Dosimetric Analysis of Breast Swelling During External Beam Radiotherapy using Biomechanical Deformation Modeling

Breast cancer is the most common cancer in women worldwide. External beam radiotherapy with MV photons remains a central treatment for breast cancer, with treatments lasting up to five weeks. Over this timeframe, breast swelling due to radiation inhibiting normal fluid drainage poses a dosimetric challenge, as the size, shape, and water equivalent depth of the treatment area can change. In current clinical practice for whole breast radiation therapy, radiation fields are created with extra margin around the tissue to ensure the breast is still inside the treatment field if swelling presents, but no estimates are made of the dosimetric consequences of the swelling. Here we will present a dosimetric study where patient CT scans used for radiation planning are deformed using a biomechanical algorithm to simulate breast swelling in-silico. These deformed reference simulation CT scans will be compared to actual replan CT scans for patients with clinically significant swelling, to validate the accuracy of the biomechanical model. Then radiation dose will be estimated using the original treatment plan on the deformed CTs for a diverse set of patients. Finally, a multiparametric model will be built to quantitatively demonstrate the dosimetric impact of varying magnitudes of swelling as a function of tumor depth, breast volume, treatment modality (static versus modulated arc therapy), fractionation scheme, and remaining fractions.