Taindra Neupane(1), Charles Shang (1,2), Wazir Muhammad(1), Theodora Leventouri(1) (1)Medical Physics, Department of Physics, Florida Atlantic University, Boca Raton, FL (2)South Florida Proton Therapy Institute, Delray Beach, FL

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PO-GePV-T-398 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: The measured dosimetry uncertainties in some clinical cases treated with diminutive MLC fields (~0.5x0.5cm2) of 10MV FFF arc beams in stereotactic radiosurgery have suggested that such variability mostly emerged from tiny shift of the MLC. This research aims to evaluate uncertainties due to slight MLC drift in very small fields at different clinical settings and to explore their clinical significance.

Methods: A Monte Carlo (MC) dose model was developed for both 6FFF and 10FFF beams of Varian EDGE radiosurgery system using the EGSnrc MC program. The phase space files provided by the vendor were used for simulations. The simulations were performed for 10x10–1x1 cm2 fields w/o MLC at (a) SSD=100cm, d=dmax and (b) SSD=95cm, d=5cm settings in water phantom until the dose uncertainty <5% was achieved. The dose profiles and depth dose curves were compared to treatment planning system (Eclipse). For the MLC field of 0.5x0.5cm2 with fixed jaws setting at 95cm SSD, d=5cm, the dosimetric parameters were analyzed with 1mm MLC shift. Major input parameters varied during simulations were the number of incident particles, directional Bremsstrahlung splitting, range rejection, particle splitting, and voxel size.

Results: Preliminary results showed acceptable agreements (≤ 4%) between MC simulated and TPS calculated dose profiles along with ≤ 4% dose uncertainty for reference fields (10x10 & 5x5) cm2 at set-ups (a) & (b) in water phantom. Similarly, good dose agreements of ≤ 4% for 6FFF and ≤ 3% for 10FFF between MC versus TPS were found for (5x5–1x1) cm2 fields at 95cm SSD, d=5cm. However, for the MLC field of 0.5x0.5cm2, early results showed relatively higher dosimetric uncertainty (~5%).

Conclusion: A MC dose model was developed for the Varian EDGE radiosurgery system and was successfully validated (≤4%) against the TPS. Evaluation of the dosimetric uncertainties in the smallest field is in progress.

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