R Badkul*, H Li, S Awan, F Wang, H Jiang, The University of Kansas Medical Center, Kansas City, KS

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  R Badkul

PO-GePV-T-378 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: Linac-based single-isocentric volumetric-modulated-arc therapy SRS-technique(SI-VMAT-SRS) is very efficient for treating multiple brain-metastases.However, it is time consuming to validate plans dosimetrically using traditional QA methods such as films/micro-chambers.In this study we use a commercially available high-definition diode-array to perform the end-to-end QA to validate this technique.

Methods: A high density SRS-MapCHECK diode-array (SRS-MC) along with StereoPHAN phantom were scanned using a Philips-brilliance-big-bore CT-scanner with 1mm slice thickness.Array and absolute dose calibration were performed per vendor guidelines. Using Varian-Eclipse TPS, 9 targets ranging from 0.5 – 1.5 cm diameters were contoured to simulate a patient with multiple brain metastases. A single isocenter VMAT plan using 5 non-coplanar arcs using a Varian-NovalisTx with 6X-SRS and HD-MLC was planned with 20Gy prescription to each lesion. After setting up the phantom with room lasers, a CBCT alignment was performed and then irradiated using planned couch angles. Four measurements at different SRS-MC planes were performed by rotating the SRS-MC at 0, 45, 90, and 315 degrees to make sure the high-dose regions of all 9 lesions were captured in the measurements. Gamma analysis in absolute dose mode were performed using both TG218 recommended 3%/2mm and tighter 3%/1mm criteria.

Results: Gamma analysis passing scores (PSs) in coronal plane (0 degree) were 98.7% and 99.3% for 3%/1mm and 3%/2mm respectively. The sagittal plane (90 Degree) PSs were 97.9% and 99.5% for 3%/1mm and 3%/2mm respectively. The 45 degree plane PSs were 98.7% and 99.8% for 3%/1mm and 3%/2 mm respectively. The 315 degree plane’s PS were 99.6% and 99.8% for 3%/1mm and 3%/2mm respectively. This comprehensive QA analysis was able to cover all 9 lesions’ high dose regions and the gamma analysis results were satisfactory even with tighter criteria of 3%/1mm.

Conclusion: Our study suggests that SRS MapCHECK with StereoPhan can be utilized efficiently for end-to-end QA and patient-specific QAs for single iso multi mets SRS plans without compromising any clinical accuracy.

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