Comparison of planned versus actual dose delivered for external beam accelerated partial breast irradiation using cone-beam CT and deformable registration.

PURPOSE: To assess the adequacy of dose delivery to the clinical target volume (CTV) using external beam (EB) accelerated partial breast irradiation (APBI).

METHODS AND MATERIALS: Sixteen patients treated with EB APBI underwent cone beam CT (CBCT) before each fraction and daily helical CT (HCT) scans to determine setup errors and calculate the dose per fraction. For 12 patients, an in-house image-intensity-based deformable registration program was used to register the HCTs to the planning CT and generate the cumulative dose. Treatment was 38.5 Gy in 10 fractions. EB APBI constraints from the National Surgical Adjuvant Breast and Bowel Project B39/Radiation Therapy Oncology Group 0413 Phase III protocol were used.

RESULTS: The mean setup error per CBCT registration was 9 ± 5 mm. Dose-volume histogram analysis showed only one patient (8%) with a decrease in the CTV V90 (8% underdosage). All other patients demonstrated adequate target coverage. PTV_EVAL V90 was on average 3% (range, 0%-16%) less than planned. For the ipsilateral breast, four patients had an increase in V50 (≤ 1% increase) and three patients had an increase in V100 (≤ 9% increase). Only one patient showed an increase >5%. Four patients had an increase in ipsilateral lung V30 (maximum 3%), and one had an increase in heart V5 (1%). Four patients had an increase in MaxDose (maximum 89 cGy).

CONCLUSIONS: The current CTV-to-PTV margin of 10 mm appears sufficient for ∼92% of patients treated with EB APBI. Although expansion of the population PTV margin to 14 mm would provide ∼97% confidence level for CTV coverage, online image guidance should be considered.