M MacPherson

Medical physics staffing for radiation oncology: a decade of experience in Ontario, Canada

The January 2010 articles in The New York Times generated intense focus on patient safety in radiation treatment, with physics staffing identified frequently as a critical factor for consistent quality assurance. The purpose of this work is to review our experience with medical physics staffing, and to propose a transparent and flexible staffing algorithm for general use. Guided by documented times required per routine procedure, we have developed a robust algorithm to estimate physics staffing needs according to center‐specific workload for medical physicists and associated support staff, in a manner we believe is adaptable to an evolving radiotherapy practice. We calculate requirements for each staffing type based on caseload, equipment inventory, quality assurance, educational programs, and administration. Average per‐case staffing ratios were also determined for larger‐scale human resource planning and used to model staffing needs for Ontario, Canada over the next 10 years. The workload specific algorithm was tested through a survey of Canadian cancer centers. For center‐specific human resource planning, we propose a grid of coefficients addressing specific workload factors for each staff group. For larger scale forecasting of human resource requirements, values of 260, 700, 300, 600, 1200, and 2000 treated cases per full‐time equivalent (FTE) were determined for medical physicists, physics assistants, dosimetrists, electronics technologists, mechanical technologists, and information technology specialists, respectively. PACS numbers: 87.55.N‐, 87.55.Qr

Evaluation of a Thermoplastic Immobilization System for Breast and Chest Wall Radiation Therapy

We report on the impact of a thermoplastic immobilization system on intra- and interfraction motion for patients undergoing breast or chest wall radiation therapy. Patients for this study were treated using helical tomotherapy. All patients were immobilized using a thermoplastic shell extending from the shoulders to the ribcage. Intrafraction motion was assessed by measuring maximum displacement of the skin, heart, and chest wall on a pretreatment 4D computed tomography, while inter-fraction motion was inferred from patient shift data arising from daily image guidance procedures on tomotherapy. Using thermoplastic immobilization, the average maximum motion of the external contour was 1.3 ± 1.6 mm, whereas the chest wall was found to be 1.6 ± 1.9 mm. The day-to-day setup variation was found to be large, with random errors of 4.0, 12.0, and 4.5 mm in the left-right, superior-inferior, and anterior-posterior directions, respectively, and the standard deviations of the systematic errors were found to be 2.7, 9.8, and 4.1 mm. These errors would be expected to dominate any respiratory motion but can be mitigated by daily online image guidance. Using thermoplastic immobilization can effectively reduce respiratory motion of the chest wall and external contour, but these gains can only be realized if daily image guidance is used.

SU‐E‐T‐411: Validation of Plan Dose Perturbation Software for Use in Patient Specific IMRT Quality Assurance

Purpose: Patient‐specific IMRTquality assurance typically compares measured and calculated dose distributions in‐phantom. The clinical importance of disagreement between measured and calculated dose is often difficult to interpret. Recently, software has been developed (3DVH Sun Nuclear, Florida USA) that calculates the “delivered” dose distributions in patients by perturbing the calculated dose using errors detected in planar dose measurements. The aim of this work was to validate the accuracy of 3DVH versus commercial treatment planningsoftware (Varian Eclipse 8.9). Methods: 17 DMLC prostate IMRT treatment plans were modified by Matlab script by adding randomly distributed positional errors of 1 or 3 mm in the planned MLC positions. These modified plans were delivered to a 2D diode array (MapCheck2, Sun Nuclear). Measured doses were compared to the dose planes from the original or recalculated MLC modified plans and 3DVH calculated a “delivered” dose in the patient. DVH comparisons were made based on mean dose and D99 for the PTV, and mean and maximum dose for the bladder and rectal for each patient and MLC error. Results: Paired Data showed good agreement for planning system and 3DVH dose volume histograms for both targets and organs at risk for both modified and unmodified plans, regardless of MLC error induced. Structure mean dose values as measured for PTV, Bladder and Rectum agreed to within 1% and dose maximum data showing a discrepancy of 2%. Conclusions: 3DVH is a potentially useful tool to calculate delivered dose distributions during patient specific IMRTquality assurance. The 3DVH data allows the user to analyze the difference between planned and delivered dose to given structures. By introducing random delivery errors, we showed that the 3DVH software was able to detect small fluence changes. This demonstrates the accuracy of 3DVH software against a commercial treatment planning system.

Simulation of a medical linear accelerator for teaching purposes

Simulation software for medical linear accelerators that can be used in a teaching environment was developed. The components of linear accelerators were modeled to first order accuracy using analytical expressions taken from the literature. The expressions used constants that were empirically set such that realistic response could be expected. These expressions were programmed in a MATLAB environment with a graphical user interface in order to produce an environment similar to that of linear accelerator service mode. The program was evaluated in a systematic fashion, where parameters affecting the clinical properties of medical linear accelerator beams were adjusted independently, and the effects on beam energy and dose rate recorded. These results confirmed that beam tuning adjustments could be simulated in a simple environment. Further, adjustment of service parameters over a large range was possible, and this allows the demonstration of linear accelerator physics in an environment accessible to both medical physicists and linear accelerator service engineers. In conclusion, a software tool, named SIMAC, was developed to improve the teaching of linear accelerator physics in a simulated environment. SIMAC performed in a similar manner to medical linear accelerators. The authors hope that this tool will be valuable as a teaching tool for medical physicists and linear accelerator service engineers. PACS number: 87.55Gh, 87.56bd

Poster — Thur Eve — 54: A software solution for ongoing DVH quality assurance in radiation therapy

PURPOSE A program has been developed in MATLAB for use in quality assurance of treatment planning of radiation therapy. It analyzes patient DVH files and compiles dose volume data for review, trending, comparison and analysis. MATERIAL AND METHODS Patient DVH files are exported from the Eclipse treatment planning system and saved according to treatment sites and date. Currently analysis is available for 4 treatment sites; Prostate, Prostate Bed, Lung, and Upper GI, with two functions for data report and analysis: patient-specific and organ-specific. The patient-specific function loads one patient DVH file and reports the user-specified dose volume data of organs and targets. These data can be compiled to an external file for a third party analysis. The organ-specific function extracts a requested dose volume of an organ from the DVH files of a patient group and reports the statistics over this population. A graphical user interface is utilized to select clinical sites, function and structures, and input users requests. RESULTS We have implemented this program in planning quality assurance at our center. The program has tracked the dosimetric improvement in GU sites after VMAT was implemented clinically. It has generated dose volume statistics for different groups of patients associated with technique or time range. CONCLUSION This program allows reporting and statistical analysis of DVH files. It is an efficient tool for the planning quality control in radiation therapy.

Poster — Thur Eve — 76: A quality control to achieve planning consistency in arc radiotherapy of the prostate

PURPOSE To report a quality control program in prostate radiation therapy at our center that includes semi-automated planning process to generate high quality plans and in-house software to track plan quality in the subsequent clinical application. MATERIAL AND METHODS Arc planning in Eclipse v10.0 was preformed for both intact prostate and post-prostatectomy treatments. The planning focuses on DVH requirements and dose distributions being able to tolerate daily setup variations. A modified structure set is used to standardize the optimization, including short rectum and bladder in the fields to effectively tighten dose to target and a rectum expansion with 1cm cropped from PTV to block dose and shape posterior isodose lines. Structure, plan and optimization templates are used to streamline plan generation. DVH files are exported from Eclipse to a quality tracking software with GUI written in Matlab that can report the dose-volume data either for an individual patient or over a patient population. RESULTS For 100 intact prostate patients treated with 78Gy, rectal D50, D25, D15 and D5 are 30.1±6.2Gy, 50.6±7.9Gy, 65.9±6.0Gy and 76.6±1.4Gy respectively, well below the limits 50Gy, 65Gy, 75Gy and 78Gy respectively. For prostate bed with prescription of 66Gy, rectal D50 is 35.9±6.9Gy. In both sites, PTV is covered by 95% prescription and the hotspots are less than 5%. CONCLUSION The semi-automated planning method can efficiently create high quality plans while the tracking software can monitor the feedback from clinical application. It is a comprehensive and robust quality control program in radiation therapy.

Poster - Thur Eve - 21: ROC analysis in patient specific quality assurance

INTRODUCTION Many institutions rely on a patient specific measurement for IMRT/VMAT patient QA. In diagnostic imaging, radiologists use Receiver Operator Curves (ROC) to help quantify the value of a diagnostic imaging test. The purpose of this work is to investigate the value or ROC methodology for patient specific IMRT QA. METHODS AND MATERIALS Beam fluences for 34 prostate IMRT patients were analyzed using gamma analysis. For half of these, measurements were done using the planned beam fluences. For the rest, perturbations to the MLC leaf positions were introduced. Gamma analysis was then used to measure fluence differences. Assuming that the unperturbed fluencies were positive measurements, distributions of true positive and false negatives were calculated. RESULTS For poorly performing beam delivery systems the choice of γ-DTA criterion has little effect on test sensitivity and specificity. The AUC is increased by about 10% for high performance beam delivery systems. For a 3%/3mm γ-DTA condition, ideal cut off values are reasonably independent of MLC performance. At a tighter γ-DTA condition of 2%/2mm, then the optimal sensitivity and specificity of the test is more dependent on MLC performance. DISCUSSION For a pass-fail test such as the γ-DTA map is, it is important to choose an optimal cut off value to maximize the sensitivity and specificity of the test. ROC methodology allows users to follow a prescriptive method to obtain ideal cut-off values for gamma analysis, and to assess improvements in sensitivity and specificity for higher performing beam delivery system.

Poster — Thur Eve — 09: Novel radiation safety challenges of a brachytherapy redevelopment at the Credit Valley Hospital

Carlo Fidani Peel Regional Cancer Center at the Credit Valley Site of the Credit Valley Hospital and Trillium Health Center is currently undergoing a redevelopment to build a brachytherapy suite and associated areas with a projected start date of April 2013. The new brachytherapy suite will be located in the PRCC, and is a redevelopment of office area into clinical space. The workload for the full brachytherapy program is expected to be 20 patients per week, in cervix, prostate, skin, lung and other sites. There were challenges for shielding due services in the slab to the renal clinic located above the redevelopment area. The presence of over 30 voids in the slab and upper walls made the solution to add shielding to the underside of the current slab unsuitable. To overcome this, a second ceiling built below the slab to allow for an uninterrupted shielding for the brachytherapy suite. The additional ceiling allows for a crawl space between slab and shielding allowing for servicing if needed for the drains from the renal clinic. The appropriate finalized shielding design is 69mm of lead brick supported by steel plate and steel beams. Final shielding for the walls is 690mm of concrete that allows public access to all hallways around the facility. The final design for the new brachytherapy site at the PRCC all services for the room are located within the shielding and all services for areas outside the room are located outside the shielding.

TH‐E‐BRB‐06: Clinical Implementation of Electron Monte Carlo for Breast Boost Radiation Therapy: A Retrospective Study to Improve Target Volume Dose Coverage

Purpose: To retrospectively compare and improve target volume dose coverage for breast boosts using eMC. Methods: In this study, 101 retrospective breast patient cases were planned and treated for an electron boost (ranging from 6Gy/3F to 16Gy/8F). All patients were planned using Varian Eclipse (v8.9). Dose contributions from tangents were not taken into account. All plans were based on CT scan datasets. Breast boost CTVs were delineated with adequate expansion for PTV and insert size for treatment. All plans were delivered with treatment intent of 90% coverage to the CTV. Actual treatment prescriptions were based on CAX percentage depth dose with no dose distributions. Without changing any parameters (eMC algorithm, contours and treatment),dose distributions were generated. Actual delivered treatment monitor units were entered into the plans and analyzed for CTV dose coverage. For inadequate CTV coverage, plans were either normalized manually or via the TPS. Boost volume coverage was compared for delivered MU and EMC dose distribution MU. Dose coverage for V90 (volume receiving 90% of the prescribed dose) was set to 98%. Dose to 90% of the CTV (D90) and difference in monitor units for both plans were compared. Results: Majority (83%) of patients had been treated with 9 or 12MeV. Data consisted of a random sample of 51 right and 49 left boosts with a mean CTV=10.04cm2. The D90 increased from a mean of 89.7% to 92.5% (p=0.000) and similarly V90 increased from a mean of 85.8% to 96.6% (p=0.000). Monitor Units between both plans increased by 3% for adequate coverage of the CTV. Verification of insert output factors using eMC were within ±0.5% of measured. Conclusions: This retrospective study indicates improvement in target coverage requires a small and significant increase in MU for breast boost treatment without changing planning parameters or technique.

TU‐C‐BRB‐07: Medical Physics Staffing for Radiation Treatment: A Robust Algorithm with Trans‐Canada Validation

Purpose: To describe an algorithm for determining appropriate physics staffing for radiation treatment. Motivation for this work came from the age of current guidelines which predate the recent evolution in techniques and technology, and also several significant adverse incidents where a lack of physics staffing was identified as a contributing factor to excessive radiation exposure of patients. Methods: Guided by published times required per procedure, we developed an algorithm adaptable to local practice which estimates staffing requirements for medical physics with parameters derived from clinical procedures and service workload, equipment inventory, training, clinical development and administration. The predictive power was evaluated using data from 32 Canadian centres. This algorithm was used to model staffing requirements for the next 10 years to aid regional, institutional and educational program planning with consideration given to the “4Rs” of human resources planning: Requirements, Recruitment, Retention and Residency. Results: For centre‐specific human resource planning, we propose a grid of coefficients addressing specific workload factors for each group. For larger scale planning, case‐based ratios were determined at 260, 300 and 600 annual radiotherapy cases for medical physicists,dosimetrists and electronics technologists respectively. Assuming a 2.5% growth in incidence of cancer and stable utilisation, our supply and demand model predicts a requirement for an additional 39 medical physicists for Ontario by the year 2020. If an additional 3% annual growth in radiation therapy utilisation is included, the number rises to 87. Conclusions: We describe a robust algorithm to determine medical physics staffing levels adaptable to centre‐specific workload and evolving local radiation treatment practice. Although annual caseload has been used in the past as a major parameter for global physics staffing determination, our results indicate that local clinical services and equipment as well as academic activity cause significant deviations from predictions based solely on caseload.