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Determination of Confidence Limits for Linac-Based SRS Point, Composite and Per-Field Measurements From a Clinically Comprehensive SRS Test Suit

G Baran1*, R Culcasi1, M Dominello2, J Burmeister1,2, (1) Karmanos Cancer Institute, Detroit, MI (2) Wayne State University School of Medicine, Detroit, MI


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

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.

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Commissioning, Stereotactic Radiosurgery


TH- External Beam- Photons: intracranial stereotactic/SBRT

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