ESTRO 2025 - Abstract Book

S2900

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

ESTRO 2025

3834

Digital Poster Can we trust our clinical alfa/beta ratio for boosting gyneacological patients? Sandra Vieira, Joep Stroom, Maria Inês Antunes, Carlo Greco Radiotherapy department, Champalimaud Foundation, Lisbon, Portugal

Purpose/Objective: Recently, updated dose constraints were published for 1 to 30 fractions [Timmerman 2022], and it became apparent that traditional a/b values generally did not fit these constraints. Our aim is to study the difference between boost dose limits for gynaecological organs-at-risk (OAR) calculated using clinical a/b and a/b fitted to dose constraints using the linear quadratic (LQ) model. Material/Methods: Dose data were collected from 90 gyneacological patients treated with radical intent. Treatment comprised of external beam radiotherapy (EBRT) given in 23 or 25 fractions followed by a hypo-fractionated boost to the vaginal cuff (2 fractions) or cervix (4 fractions). Boost dose limits were found by subtracting the 2-Gy equivalent dose (EQD2) delivered in the first course from the cumulative EQD2 dose constraint for the combined treatment. Rectum, sigmoid, and small bowel boost constraints were most challenging to be met. New a/b were fitted to Timmerman´s published dose constraint data by minimizing root-mean-square-error (RSME) for each OAR, using the LQ model. Cumulative dose constraints and boost dose limits were recalculated using the fitted a/b. Results were compared with the values from clinical practice. Results: Averaged over all OARs, fitting a/b values improved RMSE from 14.1±5.7% (clinical) to 2.1±1.6% (fitted), as shown in Fig1A for rectum and Table1. All fitted a/b (average 14.0±6.7Gy) are larger than the clinical values. Resulting differences in cumulative dose constraints and boost dose are also shown in Table1. Cumulative OAR constraints for fitted a/b are, on average, 6Gy lower (66.2±5.9Gy vs 72.5±10.3Gy). However, boosting with 2 fractions using fitted a/b allows for extra dose for all OARs (1.2±0.5Gy, for rectum). Regarding the 4 fractions, new boost limits are, in general, lower for the fitted a/b, meaning a boost constraint decrease would be needed. This is shown for the rectum by considering the boost dose limit as function of delivered dose in the first course (Fig2B); the fitted curves drop below the clinical curves earlier for the 4 fractions (at 45Gy) than for the 2 fractions (54Gy), leading to constraint decrease more frequently.