SU‐E‐T‐369: Comparison of Monte Carlo Calculations Around An Intracavitary Brachytherapy CT‐MR Compatible Fletcher Applicator with Radiochromic Film

Purpose: To calculate the dose distribution with Monte Carlo (MC) around an intracavitary brachytherapy (ICBT) ovoid type applicator and compare these calculations with radiochromic film (RCF) measurements acquired in a liquid water phantom. Methods: Detailed physical measurements of a CT‐MR compatible Fletcher ovoid applicator (Nucletron Co.) were performed in‐house to facilitate construction of a MC model of the device. MC calculations were simulated utilizing MCNPX version 2.7.b of a microSelectron version 2 (mHDR v2) Ir‐192 source (Nucletron Co.) at multiple dwell‐positions within a single ovoid. Oncentra brachytherapytreatment planning system (Nucletron Co.) was used to determine the dwell‐time required for three active dwell‐positions to deliver 10 Gy to a prescription point taken as the center of each film. RCF measurements were performed in two perpendicular planes, (a) 1 cm beyond the distal end and (b) 3.3 cm from, and parallel to, the long axis of the ovoid. RCF measurements were performed in a liquid water phantom. The ovoid, liquid water phantom, and RCF were physically aligned using in‐room patient setup lasers. The MC model application was confirmed superimposing simulated dose distributions resulting from three active dwell‐positions and comparing with RCF. Results: Brachytherapy measurements could be acquired in a liquid water phantom consistent with AAPM TG‐43 protocol. For both planes, 96% of the absolute simulated and measured dose points agreed within 2% or 2 mm distance‐to‐agreement. Conclusions: The MC model is sufficient to predict measured RCF dose distributions acquired in a liquid water phantom accurately.