ESTRO 2025 - Abstract Book

S2938

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

ESTRO 2025

Material/Methods: A total of 14 BC patients who had undergone regional nodal irradiation (4 right-sided and 10 left-sided) were enrolled in this retrospective study. Based on the number of computed tomography (CT) scans for each patient, treatment plans were devised using hypofractionated VMAT with Varian Halcyon system. The waiting time was defined as the time elapsed between surgery and the first RT fraction. Dosimetric parameters in terms of PTVs and four organs at risk (OARs) were evaluated. Results: The PTVs coverage (breast/chest wall, supra/infraclavicular and internal mammary nodes) was found to be comparable regardless of waiting time. In patients with right-sided BC, the mean heart dose was increased by 75% for waiting times ranging from 56 to 329 days. Nevertheless, more favorable cardiac sparing was evident in only 25% of patients following a waiting period of 21 days. The timing of RT did not have an impact on the Dmean and V17 Gy for the right lung, and the V4.2 Gy for the left lung. However, a proportional increase with waiting time was observed for V4.2 Gy for the left breast, reaching a maximum value of 42.19%. In the case of left-sided BC, the mean heart dose was increased in 70% of patients, while a decrease was observed in 30%. In 20% of cases, the Dmean values for the left lung did not respect the dosimetric constraint, with a value of 13.56 and 14.7 Gy for two patients. The first patient underwent a waiting period of 112 days, during which the PTV-Breast/Chestwall volume increased by 135.23 cc. The volume increased from 491.55 to 500.95 cc for the second patient. The V4.2Gy of the right breast exhibited a correlation with volume over time. A reduction in PTV-Breast/Chestwall volume resulted in a decrease in dose, and vice versa.

Figure1: Relationship between PTV volume variation and waiting times

Conclusion: Waiting times for RT-BC have a negative impact on dosimetric results, whereas reduced PTV volume, associated with reduced postoperative inflammation, improves dosimetric efficacy.

Keywords: waiting times, dosimetry, breast cancer