Surface imaging to evaluate the dosimetric impact of breast deformations in proton and photon therapy for breast cancer

Abstract

Objectives. Breast cancer patients treated with proton therapy receive weekly repeat CTs (rCT) for treatment evaluation. However, plan adaptations are rarely necessary in our department. Therefore, the goal of this study was to develop an online method using surface guided radiotherapy (SGRT) to predict the dosimetric impact of changes in breast anatomy to identify patients that require plan adaptations, in order to reduce the number of rCTs. Approach. Seven breast cancer patients, treated with proton therapy with daily 3D surface-images in treatment position used for SGRT, were included. Clinical proton and backup photon plans were available. A novel in-house developed method, Dose Impact Prediction from Surface Imaging (DIPSI), was used to quantify the deformation between the planning CT (pCT) BODY contour, and the daily 3D SGRT surface images. To validate DIPSI, known isotropic deformations of 2, 4, 6, 8, 10 and 15 mm were introduced in the pCT BODY contour surrounding the clinical target volume (CTV). Clinical proton and photon treatment plans were recalculated including this deformation to evaluate the dosimetric impact of known deformations on the CTV D98%, D2% and average dose (Dmean). Deformation thresholds were determined to detect unacceptable dose deviations, from which the corresponding sensitivity, specificity and false negative rate were determined. Main results. DIPSI showed an accuracy of 0.2 mm and perfect correlation with known deformations (R2 = 1.00). Optimal thresholds to trigger treatment plan adaptation were 6.0 mm for the proton plan and 10.0 mm for the photon plan, resulting in sensitivities >95.5%, specificities >93.9% and false negative rates <1.6% in identifying breast deformations that require plan adaptations. Significance. Breast surface deformations could accurately be determined from daily SGRT images, making them suitable in identifying patients that require plan adaptations in photon and proton therapy potentially reducing the need for rCTs.