Assessment of dose variation for accelerated partial-breast irradiation using rigid and deformable image registrations.

PURPOSE: The aim of this study was to estimate the delivered dose to the target and organs at risk (OAR) for external beam accelerated partial-breast irradiation (APBI) accounting for day-to-day setup uncertainties, using rigid and deformable image registration.

METHODS AND MATERIALS: One planning computed tomography (CT) scan and 5 cone beam CT scans for each of 25 patients previously treated with tangential breast radiation therapy were used. All cone beam CT scans were registered to the planning CT scan using 3 techniques: (1) rigid registration based on bony anatomy only, (2) rigid registration based on soft-tissue only, and (3) deformable image registration. For each patient, 4 dose distributions were calculated for APBI. The first dose distribution was the original plan. The other 3 were "dose-of-the-day" for each of the registration approaches. The effects of image registrations on estimating delivered dose to targets and OAR were determined.

RESULTS: The average reductions in V95 (percentage of the PTV that received 95% of the prescribed dose) were 6%, 7%, and 5% for bone, soft-tissue, and deformable registrations, respectively. The average increase in mean dose to the heart were 9%, 9%, and 18% for bone, soft-tissue, and deformable registrations, respectively, whereas the average increase in maximum dose to the contralateral breast were 19%, 20%, and 28%, respectively.

CONCLUSIONS: The results of this study have shown that there are differences between the planned and estimated delivered dose for APBI because of day-to-day setup uncertainties that may need to be accounted for. Estimated dosimetric impact of setup variation and breast deformation assessed using deformable registration was greater for OARs and smaller for target volumes compared to rigid registration.