S Lim*, T LoSasso , A Wild , J Mechalakos , J Yamada , D Lovelock , Memorial Sloan-Kettering Cancer Center, New York, NY

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  S Lim

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(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: To commission 10MV flattening-filter-free (10FFF) beam models with Eclipse for stereotactic radiosurgery (SRS) treatments.

Methods: AAA and AcurosXB 10FFF non-SRS customized models for a Varian Stx are used as baselines. The models are optimized in version 15.5 Eclipse (Varian, Palo Alto, CA). The focal spot, θ, and the minimum field size, FSmin, are (0.75mm,0.50mm) and 30x30mm² for both baselines. The θ, FSmin, output factors, St, and radial mean energy, , are systematically adjusted to obtain the optimized SRS models by comparing the calculated PDD’s, profiles, and outputs with measurements for 5x5 to 150x150 mm² taken with a stereotactic diode, cc01, and cc04 ion chambers in Blue Phantom. In-phantom dose distributions of a 10x4mm² clinical SRS field is calculated, using the optimized models and standard clinical mlc dosimetric parameters, and compared with radiochromic film measurements

Results: The optimized θ of AAA and AcurosXB are (0.0mm,0.0mm) and (0.50mm,0.0mm) respectively. The FSmin and the corresponding St for both algorithms are 10.0x10.0mm² and 0.771. Tuning of is deemed unnecessary for AAA. A 3.8% to 0.5% decrease in between 0 to 100.0mm is used to optimize AcurosXB. The St agreement of AAA and AcurosXB with measurements improve from the underestimation of -12.8% and -8.2% in the baseline models to the optimized values of -0.7% and +2.0% respectively for a 5x5mm² aperture. From dmax to 10cm depth, the PDD’s calculated by the optimized AAA and AcurosXB are within 1.2% and 0.9% from measurements. For the clinical field, the optimized AAA and AcurosXB agree with measurements to within 1.8% and 2.8% respectively at 5cm depth. The 80-20 penumbrae of AcurosXB and AAA are sharper than measurements by 0.4mm and 0.1mm respectively.

Conclusion: By carefully adjusting the input parameters, the accuracy of AAA and AcurosXB for small fields can be significantly improved for SRS treatments.

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