SU-F-T-574: MLC Based SRS Beam Commissioning - Minimum Target Size Investigation.

PURPOSE: To implement a MLC accelerator based SRS program using small fields down to 1 cm × 1 cm and to determine the smallest target size safe for clinical treatment.

METHODS: Computerized beam scanning was performed in water using a diode detector and a linac-head attached transmission ion chamber to characterize the small field dosimetric aspects of a 6 MV photon beam (Trilogy-Varian Medical Systems, Inc.). The output factors, PDD and profiles of field sizes 1, 2, 3, 4, and 10 cm(2) were measured and utilized to create a new treatment planning system (TPS) model (AAA ver 11021). Static MLC SRS treatment plans were created and delivered to a homogeneous phantom (Cube 20, CIRS, Inc.) for a 1.0 cm and 1.5 cm "PTV" target. A 12 field DMLC plan was created for a 2.1 cm target. Radiochromic film (EBT3, Ashland Inc.) was used to measure the planar dose in the axial, coronal and sagittal planes. A micro ion chamber (0.007 cc) was used to measure the dose at isocenter for each treatment delivery.

RESULTS: The new TPS model was validated by using a tolerance criteria of 2% dose and 2 mm distance to agreement. For fields ≤ 3 cm(2) , the max PDD, Profile and OF difference was 0.9%, 2%/2mm and 1.4% respectively. The measured radiochromic film planar dose distributions had gamma scores of 95.3% or higher using a 3%/2mm criteria. Ion chamber measurements for all 3 test plans effectively met our goal of delivering the dose accurately to within 5% when compared to the expected dose reported by the TPS (1 cm plan Δ= -5.2%, 1.5 cm plan Δ= -2.0%, 2 cm plan Δ= 1.5%).

CONCLUSION: End to end testing confirmed that MLC defined SRS for target sizes ≥ 1.0 cm can be safely planned and delivered.