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Single Isocenter SRS Plan QA for Multiple Brain Lesion Cases

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J Ho

J Ho1*, R Chin2 , (1) ,,,(2) Alta Bates Summit Medical Center, Berkeley, CA


SU-F-T-596 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Linac-based single isocenter SRS treatments for multiple brain lesions have been developed in recent years. However, it is a challenge to perform patient specific plan QA in cases where a large number of small and widely spread-out lesions are involved. We have developed a method to measure all target dose distributions in a single planar measurement.

Varian Eclipse TPS was used to create dynamic conformal arc or RapidArc© plans. Verification plans are created with a fixed gantry angle, and planar projections of 3D dose distributions are created which show high dose regions correlated to each individual lesion. Two techniques were employed for dose measurements: an actual treatment plan delivered with a phantom mounted on the gantry; and/or a verification QA plan with a phantom placed on the couch. Gafchromic films and RIT© software were used for dosimetric analysis. A high dose region was selected for absolute dose measurements.

Test plans with up to 10 targets were performed with satisfactory results in the commissioning of this method. For clinical cases, we performed QA on plans with lesion number range from 2 to 7 and size from 0.5 to 3 cm. The absolute dose measurement for dynamic conformal arc plans is within 5% for target > 1 cm and 7% for target ≤1.0 cm. Gamma analyses were performed for whole area covered as well as each focused high-dose region corresponding to individual lesions. The pass criteria is 90% or higher for 3% and 1 mm. Each plan QA took about 1-2 hrs to complete.

Our single isocenter SRS QA method has proven to be practical. The process is capable of capturing and analyzing multiple high dose regions in a single planar measurement. It takes reasonable time to verify each individual target without sacrificing accuracy and clinical relevance.

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