A Dosimetric Evaluation of the Impact of Seroma Volume During Breast Boost Radiotherapy

Abstract

Whole breast irradiation (WBI) followed by tumor bed boost is the standard of care for patients who have undergone breast conservation surgery. However, seroma which forms at the tumor bed may affect the tumor bed boost volume. This study aims to evaluate the changes in seroma volume during WBI and determine the dosimetric benefits of a re-plan for the boost. We hypothesized that re-planning on a reduced target volume resulting from a reduction in seroma volume meaningfully reduces doses to normal tissues. A retrospective study of 15 right-sided and 15 left-sided breast cancer patients treated with radiotherapy to the breast (40Gy/15# and a 10Gy/5# boost) between 2012-2019 was performed. CT simulation scans were performed before WBI (CT1) and also midway during WBI (CT2) if patient had a seroma measuring >5cm in any dimension during CT1. The tumor bed boost, referred to as the gross target volume (GTV), was contoured based on seroma and surgical clips on both CT1 and CT2. For each patient, a tumor bed boost plan was retrospectively generated on CT1 for comparison against the as-treated plan based on CT2. CT1 and CT2 treatment boosts, planned using the most appropriate 3D technique as per the contoured GTV were compared based on the 5Gy dose volume to the ipsilateral normal breast tissue (Irradiated breast-GTV), mean ipsilateral lung dose and mean heart dose (for left-sided patients). Data was analyzed using the Wilcoxon signed-rank test. Subsequently, changes in seroma volume (CT2-CT1) were correlated to the difference in irradiated breast-GTV volume, mean ipsilateral lung and mean heart dose using Spearman Rho correlation regression. Median (IQR) reduction in left and right seroma volume between CT1 and CT2 was 60.7% (36.82% -76.71%) (p = 0.001), and 45.7% (16.63% -58.31%) (p = 0.002) respectively. Median (IQR) reduction in left and right irradiated breast-GTV volume between CT1 and CT2 was 17.9% (1.41% -33.85%) (p = 0.01), and 11.9% (8.59% -33.25%) (p = 0.001) respectively. No statistically significant differences in mean ipsilateral lung and mean heart dose were observed for all patients. Similarly, the change in seroma volume between CT1 and CT2 was not significantly correlated to the differences in irradiated breast-GTV volume, mean lung and mean heart dose. Patients experienced a significant reduction in seroma volume during WBI. Re-CT and re-planning will reduce the volume of normal breast tissues irradiated, which may improve cosmesis. However, a re-CT and re-plan does not lead to a meaningful reduction in lung and heart doses.Tabled 1Abstract 2654; Table; Differences in seroma volume, irradiated breast-GTV volume, mean lung and mean heart dose (CT2-CT1).LeftRightMedian Reduction (%)/ Dose (Gy)P valueMedian Reduction (%)/ Dose (Gy)P valueSeroma Vol60.7%0.00145.7%0.0025Gy Dose Vol/ (Irradiated breast Vol – GTV)17.9%0.1111.9%0.001Mean Ipsilateral Lung Dose25.7%/ 0.08Gy0.2934.2%/ 0.05Gy0.691Mean Heart Dose19.5%/ 0.03Gy0.532 Open table in a new tab