Adaptive radiation therapy for anatomical changes of the breast during whole-breast irradiation.

PURPOSE: To evaluate the dosimetric impact of anatomical changes during whole-breast irradiation (WBI) and to assess the utility of offline adaptive radiation therapy (ART) in patients treated with the field-in-field (FIF) technique. METHODS: A dose of 42.4 Gy in 16 fractions was prescribed to the whole-breast planning target volume (PTV), followed by a sequential boost of 10.6 Gy in 4 fractions to the tumor bed. Twenty-seven patients undergoing WBI after breast-conserving surgery were selected for offline ART when anatomical changes resulted in ≥ 5 mm breast contour deviation on electronic portal imaging (EPID). Original plans were recalculated on repeat CT, and adaptive plans were reoptimized. Dosimetric parameters (Dmin, Dmean, V95 %, conformity index [CI], homogeneity index [HI]) were evaluated for the clinical target volume (CTV). RESULTS: Anatomical changes included 18 breast expansions and 9 retractions. Contour deviations ranged from 5.0 to 13.0 mm (mean 7.5 mm). CTV volume increased in 18 patients (mean 46.3 cc) and decreased in 9 patients (mean 32.5 cc). Compared with original plans (Dmin: 87.7 ± 3.6 %), test plans showed significant reductions in Dmin (74.7 ± 20 %, p < 0.001) and V95 % (96.7 ± 3.9 % vs. 98.2 ± 2.0 %, p < 0.001), with decreased CI and increased HI. Organs-at-risk doses were not significantly affected. Adaptive replanning restored target coverage. CONCLUSION: Breast contour deviations ≥ 5 mm during FIF WBI are associated with reduced CTV coverage. Offline ART provides a practical strategy for maintaining dosimetric integrity.