Real-Time Online Adaptation for Accelerated Partial Breast Irradiation Significantly Improves Target Coverage without Compromising Organs at Risk

Abstract

<h3>Purpose/Objective(s)</h3> Adaptive online radiotherapy (AORT) is an emerging technology that re-optimizes treatment plans based on cone-beam CT (CBCT) for accurate delivery of prescription and avoidance of critical organs at risk (OARs). Women with breast cancer who qualify for accelerated partial breast irradiation (APBI) are ideal patients for AORT as plans can adapt to daily positional and shape changes. Here we report the dosimetry benefit and the efficiency of patients receiving AORT APBI within the last year in our department. <h3>Materials/Methods</h3> We retrospectively reviewed 18 patients who received AORT APBI, with a total of 84 evaluable fractions. Two plans are made after OARs and targets are contoured on CBCT: a "scheduled" plan, the simulated plan overlaid on CBCT, and an "adapted" plan, where dose delivery is re-optimized to new contours. PTV and CTV volumes, PTV V30 Gy, CTV V30 Gy, ipsilateral breast V50% and V100%, skin Dmax, ipsilateral lung V9Gy, and heart V150cGy (if left sided disease) were collected for both the scheduled and adapted plans per fraction. CTV volume was created from GTV, defined as cavity delineated by surgical clips, plus a 1 cm radial margin limited from musculature of chest wall and 5 mm from skin. PTV was a 0-5 mm radial expansion of CTV volume. Time for adaptive planning was collected prospectively by radiation therapists. Three fractions were excluded from analysis due to setup error or pre-planned lack of adaptation. Paired t-test was used for parametric data and Wilcoxon rank sum test for nonparametric data; analyses were done in R v4.1.2 (www.r-project.org). <h3>Results</h3> Mean patient age was 66.4 years (range 42-80). All patients met ASTRO APBI "suitable" or "cautionary" category. 15 patients had pathologic T1-2N0M0 invasive ductal carcinoma and 3 patients had ductal carcinoma in situ (1/3 grade 2 and 2/3 grade 3). 82% (69/84) of fractions were delivered adapted. CTV and PTV volumes ranged from 15.7-197 cm³ (mean 80.7 cm³) and 33.4-227 cm³ (mean 119.8 cm³). Scheduled PTV V30 was <95% in 60 fractions (71%) and <90% in 30 fractions (36%). On adapted fractions, PTV V30Gy and CTV V30Gy coverage were significantly improved comparing scheduled versus adaptive plans, median (IQR) 91.4 (85.9-94.4%) v 95% (95-95%) and 98 (93.4-99%) v 99.7% (98.8-99.8%), respectively, p < 0.0001. There were no differences in PTV/CTV coverage when scheduled plans were chosen over adapted plans. Only skin Dmax was statistically decreased among the OARs (635.3 vs 629.1 cGy per fraction, p = 0.018); all other OAR coverages were not statistically different. Contouring and plan generation took on average (IQR) 9.4 minutes (6-12 min) and 5 min (4-6 min) respectively. <h3>Conclusion</h3> Adaptation with AORT significantly improved PTV/CTV prescription coverage and skin dose in APBI with <15 min added treatment time. AORT may improve outcomes for ABPI due to improved PTV/CTV prescription delivery.