ESTRO 2020 Abstract book

S1035 ESTRO 2020

AB, Stockholm) has been designed and manufactured in our institution. A frame supports a bolus curtain between the treatment area and areas at risk from high ESE doses, enabling reduction of out of field doses to acceptable levels. Material and Methods

The template, tuned on the first clinical patient, was efficient in generating good dose distributions with low manual interaction on two further patients. In Figure 1a an example of dose distribution is reported. Concerning the comparison between calculation and measurement, average gamma passing rate was 95.0%±0.5% with a very good agreement both in the low dose and high dose regions (in Figure 1b an example of calculated and measured isodose lines).

The accumulated dose distribution over the 7 fractions is comparable with the planned one. In Figure 2 the comparison between planned and accumulated DVH parameters for both target (CTV and PTV) and OARs is presented. None of the differences resulted to be statistically significant at the significance level of 0.05.

Malkov et al. suggests that 1cm of water-equivalent bolus will provide sufficient dose attenuation. Figure 1 shows confirmation of this in a Monaco simulation with a 1cm thick ROI with water density applied (v5.40.01, Elekta). A custom bolus support was designed to fix a 1cm sheet of Superflab bolus (Eckert & Ziegler) between two rigid perspex plates. The support slots into the coil cable rails on the Unity couch and can be pushed directly against the Unity MR coil holder to sit between the treatment area and ESE high risk areas (Figure 2). For the initial application of this shielding, an asymmetric bolus is used which shields the head, midline and ipslateral arm in partial breast treatments. This asymmetric approach reduces the amount of bolus that is needed, with corresponding advantages of reduced bolus weight and potential reduction of patient claustrophobia. Image quality was evaluated using the ACR MR phantom, Philips PIQT phantom and B0 mapping, comparing results with and without the bolus support. The bolus was checked for risk of increased radiofrequency (RF) heating by assessing the bolus conductivity using B1 mapping, comparing against baseline QA measurements. Additionally a direct measurement of heating was taken using temperature indicators placed on and under a sheet of bolus which lay directly on a large MR phantom, subjected to continuous high RF imaging for a period of 60minutes.

Conclusion The treatment of scalp and face with helical Tomotherapy resulted to be feasible. In spite of the very superficial dose distribution with high dose gradients, the plan resulted robust enough respect to the residual setup errors. The agreement between measured and calculated dose distribution was satisfactory. TSFI can be safely delivered to patients eligible for this treatment. PO-1766 A clinical solution for electron streaming shielding for partial breast treatments on Unity MR- linac J. Chick 1 , A. Mitchell 1 , J. Sullivan 1 , T. Herbert 2 , R. Lawes 2 , H. McNair 3 , M. Schmidt 1 , S. Nill 1 , A. Kirby 2 , U. Oelfke 1 1 Royal Marsden Hospital NHS Trust and the Institute of Cancer Research, Joint Department of Physics, Sutton, United Kingdom ; 2 Royal Marsden Hospital NHS Trust, Radiotherapy, Sutton, United Kingdom ; 3 Royal Marsden Hospital NHS Trust and the Institute of Cancer Research, Radiotherapy, Sutton, United Kingdom Purpose or Objective In magnetic resonance guided radiotherapy, there can be an increased risk of out of field skin doses due to the Electron Streaming Effect (ESE). A practical, non patient- specific, shielding solution for the Unity MR-linac (Elekta