B Erickson¹*, Y Cui², C Wang², W Giles², M Alber⁶, J Adamson², (1) Duke University, Durham, NC, (2) Duke University Medical Center, Durham, NC, (3) ScientificRT, Munich, Germany

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  B Erickson

WE-C930-IePD-F4-5 (Wednesday, 7/13/2022) 9:30 AM - 10:00 AM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 4

Purpose: Single isocenter multitarget (SIMT) SRS presents difficulties for QA as there is no efficient way to measure multiple targets. We recently commissioned a secondary Monte Carlo (MC) dose calculation algorithm customized for SIMT SRS. However, TG-219 on independent dose calculation for IMRT includes little guidance on MC algorithms or volumetric action criteria (versus point dose). We aim to establish appropriate action criteria as well as to investigate the cause(s) and frequency of replanning for SIMT SRS cases.

Methods: 175 targets from 21 SIMT SRS cases treating 2-20 brain metastases were analyzed, with target volumes ranging from 0.03-34cc and off-axis distances ranging from 0.9-8.4cm. Differences between the original AAA calculation and the MC calculation were tabulated (D99%, D95%, Dmean, and D1%) as well as 3D Gamma Index (GI, 3%/1mm). Cases exceeding the TG-219 recommended action criterion of 7% for high-gradient composite dose in heterogeneous media were (1) re-calculated without heterogeneity, (2) re-calculated on SRS verification phantom geometry, and (3) measured with an SRS diode array to quantify agreement. Average MLC field size and MU-weighted modulation complexity score (MUMCS) were recorded for correlation analysis.

Results: Using the 7% criterion for Dmean resulted in 4.6% of targets failing, which persisted even in absence of heterogeneity corrections. For D99%, D95%, and D1%, failing percentages were 17.1%, 10.9%, and 2.3%, respectively. For targets exceeding the 7% Dmean threshold, 62.5% still exceeded a 5% threshold on phantom geometry. GI did not correlate with failures, with >97% passing for all cases. Off-axis distance, smaller target volumes, and smaller MUMCS values tended to increase the magnitude of the dosimetric differences.

Conclusion: Secondary MC offers a uniquely comprehensive verification for SIMT SRS which can complement diode array measurements. Dmean serves as an effective surrogate to establish pass/fail criteria. Limiting modulation can reduce the need for replanning.

Funding Support, Disclosures, and Conflict of Interest: Dr. Justus Adamson received grant funding from Radialogica. which is unrelated to this study. Dr. Justus Adamson reports partial ownership of Clearsight RT, LLC which is unrelated to this study. Markus Alber works for ScientificRT, who developed the Monte Carlo algorithm used in the study.

Keywords

Stereotactic Radiosurgery, Quality Assurance

Taxonomy

TH- External Beam- Photons: intracranial stereotactic/SBRT

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