A Protocol for 3D IMRT Quality Assurance of Prostate Radiotherapy
G Gueorguiev1,2*, G Sharp2, M MahD1, J Turcotte 2, B Crawford 2, (1) University of Massachusetts Lowell, Lowell, MA, (2) Massachusetts General Hospital, Boston, MASU-E-T-189 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: Define a set of statistical parameters and plan structures that will characterize the pass/fail criteria for a 3D prostate pre-treatment IMRT quality assurance protocol.
Methods: For this study, 3D IMRT QA measurements were performed on thirteen IMRT prostate patients. All plans were for an initial treatment course to the prostate and the seminal vesicles with prescription dose of 45Gy, and planned with step-and-shoot IMRT. All patients plans passed traditional QA methods, including point measurement with ion chamber and 2D gamma analysis at 3mm/3%. A total of 75 3D IMRT QA measurements were performed on all patients, on different days and on different beam matched linear accelerators. IBA COMPASS system was used for 3D QA measurements. For each patient, nine structures of interest were selected: femoral heads, two planning target volumes (PTV1, PTV2), prostate, seminal vesicles, anterior and posterior rectal wall and bladder. For each structure the following statistical parameters were evaluated: average dose, 3D gamma test, volume at 4% and 6% difference, and volume that receives between 5%-105% of the prescription dose. Two QA protocols are proposed, a conservative with pass/fail QA thresholds the maximum of these 75 passing measurements; standard protocol with pass/fail QA threshold value at the maximum of the 75 measurements +50% increase.
Results:Out of the 153 statistical parameters investigated, 65 were excluded from the QA protocol. The reason is that their values were constant for all 75 measurements. Assessment of the validity of the conservative protocol and standard protocol are ongoing.
Conclusion:3D IMRT QA is a powerful and versatile tool for pre-treatment prostate IMRT QA, with ability to perform variety of statistical tests and overlay dose and anatomy. However, because 3D QA provides more information than single ion chamber measurement and/or 2D gamma analysis, a protocol is needed to establish pass/fail thresholds.
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