SU‐GG‐T‐534: The Impact of Linac Static Jaw Setting on Dose Output from Small Field SRS/SRT Using an Add‐On Micro‐Multileaf Collimator

Purpose: In small field SRS/SRT using an add‐on MMLC, the default jaw setting from the BrainLAB treatment planning is larger than the furthest end of MMLC‐defined segment. The purpose of this study is to evaluate how the dose output changes with varying static jaw setting. Methods and Material: The BrainLAB micro‐multileaf collimator (BrainLAB M3) was mounted on a Varian 2100EX linear Accelerator. Output factors were measured for six different MMLC and jaw field sizes (12×12mm² ∼ 100×100mm²) and compared with different MMLC and fixed jaw setting (100×100mm²). A PTW 31016 PinPoint ionization chamber (0.016cc) was employed to measure the field sizes less than 42 × 42 mm and a PTW 30013 farmer chamber (0.6cc) was used to measure field sizes larger than 42 × 42mm. The measurement depth was 5 cm using solid water (Gammex). Source to surface distance was 100cm and 300 monitor units were used for each measurement. The measurements were repeated twice for each condition. Results: For 6 MV photon beams, compared to the output from the same static jaw settings as MMLC defined field size (12×12mm² ∼ 100×100mm²), the dose percentage difference varies from 10.2% down to the 1.2 % for MMLC defined field size from 12×12mm² to 80×80mm² with the fixed static jaw setting(100×100mm²). For 18 MV photon beams, the dose percentage difference varies from 9.1% to 1.4%.Conclusion: The static jaw setting should be manually adjusted as close to the furthest end of MMLC segment to reduce summated leakages through the MMLC and reduce unintentional over‐irradiation to target and organs at risk. Future work: More retrospective studies will be conducted to include various tumor sites in order to quantitize dose deviations from beam limiting jaw settings.