S Becker

PROSPECTIVE STUDY OF CONE-BEAM COMPUTED TOMOGRAPHY IMAGE-GUIDED RADIOTHERAPY FOR PRONE ACCELERATED PARTIAL BREAST IRRADIATION

PURPOSE To report setup variations during prone accelerated partial breast irradiation (APBI). METHODS New York University (NYU) 07-582 is an institutional review board-approved protocol of cone-beam computed tomography (CBCT) to deliver image-guided ABPI in the prone position. Eligible are postmenopausal women with pT1 breast cancer excised with negative margins and no nodal involvement. A total dose of 30 Gy in five daily fractions of 6 Gy are delivered to the planning target volume (the tumor cavity with 1.5-cm margin) by image-guided radiotherapy. Patients are set up prone, on a dedicated mattress, used for both simulation and treatment. After positioning with skin marks and lasers, CBCTs are performed and the images are registered to the planning CT. The resulting shifts (setup corrections) are recorded in the three principal directions and applied. Portal images are taken for verification. If they differ from the planning digital reconstructed radiographs, the patient is reset, and a new CBCT is taken. RESULTS 70 consecutive patients have undergone a total of 343 CBCTs: 7 patients had four of five planned CBCTs performed. Seven CBCTs (2%) required to be repeated because of misalignment in the comparison between portal and digital reconstructed radiograph image after the first CBCT. The mean shifts and standard deviations in the anterior-posterior (AP), superior-inferior (SI), and medial-lateral (ML) directions were -0.19 (0.54), -0.02 (0.33), and -0.02 (0.43) cm, respectively. The average root mean squares of the daily shifts were 0.50 (0.28), 0.29 (0.17), and 0.38 (0.20). A conservative margin formula resulted in a recommended margin of 1.26, 0.73, 0.96 cm in the AP, SI, and ML directions. CONCLUSION CBCTs confirmed that the NYU prone APBI setup and treatment technique are reproducible, with interfraction variation comparable to those reported for supine setup. The currently applied margin (1.5 cm) adequately compensates for the setup variation detected.

Collision indicator charts for gantry-couch position combinations for Varian linacs

The use of non‐coplanar radiation fields can potentially lead to collisions between the gantry and the couch or patient. The collisions are often not realized until the plan is finished and the fields are checked on the machine, or even later when the patient is already on the table. This paper presents an easy method of gauging if a collision is likely between the gantry and couch or patient during treatment planning. The method involves creating a chart of allowable gantry and couch combinations. The charts contain curves on a polar graph of the gantry and couch angle “plane”. The curves display the limits of collisions for each gantry and couch combination for vertical couch positions 10, 15 and 20 cm below isocenter and for couch lateral positions of −10,0, +10cm, covering the majority of couch positions encountered in patient treatments. All combinations in the region within the curves (containing the origin) are valid, while all combinations outside the curves will result in a collision. The data for the charts are collected from measurements of the gantry angle that just clears each couch angle. The patient presence was modeled by placing a stereotactic body frame on the top of the couch. Separate charts were created for couch angles between 0° and 90° and between 360° and 270° over all gantry angles. The graphs are easy to create, implement, and use in the clinic and help reduce the time, complications, and uncertainties of planning with non‐coplanar fields. PACS numbers: 87.55.‐x; 87.56.‐v

Breast radiotherapy in the prone position primarily reduces the maximum out-of-field measured dose to the ipsilateral lung.

PURPOSE To quantify the potential advantages of prone position breast radiotherapy in terms of the radiation exposure to out-of-field organs, particularly, the breast or the lung. Several dosimetric studies have been reported, based on commercial treatment planning software (TPS). These TPS approaches are not, however, adequate for characterizing out-of-field doses. In this work, relevant out-of-field organ doses have been directly measured. METHODS The authors utilized an adult anthropomorphic phantom to conduct measurements of out-of-field doses in prone and supine position, using 50 Gy prescription dose intensity modulated radiation therapy (IMRT) and 3D-CRT plans. Measurements were made using multiple MOSFET dosimeters in various locations in the ipsilateral lung, the contralateral lung and in the contralateral breast. RESULTS The closer the organ (or organ segment) was to the treatment volume, the more dose sparing was seen for prone vs supine positioning. Breast radiotherapy in the prone position results in a marked reduction in the dose to the proximal part of the ipsilateral lung, compared with treatment in the conventional supine position. This is true both for 3D-CRT and for IMRT. For IMRT, the maximum measured dose to the lung was reduced from 4 to 1.6 Gy, while for 3D-CRT, the maximum measured lung dose was reduced from 5 to 1.7 Gy. For the proximal part of the ipsilateral lung, as well as for the contralateral lung and the contralateral breast, there is little difference in the measured organ doses whether the treatment is given in the prone or the supine-position. CONCLUSIONS The decrease in the maximum dose to the proximal part of the ipsilateral lung produced by prone position radiotherapy is of potentially considerable significance. The dose-response relation for radiation-induced lung cancer increases monotonically in the zero to 5-Gy dose range, implying that a major decrease in the maximum lung dose may result in a significant decrease in the radiation-induced lung cancer risk.

Collision indicator charts for gantry-couch position combinations for Siemens ONCOR and Elekta Infinity linacs

Noncoplanar radiation fields from a linear accelerator can be used to deliver radiation dose distributions that are superior to those delivered using coplanar radiation fields. Noncoplanar radiation field arrangements are especially valuable when delivering stereotactic body radiation therapy (SBRT). Noncoplanar radiation fields, however, are geometrically more challenging to deliver than coplanar radiation fields, and are associated with a greater risk of collisions between the gantry, treatment couch, and patient. Knowledge of which treatment couch offset, treatment couch angle, and gantry angle combinations provide a collision‐free radiotherapy delivery is useful in the treatment planning process, as the risk of requiring replanning due to improperly selected treatment parameters can be minimized. Such tables are by default specific to the linear accelerator make and model used for treatment. In this work a set of plots is presented indicating which combination of treatment couch lateral offsets (‐10 cm to 10 cm), couch angles (270° to 90°), and gantry angles (0° to 360°), will result in collision‐free radiation delivery using Siemens ONCOR linear accelerators equipped with a 160‐leaf multileaf collimator and a 550 TxT treatment table, and a Elekta Infinity linear accelerator with an MLCi2 and Elekta iBEAM evo Couchtop EP. The patient was assumed to have a width of 50 cm and a height of 25 cm. PACS numbers: 87.55.‐x, 87.56.‐v

SU‐FF‐T‐218: Skin Dose Due to a Supporting Pad in Prone Breast Treatments

Purpose: To quantify the skin dose on the medial side of the breast due to the use of a foam pad to support the patient in the prone treatment position. Method and Materials: A phantom was constructed to measure the skin dose with a parallel‐plate chamber. The chamber side of the phantom faced medially so that it faced the pad. The phantom was then treated repeatedly with the pad at various distances from the phantom. A buildup experiment was also performed on the phantom without the pad and with the pad set at 2.5cm away from the surface of the chamber. Performance was assessed by measuring the skin dose as a function of distance and as a percentage of the target dose. It was also assessed by the buildup region created by the pad to the one without a pad present. Results: Examination of the skin dose versus the distance to the pad revealed a dependence on distance. This can result in the total dose increasing as much as 50%. The pad when positioned 2.5cm from the breast adds a bolus to the breast of about 3mm on the medial side of the breast. Conclusion: This project revealed that skin dose increases through the use of the supportive pad. The experiment however is a worst‐case scenario. A normal treatment would have reduced skin dose due to the use of multiple beam angles. In the future, skin dose has to be considered for prone treatments with a support pad. Conflict of Interest: Funded in part by TomoTherapy, Inc.

MO-E-16A-01: Preparing for the ABR Therapy Medical Physics Exam

Preparing for all three parts of the Therapy ABR Physics boards is more than just studying as much material as possible. There will always be material that is missed and gaps in ones knowledge. Therefore it is crucial to understand how the materials relate to each other and to clinical experiences in order to fill those gaps. Each part of the board exam presents its own difficulties: Part I: Determining what material to study, most of which have been learned a long time ago. Part II: Solving a vast array of clinical calculations rapidly by hand Part III: Gaining crucial clinical experiences and being able to explain orally how they are performed and what they mean. All three require different skill sets and preparation, from memorizing vast amounts of material, to rapidly recognizing and solving calculations, to being able to easily and confidently respond to oral questions about all aspects of working in the clinic and why those clinical methods and procedures are performed that way. This symposium is not a comprehensive review of all required study material. Instead it focuses on the previously mentioned problems and required skill sets that are needed for each part of the board exam. Expert Medical Physicists will share their experiences and methods to help students best prepare for the challenges of each individual exam. LEARNING OBJECTIVES 1. How to Prepare for Part 1 by determining study material 2. How to Prepare for Part 2 by learning to solve clinical calculations rapidly 3. How to Prepare for Part 3 by determining critical clinical experiences and learning how to answer questions orally.

SU-E-T-532: Validation and Implementation of Model-Based Patient Specific Quality Assurance Using Mobius3D and MobiusFX in a Clinical Setting

PURPOSE This work carries out the commissioning and validation of the Mobius3D and MobiusFX software tools, which can replace the time-consuming measurement-based patient specific quality assurance (PSQA). METHODS The beam model supplied by Mobius3D was validated against a 21EX linacs beam measured data. Complex patient (VMAT) plans using Eclipse treatment planning system (TPS) was used to test the consistency between Mobius3D (calculates dose using patient image and field data) and MobiusFx (calculates dose using treatment dynalog files). Dose difference and gamma analysis (3%/3mm) between Mobius3D and MobiusFx were used to assess treatment plan and treatment delivery consistency. An end-to-end test was performed to validate Mobius3D and MobiusFx against ion chamber measurements. Effect of the dosimetric leaf gap (DLG) on Mobius3D dose calculation was additionally investigated. RESULTS Mobius3D beam model parameters matched within 1%-3% with our beam measured data. A comparison of Mobius3D and MobiusFx dose matrices for VMAT planned prostate cases showed (0.33±0.07)% mean dose difference with gamma values above 95%. The end-to-end test showed dose differences of 1% between Mobius3D and MobiusFx. Dependence of Mobius3D dose calculation upon DLG was explored by introducing a ±0.5 mm change in the default value for DLG. This change resulted in agreement differences above 2% CONCLUSION: Use of reference beam data would appear to speed up commissioning process for the clinical implementation of Mobius3D. However, careful consideration is required when comparing the information provided by the software, since large dose variations can be seen when the proper parameters are not optimized. The plan and delivered dose were in good agreement; hence MobiusFx has the potential to significantly speed up the PSQA process and at the same time helps to verify treatment parameters that are not possible with measurement-based PSQA.

SU-E-E-02: Dashboard for Tracking Physics Resident Progress

PURPOSE Design a system to easily and securely track the progress of medical physics residents through their residency. Paper sign-offs while offering a real signature are not easily updated or summarized. A resident or mentor needs to be able to quickly assess what the current assignments are, what are overdue, and whether the resident is on track to complete all the tasks in a timely fashion. An electronic version can accomplish all these goals. METHODS An electronic dashboard was created in excel to not only house the tasks and sign-off but to succinctly summarize the residents progress. The first tab contains the dashboard which displays tables of the progress of the residents in each rotation, their current task, and overdue tasks. It also displays the last meetings with the residents, and timeline of important items, and a burn-down chart of the remaining tasks. This are all tied to the data and current date which auto fills the tables. The second tab contains the data. This is comprised of lists of rotations and their associated tasks along with their due dates. A signature column was also created which is password protected but allows special subset users i.e. mentors to alter without using a password. RESULTS The dashboard has allowed residents to better track their progress and tells them what they should be working on. It has also allowed the mentors and the program director to rapid assess their progress. CONCLUSION The dashboard is successful and has been created to allow easy addition and subtraction of required tasks as the residency evolves. The next step is to create a web app version of the excel sheet with logins.

SU-E-T-256: Evaluation of Silver Dressing as Bolus

Purpose: Anti‐microbial silver wound dressing may be useful as a bolus material. This study measures the water‐equivalent‐thickness of a silver wound dressing to characterize it for clinical use. This nylon‐mesh dressing, with its permanently plated silver surface (546 mg Ag/100 cm2) may meet the need for bolus with fine variability of thickness and self‐sterilization. Methods: A percent depth dose (PDD) curve was measured in plastic water (CNMC Best Medical) using a parallel plate chamber (PTW N34001) for an 8×8 cm field size, 6 MV beam, 100 cm SSD with a linear accelerator. Measurements were repeated under varying thicknesses (0.5 mm, 1mm, and 2 mm) of silver dressing material, 10 cm × 11 cm (Silverlon™ Antimicrobial silver wound contact dressing WCD‐466). Control measurements were also made in a small water tank. Results: For every layer of dressing added (∼0.5mm) the PDD shifts to the left along the depth axis. A 2 mm offset applied to the position values of the 2 mm (4‐layers) PDD curve causes superposition with the control PDD. This suggests that 2 mm of silver dressing performs similarly to 2 mm of plastic water. This was verified with measurements under clinical setup conditions, 100 cm to the top of the parallel plate chamber, ie. to skin, with bolus placed on top, under 2 mm plastic water (73.40%) and under 2 mm silver dressing (73.41%). Measurements in a water tank for this equivalent depth agree within 1%. Conclusion: The bolus properties of silver‐mesh dressing appear to be water‐equivalent. Silver dressings can be used as bolus and may be particularly beneficial when fine variations are desired or when maintaining an anti‐microbial environment is of particular value.

WE‐A‐105‐01: Preparing for the ABR Therapy Exam

Preparing for all three parts of the Therapy ABR Physics boards is more than just studying as much material as possible. There will always be material that is missed and gaps in ones knowledge. Therefore it is crucial to understand how the materials relate to each other and to clinical experiences in order to fill those gaps. Each part of the board exam presents its own difficulties: Part I: Determining what material to study, most of which might have been learned a long time ago. Part II: Solving a vast array of clinical calculations rapidly by hand, which are usually done by a computer. Part III: Gaining crucial clinical experiences and being able to explain orally how they are performed and what they mean. All three require different skill sets and preparation, from memorizing vast amounts of material, to rapidly recognizing and solving calculations, to being able to easily and confidently respond to oral questions about all aspects of working in the clinic and why those clinical methods and procedures are performed that way. This symposium is not a comprehensive review of all required study material. Instead it focuses on the previously mentioned problems and required skill sets that are needed for each part of the board exam. Expert Medical Physicists and Medical Faculty will share their experiences and methods to help students best prepare for the challenges of each individual exam along with helping students cope with the anxiety of the exam process. LEARNING OBJECTIVES 1. How to Prepare for Part 1 by determining study material 2. How to Prepare for Part 2 by learning to solve clinical calculations rapidly 3. How to Prepare for Part 3 by determining critical clinical experiences and learning how to answer questions orally.