ESTRO 2024 - Abstract Book

S3532

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

This phantom study is to evaluate each treatment planning technique in Accuray Radixact® System Precision TPS (Precision) and Monte Carlo (MC) Calculation on metallic implant cases.

Material/Methods:

A tailor-made cylindrical PMMA phantom with insert of four stainless steel rods (see Fig. 1a & 1b) was scanned using a CT simulator (SOMATOM Definition Edge, Siemens, US) with iterative metallic artifact removal (iMar). The rods were contoured based on its physical dimension. The phantom was planned in Accuray Precision Treatment Planning Software with TomoDirect beams of 90 and 270 angles parallel opposing fields to maximize the effect of these planning techniques (StOv, StNoOv, ExNoOv) on dose underneath metals causing maximal dose variation. (see Fig. 1c) These plans were exported to MIMS SureCalc software for MC calculation using SciMoCa algorithm. (ScientificRT GmbH, Munich, Germany) Gamma analysis (1%/1mm) for plan comparison was done to evaluate minor dose difference between plans in Precision and MC (see Fig. 1d). The plans are delivered in Radixact® machine and absolute dose measurement near metal was performed using A1SL ionization chamber and 1D profile underneath metal in the beam path was measured by film. Dose distribution was compared with the planned dose of different planning techniques in Precision and MC.

Results:

From ionization chamber measurement, absolute doses of different MC and Precision plans were comparable with results <1%. The chamber measurement shows no significant difference between plans as the chamber is not at the same axial cut with the metal rods. The effect on chamber is minimal as the chamber is not blocked by any implant in parallel opposing beam path. From 3D gamma analysis (1%/1mm) of dose comparison in Precision and MC, Gamma comparison fails mainly in tissues near and in between metal rods (See Fig. 1d). Different planning techniques have different behaviour in MC and Precision. Fig. 2a shows 3D gamma analysis comparisons between different techniques in MC and Precision respectively. In general, using NoExNoOv technique can result a larger dose underneath metals than the other two techniques. It can be due to the HU saturation of stainless steel metal in standard CT scan causing inaccurate enhancement of dose underneath metals. This finding was further verified by plotting profile underneath metals of the measured film dose and calculated dose in both Precision and MC. (See Fig. 2b & 2c) ExNoOv is comparable with NoExOv technique with Gamma passing rate >97% in tackling small metal implants in