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Program Information

Cranial Multi-Metastases SRS Commissioning with M120 MLC

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S Lim

S Lim*, L Kuo , X Li , A Ballangrud , T LoSasso , Memorial Sloan-Kettering Cancer Center, New York, NY


SU-I-GPD-T-593 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To commission a custom VMAT multi-lesions and a DCA single lesion 6MV AAA Eclipse beam models for cranial SRS treatments on a Varian TrueBeam with M120 multi-leaf collimator (MLC)

Methods: Seven clinical VMAT plans, with 1-6 targets each, and three single target DCA plans are created using the custom models with (1.75,0.75) and (0,0) focal spots respectively. The respective targets of the VMAT and DCA plans range are 0.7-4.1cc, and 0.1-10.9cc. All VMAT plans are measured with EBT3 films in a 30x30x10cm solid water phantom on treatment couch with rotating gantry on a Varian TrueBeam with M120 MLC. Exposed films are compared with the calculation using average dose-difference,
, and gamma function, γ. The γ criteria used in this study is 3% of the local dose-difference and 2mm distance-to-agreement (DTA). A region-of-interest (ROI) of 80% of maximum plan dose is used. For DCA plans, an IBA razor diode, positioned at SAD 100cm and 5cm depth in a 25x25x30cm polystyrene phantom, is used to measure the central axis dose at gantry 0. The measurements are normalized to the 3x3cm² output measured with an IBA cc04 ion chamber. The dose-difference, DD, between the normalized measurements and the calculation is used for comparison.

Results: The respective
and γ for all films are within ± 5% and >94.5%. The
uncertainty increases with decreasing target size. At around 0.7cc, the VMAT calculation underestimates by about 5%. With DCA, the DD are 2.5% and 0.7% for 0.1 and 0.6-10.9cc targets respectively.

Conclusion: The custom models are capable to generate acceptable dosimetry from 0.1 to 10.9cc targets with the combination of VMAT and DCA techniques. Multiple targets larger than 0.7cc should be grouped and treated with a singe isocenter VMAT plan. A separate DCA plan should be considered for targets between 0.1 and 0.7cc.

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