A comparison of measured and treatment planning system out-of-field dose for a 1.5 T MR linac

Phys Med Biol. 2023 Sep 12. doi: 10.1088/1361-6560/acf912. Online ahead of print.ABSTRACTObjective
Dose due to the electron streaming effect (ESE) is a significant contribution to out-of-field dose on the Elekta Unity MR-Linac. The aim of this work is to provide a systematic comparison of calculated and measured streaming dose for this system. 
Approach
Beams 1.0 × 1.0 cm2 to 5.0 × 5.0 cm2, gantry 90.0°, 1000 MU, were incident on an in-house phantom. At the beam entrance and exit surfaces of the phantom, ESE was generated in the Y-direction (IEC 61217). EBT3 film, orientated within the X-Z plane and at 14.0 mm depth in a solid water block, was used to determine ESE dose 5.0 cm beyond the phantom. The experimental arrangement was simulated in the Monaco v5.4 treatment planning system (TPS), utilising a CT phantom dataset with differing relative electron densities (RED) for the surrounding air. Horizontal (X direction) and vertical (Z direction) film dose profiles were compared to the corresponding TPS profiles. 
Main results
For each field, the maximum ESE dose was observed at the beam exit, the magnitude of which decreases with decreasing field size. For the 5.0 × 5.0 cm2 field, the exit and entry ESE doses were 19.6 % and 7.0 % of the Dmax dose to water, respectively. Across horizontal profiles, differences (simulated - measured) were reduced with smaller fields and lower RED. The maximum absolute profile difference was 1.7 % of th...
Source: Physics in Medicine and Biology - Category: Physics Authors: Source Type: research