6X FFF and 10X FFF Portal Dosimetry Output Factor Verification: Application for SRS/SBRT

Purpose: To enhance portal dosimetry of high dose rate SRS/SBRT plan verifications with extensive imager measurement of output factors (OF). Methods: Electronic portal image dosimetry (EPID), implemented on the Varian Edge allows for acquisition of its two energies: 6X FFF and 10 FFF (1400 and 2400 MU/min, respectively) at source to imager distance (SID) =100cm without imager saturation. Square and rectangular aSi OF following EPID calibration were obtained. Data taken was similar to that obtained during beam commissioning (of almost all field sizes from 1×1 to 15×15 and 20×20 cm{sup 2}, [Trilogy] and [Edge], respectively) to construct a table using the OF tool for use in the Portal Dosimetry Prediction Algorithm (PDIP v11). The Trilogy 6x SRS 1000 MU/min EPID data were taken at 140 SID. The large number of OF were obtained for comparison to that obtained with diode detectors and ion chambers (cc13 for >3×3 field size). As Edge PDIP verification is currently ongoing, EPID measurements of three SRS/SBRT plans for the Trilogy were taken and compared to results obtained prior to these measurements. Results: The relative difference output factors of field sizes 2×2 and higher compared to commissioning data were (mean+/-SD, [range]): Edge 6X (−1.9+/−2.9%, [−5.9%,3.1%]), Edgemore » 10X (−0.7+/−1.2%, [− 3.3%,0.8%] and Trilogy (0.03+/−0.5%, [−1.4%,1.1%]) with EPID over predicting. The results for the 140 SID showed excellent agreement throughout except at the 1×1 to 1×15 and 15×1 field sizes where differences were: −10.6%, −6.0% and −5.8%. The differences were also most pronounced for the 1×1 at 100 SID. They were −7.4% and −11.5% for 6X and 10X, respectively. The Gamma (3%, 1mm) for three clinical plans improved by 8.7+/−1.8%. Conclusion: Results indicate that imager output factor measurements at any SID of high dose rate SRS/SBRT are quite reliable for portal dosimetry plan verification except for the smallest fields. This work was not funded by Varian Oncology Systems. Some authors have other work partly funded by Varian Oncology Systems.« less