Room: AAPM ePoster Library
Purpose: To showcase the potential multi-institutional use of a standardized set of SRS test cases by establishing single-institution confidence limits (CLs) for point, composite and per-field measurements for evaluating linac-based SRS commissioning delivery accuracy.
Methods: A set of 5 test cases with increasing complexity were developed to establish CLs similar to those defined within AAPM TG-119 for IMRT commissioning. Standardized structure sets, planning goals, and delivery requirements were specified for each of the cases including a small sphere target, irregular target, irregular target placed off-axis, multi-target, and abutting targets. All cases were calculated and measured using SunNuclear’s StereoPHAN with Standard Imaging’s A16 microchamber for point measurements, Gafchromic EBT-XD film for composite measurements, and SRS MapCHECK’s diode array for composite and per-field measurements. Composite and per-field measurements were evaluated with 3%/1mm gamma criterion and 10% dose threshold.
Results: A TG-119 equivalent analysis for cumulative point measurements in high-dose target regions yielded a mean difference between planned and measured dose of 0.14±4.35% with a 95% CL of 8.7%. Removing two measurements taken in a steep local dose gradient, this CL reduces to 4.8%. Composite film and MapCHECK measurements resulted in 95% CLs of 98.0% and 98.4%, respectively. 87.5% of per-field measurements had gamma analysis values of greater than 95%. Low individual per-field passing rates occurred for complex multi-arc arrangements where some fields contribute a low fraction of the total dose delivered and require further planning system optimization.
Conclusion: We have performed comprehensive dosimetric measurements on a standardized set of SRS optimization problems to establish CLs to fine-tune and understand the limitations of our SRS planning and delivery system. This standardized approach to linac-based SRS commissioning provides a mechanism for the medical physics community to evaluate the implementation and delivery accuracy of SRS systems, similar to TG-119 for IMRT.