ESTRO 2022 - Abstract Book

S1665

Abstract book

ESTRO 2022

PO-1880 Determining treatment margins in hypo-fractionated (Fast Forward) breast treatment.

L. van der Heijden 3 , P. Brouwers 4 , B. Jones 2 , B. Wachters 4, F. Van den Heuvel 1,2

1 Zuidwest Radiotherapeutisch Instituut, Physics, Vlissingen, The Netherlands; 2 University of Oxford, Oncology, Oxford, United Kingdom; 3 Zuidwest Radiotherapeutisch Instituut,, DPK, Vlissingen, The Netherlands; 4 Zuidwest Radiotherapeutisch Instituut,, Clinic, Vlissingen, The Netherlands Purpose or Objective A reduced number of fractions for breast cancer treatment is indicated . Using five fractions has been shown to be safe. The techniques used, were direct translations of the standard, the only change was the number and size of the various fractions. However, margins used with a high number of fractions were used, but the statistical power of the various clinical trials was likely to be insufficient to detect any inadequacies. 1. Margin calculations rely on the concepts of normal distributions, an invalid assumption. 2. Margins impact healthy tissue, which can vary depending on the radio-biological properties of specific normal tissues Here we analyse the efficacy of the current margins used in whole breast treatments. Materials and Methods 20 patients (10 left, 10 right) treated to the whole breast were entered in an audit study. Plans were generated for treatments 2.67Gy x 15 frx and 5.2Gy x 5 fx, using the recommended constraints for normal tissue. For each patient 20 systematic errors are generated, and doses recalculated. Shifts shifts from this position is determined by the number of fractions. For each treatment instance the Effective Uniform Dose (EUD) for all structures is calculated. Here we report on Heart, Lung ( α / β = 3.0 late toxicity) and Liver ( α / β = 1.0). Results Figure 1 shows the variations of EUD (Gy) in a boxplot as a function of the systematic and random errors ( Σ and σ ) . In the fast forward arm more outliers are noted indicating “unlucky” patients. However the outliers only occur with Σ and σ larger than 5mm.