTION 2. OVERVIEW OF DOSE RESULTING FROM IMAGE GUIDANCE PROCEDURES 2.A. Megavoltage beam imaging 2.B. Kilovoltage beam imaging 3. DOSE CALCULATION ALGORITHMS FOR KVAND MV IMAGING DOSE 3.A. Monte Carlo-based methods 3.B. Model-based methods 4. KILOVOLTAGE IMAGING BEAM DOSIMETRY 4.A. Input data for kv x-ray beam characterization 4.B. Output of a kv imager 4.C. In vivo dosimetry 5. METHODS OF ACCOUNTING FOR IMAGING DOSE 5.A. Patient-specific imaging dose calculations 5.B. Nonpatient-specific imaging dose estimations 6. RECOMMENDATIONS 6.A. General recommendations 6.B. Imaging dose output and consistency checks 6.C. Accounting for imaging dose to RT patients 6.D. Available techniques to reduce imaging dose to patients 7. ACKNOWLEDGMENTS 1. INTRODUCTION Image-guided radiation therapy (IGRT) has rapidly been adopted as the standard of care to improve the geometric accuracy of patient positioning during radiotherapy. 1 6 IGRT can significantly reduce target positioning errors, therefore, enabling highly conformal treatments. The acquired images during treatment delivery can be used for monitoring patient and target geometry changes, potential adaptive planning, 7 9 or margin reduction. 10 During the course of IGRT, the image guidance procedure is typically performed for each treatment fraction. The patient may occasionally be imaged more than once during any fraction in order to ensure that the patient s position is correct, and to reposition if necessary. Since these imaging procedures deliver additional radiation dose to patients, understanding the magnitude of this dose has become increasingly important in order to minimize its risk. While the commonly adopted radiation protection safety philosophy of As Low As Reasonably Achievable (ALARA) is also applicable for imaging dose, minimizing imaging doses by employing reasonable methods should not compromise target localization. Currently, imaging dose is not accounted for in RT treatment planning, 11 and the purpose of this report is to provide imaging dose data and develop guidelines for clinicians to make informed decisions regarding the risk and benefits of x-ray image guidance. The report of AAPM TG-75 1 provided dose estimates for a variety of image guidance techniques and recommended strategies for minimizing imaging dose while improving treatment delivery. TG-75 also identified the need to manage imaging dose for the large number of current and emerging imaging techniques in RT. These techniques include CT; 4D- CT; diagnostic x-ray imaging; in-room CT; dual radiographic x-ray imaging; fluoroscopy; and portal imaging, either using film or an electronic portal imaging device (EPID), in various modes. This report is intended to complement the AAPM TG-75 report. 1 It contains updated dose data resulting from current image acquisition procedures and modalities and addresses current issues in estimating and accounting for imaging dose during treatment planning when required. It also addresses how to minimize dose due to imaging procedures, and provides recommendations for incorporating suggestions made in AAPM TG-75 1 and ICRP This report also offers guidelines for commissioning imaging beams in order to provide patient-specific imaging dose calculations when needed. The imaging dose referred to in this report is absorbed dose to medium (e.g., dose to bone or dose to soft tissue), which differs from the effective dose metric used in TG-75. Effective dose, as defined by the ICRP, 13 is based on an estimate of biological effect integrated over the entire patient body, requiring a detailed knowledge of the radiation energy spectrum and irradiation geometry. Most treatment planning systems (TPSs) are not capable of calculating and displaying kv energy range dose distributions without special research tools, nor are they capable of converting calculated absorbed dose to effective dose and displaying the results. Thus, in order to avoid this additional level of complexity, absorbed dose is used in this report rather than effective dose. The magnitude of imaging dose is dependent on many factors, including the frequency of imaging and the technique used. For a single treatment fraction, two or more planar images or one or more volumetric acquisitions may be acquired. In the case of the Brainlab AG (Feldkirchen, GER- MANY) ExacTrac 14,15 or Accuray, Inc. (Sunnyvale, CA) CyberKnife systems, 16,17 the number of planar image acquisitions per session can potentially be well over 80, even for none of the SRS/SBRT treatments, as frequent imaging is typical for monitoring patient position. Each of these imaging procedures deliver additional radiation dose to normal tissue. 1,18 25 Depending on the imaging protocols and techniques used there are large variations in the imaging dose delivered to the patient. In general, MV imaging delivers higher doses than kv imaging. 26 With the exception of MV volumetric imaging, a single image acquisition can deliver a dose of cgy to the patient depending on the imaging modality. However, even with demonstrated progress in dose reduction, 21,25,27 41 the kv-cbct procedure employed for pelvic imaging can add a cumulative dose of 1 3% of the prescription dose during the course of treatment. Since the photoelectric effect is the dominant photon interaction process for kv imaging, the imaging dose to bony structures is a factor of 2 4 greater than that to soft tissue. 20,21,29 For an MV-CBCT image acquisition, the imaging dose can be greater than 10 cgy, depending on the imaging site and clinical protocol. 23,42 45 With all imaging procedures, the imaged volume is generally larger than the treatment volume, and tissues and organs outside the

3 e86 Ding et al.: Image Guidance Doses to Radiotherapy Patients e86 therapeutic beams are exposed to imaging radiation. These imaging doses to organs outside the treatment volume also need to be managed, as they may present an increased risk, especially in the case of pediatric patients. In this report, we review a variety of methods for determining x-ray imaging dose. For kv-cbct imaging dose, these include experimental phantom measurements, 19,38,46,47 in vivo measurements on patients, 48 and Monte Carlo (MC) calculations. 21,22,25 27,29,30,40,49 52 Commercially available treatment planning systems with user modifications, as well as mathematical models, 39 have been used to calculate the MV-CBCT 42,45 and kv-cbct 53,54 dose. Measurements have been used to estimate doses from 2D kv radiographs, 32,38 kv-cbct, 21,27 41 MV portal images, 26,32,38 MV- CBCT, 23,42,44 and MVCT. 55,56 In radiation therapy the prescribed therapeutic dose represents the minimum dose to part or all of the planning target volume. In developing strategies for managing the imaging dose, this task group considers 5% of the therapeutic target dose to be the threshold beyond which imaging dose should be accounted for in the treatment planning process. Dische et al. 57 stated that there was evidence from published clinical data and a suggestion from an analysis of the Continuous Hyperfractionated, Accelerated Radiotherapy (CHART) pilot study data 58 that dose variations as small as 5% may lead to real variations in both tumor response and the risk of morbidity. 59 Many studies on accuracy requirements in radiotherapy have recommended an accuracy level of 5% in the delivery and determination of dose to tumors and normal tissue Hence, the choice of a 5% threshold is based on considerations of clinical relevance, accuracy of dose calculation and delivery, dose tolerances for critical organs, and feasibility in clinical practice. Data currently available in the literature, and quoted throughout this report, for patient populations undergoing IGRT indicate that imaging dose is generally less than 5% of the therapeutic target dose, 26,32,66,67 except for some imaging procedures that use MV beams, particularly MV-CBCT. 42 Balancing ALARA principles with the requirement for effective target localization, however, requires the imaging dose to be managed on the consideration of weighing risks and benefits to the patient. 2. OVERVIEW OF DOSE RESULTING FROM IMAGE GUIDANCE PROCEDURES 2.A. Megavoltage beam imaging Megavoltage imaging modalities capture projection images using either electronic portal imaging devices (EPIDs) [Fig. 1(a)] or, in the case of MVCT in the Tomotherapy Hi- Art Radixact system (Accuray Inc., Sunnyvale, CA), 55,68,69 a single-row CT detector. A typical pair of orthogonal 6 MV portal images acquired using an EPID results in a dose distribution like the one shown in Fig. 1(b), and organ doses of 1 5 cgy (Tables Ia Ic) while the imaging dose from a 2.5-MV image beam 70 is about 50% of that from a 6-MV beam. Volumetric MV-CBCT images are reconstructed using projections acquired with the EPID and result in a greater dose than a pair of orthogonal MV portal images. Reported doses per monitor unit for MV-CBCT delivered to regions and organs typically considered are listed in Table Id. 45 Monitor unit values ranging from 2 to 15 have been reported in the literature ,71 Typically the head and neck region is imaged with lower monitor unit protocols (2 5 MU) than the thoracic or pelvic regions, which may be imaged at up to 15 MU. Figure 2 shows an MV-CBCT dose distribution for a pelvic patient imaged at 15 MU for reference. The Siemens kview system provides the option to improve MV-CBCT image quality per unit dose by generating the imaging beam with a low Z (carbon) electron target and a 4.2-MeV electron beam, increasing the percentage of kilovoltage photons in the imaging beam considerably relative to MV-CBCT acquired with a conventional 6-MV beam generated with a tungsten target. 72 With the kview system it is possible to obtain the same contrast-to-noise ratio as conventional MV-CBCT at about one-third the imaging dose. 73 Organ-specific kview CBCT doses reported by Beltran 74 and Dzierma et al. 75 ranged from 0.6 cgy to 1.2 cgy/ MU. Image doses from kview based MV-CBCT per monitor unit are generally less than those of conventional 6-MV beams. 44,73 80 Table Ie shows measured Tomo MVCT doses at the center of a 30-cm water phantom. Doses range from 0.8 to 2.5 cgy and depend on the acquisition mode. 2.B. Kilovoltage beam imaging Current kv imaging devices are generally integrated into linear accelerators and are capable of acquiring both 2D radiographs and 3D volumetric kv-cbct images. 8,81 83 Examples of kv-cbct scanners integrated into a Varian Medical Systems, Inc. (Palo Alto, CA) On Board Imaging (OBI) system and an Elekta (Stockholm, Sweden) X-Ray Volume Imaging (XVI) system are shown in Fig. 3. Using MC methods to simulate an earlier version of the kv-cbct scanner on the Varian OBI system (Varian OBI 1.3), Ding et al. reported 21 the imaging doses received by patients from kv-cbct scans of different treatment sites, including head and neck, chest, and abdomen. The imaging doses resulting from a single kv-cbct procedure were 1 9 cgy to soft tissues and 6 29 cgy to bones and depended on the patient size and the site of the scan. 21 Since the introduction of kv imaging systems, progress has been made by the vendors to reduce the imaging dose 29 while maintaining or improving image quality. Examples of these efforts include better reconstruction techniques, improved software implementation, and the use of lower beam energies (100 and 110 kvp, in addition to the standard 125 kvp x rays), where beam energy is optimized for the size of the patient and the atomic number of the structures being imaged. 85 Newer kv-cbct acquisition techniques, which utilize 200 degree scans (instead of 370 degree ones), were introduced to reduce the imaging dose to patients for head and pelvis scans. 29

4 e87 Ding et al.: Image Guidance Doses to Radiotherapy Patients e87 (a) (b) FIG. 1. (a) Conventional electronic portal imaging device (EPID) and (b) typical dose distributions and organ dose-volume histograms (DVHs) resulting from an orthogonal pair of 6 MV portal images (2 MU per image). Reproduced from Ding and Munro. 26 TABLE IA. Typical organ doses for the head and neck treatment site with MV EPID portal imaging. D50 is minimum dose delivered to 50% of the organ volume (from Reference [26] for 6 MV beam and Reference [70] for 2.5 MV beam). These are for a typical pair of orthogonal setup fields (2 MU/field for 6 MV and 1 MU for 2.5 MV). Brain TABLE IB. Typical organ doses for the chest treatment site with MV EPID portal imaging (from References [26] and [70]) for a typical pair of orthogonal setup fields (2 MU/field for 6 MV and 1 MU for 2.5 MV). Chest D50 range (cgy) D50 range (cgy) Organ 6 MV 2.5 MV Organ 6 MV 2.5 MV Brain Brainstem Chiasm Eyes Optic Nerves Pituitary Aorta Lungs Esophagus Kidney Heart Liver Spinal Cord This not only reduces CBCT acquisition times, but results in a nonuniform exposure of the patient with the potential to avoid irradiating superficial critical structures. 29 These improvements have reduced imaging dose by more than an order of magnitude in some cases. Figures 4 6 compare dose distributions between kv-cbct scanners on newer

5 e88 Ding et al.: Image Guidance Doses to Radiotherapy Patients e88 TABLE IC. Typical organ doses for the pelvis treatment site with MV EPID portal imaging (from Reference [26] and [70]) for a typical pair of orthogonal setup fields (2 MU/field for 6 MV and 1 MU for 2.5 MV). Pelvis Organ D50 range (cgy) 6 MV 2.5 MV Bladder Bowel Femoral heads Prostate Rectum TABLE ID. MV-CBCT doses per monitor unit using a 6 MV treatment beam with an acquisition arc of 200 degrees, starting at 270 degrees and stopping at 110 degrees (from Reference [45]). Location Isocenter dose (cgy/mu) Average organ dose (cgy/mu) Maximum organ dose (cgy/mu) Cranium Total-brain Left lens Right lens Left eye Right eye Thorax Left lung Right lung Total lung Spinal canal Heart Vertebral bodies Soft Tissue Pelvis Femoral heads TABLE IE. Tomo MVCT dose at the center of a 30-cm water phantom and its dependency on acquisition protocols. MVCT in Tomo Acquisition mode Fine pitch (4 mm couch travel/rotation) Normal pitch (8 mm couch travel/rotation) Coarse pitch (12 mm couch travel/rotation) Dose (cgy) 2.5 cgy 1.2 cgy 0.8 cgy From Edward Chao, Accuray Incorporated and T. Rock Mackie, UW, Madison, WI. Varian OBI systems (OBI 1.4/1.5) and on a previous version of the Varian OBI system (OBI 1.3). 26,29 When two orthogonal planar kv images are sufficient for an image guidance task, 2D radiographs are often used. The doses resulting from 2D kv imaging, investigated using multiple methods, 26,32,38 have been shown to be much lower compared to those from volumetric kv-cbct 26 procedures. Figure 7 shows typical dose distributions resulting from a pair of orthogonal planar images in head, thorax, and pelvis scans. 26 Similar lower doses, on the order of 0.1 cgy, were reported for fixed double x-ray tube systems that use projected images for patient localization, such as the Brainlab ExacTrac 14,15 and the Accuray CyberKnife. 16,17 3. DOSE CALCULATION ALGORITHMS FOR KV AND MV IMAGING DOSE 3.A. Monte Carlo-based methods Monte Carlo (MC) techniques for simulating dosimetry problems have evolved considerably over the last three decades 86 and are regarded as the gold standard in dose calculations. The development of a special purpose MC code, BEAM, 87,88 made it practical to simulate megavoltage and kilovoltage beams. With many improvements in both accuracy and computational efficiency, 87,89 MC techniques have been used to characterize therapeutic megavoltage beams from linear accelerators and kilovoltage photon beams from x-ray units. 21,25,51,84,86,88,90 92 Given these capabilities, MC simulations have been used to calculate realistic imaging dose distributions in patients resulting from different x-ray imaging systems. 21,22,25,27,29,30,40,49 51 These studies provide detailed information about patient organ dose resulting from different image guidance procedures. Although a number of commercial treatment planning systems incorporate MC calculations for MV beams, none currently allow MC calculations for kv beams. 3.B. Model-based methods FIG. 2. Dose distribution resulting from an MV-CBCT localization procedure of a prostate cancer patient using a 15 MU imaging protocol with a 6 MV beam. Reproduced from Miften et al. 42 It is worth noting that the asymmetric dose distribution is because the scan is acquired with a gantry rotation of 200 degrees. Model-based dose calculation algorithms were developed for accurate MV beam dose calculations and are commonly implemented in commercial treatment planning systems. 93 When an imaging procedure uses MV beams, these algorithms are capable of accurately calculating the imaging dose.

6 e89 Ding et al.: Image Guidance Doses to Radiotherapy Patients e89 (a) (b) wedge to mimic the isodose tilt of a half fan/half-bow tie imaging beam in the treatment planning system. Further work in this area has demonstrated that this could also be achieved by inserting a compensator in the beam. 99 The same commercial TPS has also been used to model kv beams from Elekta XVI and Siemens kvision imaging systems. 100,101 In general, use of these algorithms produces dose distributions of sufficient accuracy, except in and around bony structures. A proposed correction method, the Medium-Dependent Correction (MDC) approach, 102 accounts for atomic number dependency when computing kv dose distributions and can potentially increase the accuracy of kv imaging dose calculations to an acceptable 10 20%. 94,95 With additional improvements in commercial TPSs, therefore, it may become feasible to use the same model-based algorithms to calculate doses from both an MV therapeutic beam and a kv imaging beam. 4. KILOVOLTAGE IMAGING BEAM DOSIMETRY Unlike MV imaging beams that can be easily added to treatment plans for dose calculation purposes, kv imaging beams generally require commissioning in treatment planning systems. As indicated above, current commercial treatment planning systems do not accommodate the addition of kv imaging beams in routine clinical practice. Considering that kv imaging dose calculation in treatment planning systems may become possible in the future, however, guidance is provided in Sections 4.A and 4.B to address kv imaging beam data acquisition. FIG. 3. kv image devices integrated into linear accelerators: (a) Varian OBI system on a Varian Trilogy treatment unit, Reproduced from Ding at el. 84 ; (b) Elekta XVI on an Elekta Synergy treatment unit. Morin et al. 43 and Miften et al. 42 calculated the imaging dose from MV-CBCT using two different treatment planning systems and reported dose calculation accuracy of better than 3%. The dose from the Siemens kview system has been computed using a commercial treatment planning system 44,76 by implementing the kview beam spectrum given by Faddegon et al. 72 in the TPS. It is feasible to use model-based dose calculation algorithms to perform dose calculations for kv-range beams, but there are inherent inaccuracies in the approach. Commercially available model-based algorithms underestimate dose to bone by up to 300% 94,95 when used for kv beams due to the fact that they do not account for photoelectric effect. 21,93,96,97 Alaei et al. demonstrated the feasibility of modifying an existing MV model-based dose calculation algorithm in a commercial TPS to calculate kv-cbct dose. 53 This required the addition of low-energy deposition kernels 98 which accounted for the density effect of bone but not the photoelectric effect. In addition, unlike MV beams, modeling kv beams in a planning system requires beam data for the respective kv system, a topic which is further discussed in Section 5. In reference 51, Alaei et al. used a 4.A. Input data for kv x-ray beam characterization Characterizing kv imaging beams in a planning system requires collecting beam data, including depth-dose curves, cross-profiles, and the absolute dose resulting from an image acquisition procedure. There are inherent challenges in collecting data for kv imaging beams due to the low radiation dose rate and the strong dependence on the medium in which the measurements are made. To overcome these challenges, experimentally validated MC-simulated beam data can be used. 53,84,103 Caution should be taken when using the beam data generated from MC simulations, as they will depend on the simulation parameters. 4.B. Output of a kv imager The beam output should be collected by the clinical physicist using a proper detector calibration and data collection protocol. Although a method for determining kv absorbed dose with a calibrated ionization chamber based on x-ray beam specifiers, such as half-value layer (HVL) and kvp, is available from dosimetry protocols, the calibration conditions recommended in these dosimetry protocols are often not applicable to imaging acquisition procedures, especially when the x-ray source is moving during the scan. It is known that water is the most suitable medium for kv x-ray beam measurements. However, plastic phantom materials are more convenient. A method to determine the output

7 e90 Ding et al.: Image Guidance Doses to Radiotherapy Patients e90 (a) (b) (c) % volume (d) (e) (f) Head Head Scan: OBI 1.3 Standard Dose Head: OBI OBI Spinal cord 80 Spinal cord Left eye Bone Right eye Left eye Brain Brain Right eye Body 40 Right eye Bone 40 Spinal cord Body Left eye dose /cgy dose /cgy % volume % volume dose /cgy FIG. 4. Typical doses for a head scan shown in color wash along with dose-volume histograms (DVHs) for radiosensitive organs for the Varian OBI 1.3 (a,d); for the OBI 1.4 during a Standard-Dose Head scan (b,e), where the x-ray source is positioned below the patient; and for the OBI 1.4 during a head scan with the x- ray source positioned above the patient (c,f dashed lines). The solid lines in (f) are reproduced from (e) and show the quantitative effect of rotating the x-ray source from the back to the front of the patient. Note that the abscissa in (d) is 10 times larger than in (e). Reproduced from Ding et al. 29 (Scanning parameters are listed Table IId). resulting from a specific image acquisition procedure has been described 103 where the effect of using different plastic phantom materials in the kv energy range is investigated. This study includes two water equivalents, Plastic Water Low Energy Range (PW-LR CIRS, Inc., Norfolk, VA) and Solid Water (Gammex, Inc., Middleton, WI), along with the less water-equivalent polymethyl methacrylate (PMMA). Caution should be used when interpreting the measured data from solid water-equivalent and PMMA phantoms to determine the output of a kv beam, as the uncertainties can be significant (8 20%). 103 The considerations for a suitable phantom include phantom size relative to x-ray field size, beam attenuation by the phantom, and availability. For cases where fluoroscopy mode is used instead of radiography mode for acquiring 2D kv planar images, the imaging dose is proportional to the x-ray exposure times for the selected imaging protocol parameters (kvp and ma), and doses can be estimated based on the product of dose rate and scan time for the kv beam. The output of a kv beam can be obtained in air or in a phantom as recommended in the AAPM dosimetry protocol for kv beams. 106 When measured in air, the beam output is insensitive to the field size used. When measured in a phantom, the beam output is affected by the phantom scatter, which depends on the field size. If the beam cannot be delivered in static mode, rotational absorbed dose can be measured utilizing a cylindrical chamber placed at the isocenter. 4.C. In vivo dosimetry If patient-specific imaging dose verification is desired, in vivo patient dose measurements can be performed using available detectors such as diodes, thermoluminescent dosimeters (TLDs), or optically stimulated luminescent dosimeters (OSLDs). 48,107,108 Dosimeters, such as diodes, that are intended for use in MV beam in vivo measurements are not suitable for kv beams as they include inherent buildup. In addition, their response to kv radiation is significantly different from MV beams. Dosimeters that are used for kv beams should be calibrated for the beam energy used. 48 In vivo measurements using detectors placed on the patient s skin may be used to estimate organ dose once detector response has been scaled by the known dose distribution inside the patient. 48 There have been many publications discussing the use of TLDs, OSLDs, MOSFETs, and other detectors in kv beams. 48, METHODS OF ACCOUNTING FOR IMAGING DOSE Many different methods have been used to calculate the imaging dose delivered to RT patients. 1,11,18,19,21 23,25,51 When there is reasonable expectation that the imaging dose will exceed 5% of the total prescribed dose, two methods can

8 e91 Ding et al.: Image Guidance Doses to Radiotherapy Patients e91 (a) (b) % volume (c) (d) (e) 100 Pelvis: Full scan length Pelvis: 5 cm scan length Left femur head Righ femur head Body Left femur head Righ femur head Body Rectum Bladder 30 Rectum Bladder Prostate 10 Prostate dose /cgy % volume dose /cgy dose /cgy Dose profiles in axial direction Standard scan length (17 cm) Scan length set to 10 cm Scan length set to 5 cm z-axis /cm FIG. 5. The effect of reducing the scanned length: (a) Dose distributions shown in colorwash for default Pelvis scan length (16 cm); (b) reduced Pelvis scan length (5 cm); (c, d) Corresponding dose-volume histograms for the specific organs resulting from respective scans; (e) Dose profiles in the inferior superior direction along the line AB shown in Figure (a,b) across the irradiated volume for 16 cm (standard scan length), 10 cm, and 5 cm, respectively. The direction of z-axis is from inferior to superior in (e). The peak at the right in (e) represents the dose as the line AB crosses into the sacral vertebral body (bone). Note that reducing the scan length of the CBCT scan reduces both the maximum dose and the volume that is exposed to radiation. Reproduced from Ding et al. 29 (Scanning parameters are listed Table IId). be used to manage it: patient-specific dose calculations and nonpatient-specific dose estimations. 5.A. Patient-specific imaging dose calculations Patient-specific imaging dose calculations are based on patient CT images 67,113 and provide individualized organ doses, since the dose resulting from image guidance procedures will vary depending on the patient size as well as image location. 21,54,67 In order to perform patient-specific imaging dose calculations in a treatment planning system, the beams used for imaging procedures must be characterized as part of the commissioning process. The process of characterizing MV imaging beams in a treatment planning system is straightforward in cases where the imaging beam is the same as the therapeutic beam, allowing imaging dose to be calculated according to the monitor units and field sizes used in image acquisition. Imaging dose can then be added to therapeutic dose directly during treatment planning. Moreover, if the total number of imaging procedures is known at the planning stage, the imaging dose can be accounted for when optimizing patient treatment plans for total dose to the target and OARs. Miften et al. demonstrated this approach 42 by showing optimized IMRT plans with and without MV-CBCT included in the process. This, along with the lack of MC simulations or model-based algorithms capable of handling kv beams in commercial TPSs, 53 currently precludes patient-specific imaging dose calculations for any other than therapeutic MV beams.

9 e92 Ding et al.: Image Guidance Doses to Radiotherapy Patients e92 (a) (b) (c) Low Dose Thorax: OBI 1.4 (d) Bone Chest: OBI 1.3 % volume Body Bone Heart Left lung Right lung Spinal cord % volume Body Heart Left lung Right lung Spinal cord dose /cgy dose /cgy FIG. 6. Axial, frontal, and sagittal views showing dose distributions resulting from (a) OBI 1.4 low-dose thorax scan and (b) OBI 1.3 scan. The corresponding dose-volume histograms for different organs are shown for the low-dose scan (c) and the OBI 1.3 scan (d). Note that the horizontal scales in (c) and (d) differ by a factor of 5. Reproduced from Ding et al. 29 (Scanning parameters are listed Table IId). 5.B. Nonpatient-specific imaging dose estimations Given the small magnitude of imaging dose relative to therapeutic dose, 26,113 it may be adequate to use simpler approaches that provide reasonable estimates of the imaging dose. It has been shown Ref. [54,67] that interpatient variation and geometry dependence are small in most cases and that dose estimates could be provided in the form of simple look-up tables, which may be accurate enough to estimate the dose from repeat imaging procedures. Such tabulated values 67,113 can provide clinicians with adequate estimates of imaging dose to organs. Tables IIa, IIb, IIc, IId, IIe, and IIIa, IIIb, IIIc list representative organ doses resulting from kv- CBCT scans in the Varian OBI and Elekta XVI systems for different treatment sites and scan protocols. Note that bow tie filters used in the kv-cbct acquisition not only improve image quality but also significantly reduce imaging dose. 26 These tabulated values, when scaled by the mas used for image acquisition, are sufficient to estimate imaging dose to within 20% 67 and can assist the clinician in: (a) determining if the imaging doses are expected to be close to the 5% threshold, (b) choosing a suitable IGRT protocol, and (c)

10 e93 Ding et al.: Image Guidance Doses to Radiotherapy Patients e93 (a) (b) (c) FIG. 7. Imaging dose from a pair of orthogonal planar kv images for (a) head, (b) chest, and (c) pelvis images using the Varian OBI system. 26 (Parameters for kv radiographs for specified acquisition techniques are listed in Table IId).

11 e94 Ding et al.: Image Guidance Doses to Radiotherapy Patients e94 TABLE IIA. Organ doses for the head and neck and brain treatment sites from Varian OBI v1.4 using Standard Head kv-cbct scan. D50 and D10 are minimum dose delivered to 50% and 10% of the organ volume, respectively. (From Reference [67] and kv-cbct scan parameters for Varian OBI 1.4 shown in Table IId). Organ Standard head, brain D50 range (cgy) D10 range (cgy) Standard head, head and neck Organ D50 range (cgy) D10 range (cgy) Brain Brain Brainstem Larynx Chiasm Oral cavity Eyes Parotids Optic Spinal cord Nerves Pituitary Thyroid Spinal Esophagus Cord Skin Skin Bones Bones TABLE IID. Parameters for kv-cbct-specified acquisition techniques in Varian OBI 1.4 (from Reference [26]). kv-cbct Name Bow tie filter (kv) (mas) Gantry rotation (degrees) OBI Standard-dose Full fan head OBI Low-dose Full fan head OBI High-quality Full fan head OBI Pelvis Half fan OBI Pelvis spot Full fan light OBI Low-dose Half fan thorax TrueBeam Head Full fan TrueBeam Pelvis Half fan TrueBeam Spotlight Full fan TrueBeam Thorax Half fan TABLE IIB. Organ doses for the chest treatment site from Varian OBI v1.4 using the low-dose thorax kv-cbct scan (from Reference [114] and kv- CBCT scan parameters for Varian OBI 1.4 shown in Table IId). Low-dose thorax Organ D50 range (cgy) D10 range (cgy) Aorta Lungs Small bowel Esophagus Kidney Heart Liver Spinal cord Spleen Stomach Trachea Skin Bones TABLE IIC. Organ doses for the pelvis treatment site from Varian OBI v1.4 using Pelvis kv-cbct scan. (From Reference [114] and kv-cbct scan parameters for Varian OBI 1.4 shown in Table IId). Pelvis scan, prostate isocenter Organ D50 range (cgy) D10 range (cgy) Bladder Bowel Femoral heads Prostate Rectum Skin Bone TABLE IIE. Parameters for kv radiographs for specified acquisition techniques in Varian OBI 1.4. Name (kv) (mas) Head-AP Head-Lat 70 5 Thorax-AP 75 5 Thorax-Lat Pelvis-AP-Med Pelvis-Lat-Med The clinical default OBI blades are set to X1 = X2 = 13.3 cm and Y1 = Y2 = 10.3 cm in all acquisition techniques. All six techniques were modeled with and without a full fan bow tie filter. TABLE IIIA. Organ doses for the head and neck treatment site from Elekta XVI kv-cbct scan using S cassettes, 100 kvp, 0.1 mas/acquisition, 360 acquisitions, degree (IEC) rotation (from Reference [54]). Head and neck Organ D50 range (cgy) Brainstem Rt eye Lt eye Rt parotid Lt parotid Rt cochlea Lt cochlea Oral cavity accounting for the organ dose resulting from a specific image acquisition procedure over the course of treatment. Until kv dose calculations become available in commercial treatment planning systems, using estimated organ dose

12 e95 Ding et al.: Image Guidance Doses to Radiotherapy Patients e95 TABLE IIIB. Organ doses for the pelvis treatment site from Elekta XVI kv- CBCT scan using an M cassette without a bow tie filter, 120 kvp, 1.0 mas/ acquisition, 650 acquisitions, full 360 degree rotation (from Reference [54]). Pelvis Organ D50 range (cgy) Bladder Rectum Small bowel TABLE IIIC. Organ doses for the pelvis treatment site from Elekta XVI kv- CBCT scan using an M cassette with bow tie filter, 120 kvp, 1.6 mas/acquisition, 650 acquisitions, full 360 degree rotation (from Reference [54]). Pelvis Organ D50 range (cgy) Bladder Rectum Small Bowel that measured dose is within the manufacturer-stated specifications at the time of acceptance of the image device. Image acquisition procedures should include both those at MV and kv energies. The phantom and detectors used should be appropriate for the beam energy. Phantom sizes should be large enough to provide full x-ray scattering. (b) Consistency checks should be performed annually and after each system upgrade, and the recommendations from AAPM quality assurance reports, such as those from AAPM Task Group 142, 115 should be used. Checks for imaging dose consistency in air can also be performed using commercially available tools for measuring beam parameters (i.e., kvp, mas, etc.) that uniquely define the specific procedure. (c) If patient-specific imaging dose verification is desired for a particular patient, in vivo patient dose measurements should be performed with suitable detectors. The limited accuracy of patient-specific measurements should be taken into account in the review of the measured data. tables from various imaging procedures may be an acceptable option given the expected low-dose level. 6. RECOMMENDATIONS Unlike diagnostic imaging procedures, IGRT image acqusitions are more frequent, repeated on a daily basis, and include a volume that is larger than the treated one. Proper management of imaging dose in IGRT includes adherence to ALARA principles by minimizing the dose as much as possible and accounting for it when necessary. 6.A. General recommendations (a) Create local imaging protocols, including imaging modality, technique, and frequency, that are suitable for the imaging requirements of the clinic. 26 Consulting with a diagnostic imaging physicist may be helpful in this process. (b) Develop protocols that are specific for pediatric patients. 29,40 (c) Communicate the imaging dose associated with IGRT protocols by site (head, thorax, abdomen, pelvis) to radiation oncologists. This enables informed decision-making for selecting imaging protocols and ensures the clinicians are aware of the imaging doses being delivered to their patients. 6.B. Imaging dose output and consistency checks (a) The anticipated imaging dose for each image acquisition procedure, with specified protocol parameters, should be measured in air or in-phantom, according to the AAPM dosimetry protocols for kv and MV beams, to confirm 6.C. Accounting for imaging dose to RT patients It is recommended that imaging dose be considered part of the total dose at the treatment planning stage if the dose from repeated imaging procedures is expected to exceed 5% of the prescribed target dose. Patient organ doses can be calculated or estimated by using either patient-specific or nonpatient-specific methods. Facilitation of patient-specific imaging dose calculations may require implementation of new algorithms for MV and kv beams in commercial treatment planning systems. It is acceptable for the uncertainties of calculated imaging doses to reach 20%, because imaging dose is generally only a few percent of the prescribed target dose. As a result, the uncertainty of the summed dose (therapeutic + imaging) will still be at a level of 2 3%. 6.D. Available techniques to reduce imaging dose to patients Depending on the imaging modality, a variety of techniques are available to reduce the imaging dose to organs at risk, as recommended by AAPM TG-75 1 and AAPM TG ALARA should always be the guiding principle applied in practice. At the time of this report, the following techniques are recommended: (a) Reduce the imaging field size as much as possible. This will reduce the volume of irradiated tissue surrounding the target. Reducing the cranial-caudal extent of kv- CBCT scans can also significantly reduce the integral as well as scattered dose in the volume. 29 (b) During patient setup for MV portal images, minimize the imaging field size without removing reference

13 e96 Ding et al.: Image Guidance Doses to Radiotherapy Patients e96 structures needed for patient alignment. For images acquired for documenting delivery, select image during treatment to avoid adding additional dose to the patient. (c) For Tomotherapy units, select MVCT scan pitch parameters that balance imaging dose with clinical need (i.e. patient positioning or adaptive planning). The imaging dose differs significantly when different pitch parameters are selected (Table Ie). 56 (d) For MV-CBCT, select a patient-specific MV imaging protocol and restrict the imaging field of view (FOV). The imaging dose can be reduced if bony anatomy rather than soft tissue is used for treatment localization. 78 Note that the degree of dose reduction possible will depend upon the image quality requirements of the clinicians. (e) When deciding between 2D radiographs or 3D volumetric images, consider the image requirements. As ALARA is the guiding principle, consider 2D if two planar orthogonal kv images are sufficient for the task. The organ doses from image guidance can be reduced by a factor of 10 using 2D kv imaging compared to 3D kv-cbct. 26 (f) Optimize imaging parameters (e.g., kvp, mas) and select appropriate manufacturer-provided default clinical protocols (pelvis, abdomen, thorax, head and neck) for different normal adult body sites. In the case of pediatric patients with a small body size, default low-dose protocols for a head and neck kv-cbct scan can be used to image a pelvic site. This reduces imaging dose by a factor of 2 3 without compromising the image quality. 29 (g) The kv-cbct scan protocols that use partial rotation provide the opportunity to selectively avoid irradiating superficial organs. Partial rotation during a head scan can be used to dramatically reduce the dose to the eyes. 29 The technique can also be applied to reduce dose to the bladder or rectum for kv-cbct scans. (h) Since the beam exit dose is only a few percent of the entry dose for kv x rays, the beam directions used for orthogonal planar images can be selected to minimize dose to critical organs. 26 For a fixed orthogonal pair, consider not only 0 and 90 but also 180 and 90, 0 and 270, and 180 and 270 beam angles to minimize dose to organs at risk. (i) Consider the use of full bow tie filters when acquiring planar kv images. Bow tie filters can significantly reduce skin dose and dose to organs at risk. Always use the appropriate bow tie filter for kv-cbct acquisition when manual placement of the filter is required, since failure to employ the filter increases the imaging dose by factors of ACKNOWLEDGMENTS The members of this task group thank Greg Sharp (TISC Lead Reviewer), Ping Xia (TPC Lead Reviewer), Jeffrey Siebers (External Lead Reviewer), Jeff Colvin, and Sonja Dieterich for their very helpful comments and suggestions. 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