In Silico Assessment of the Radiation Dose Range for Definitive Stereotactic Body Radiation Therapy of Primary Breast...

The maximal dose for partial breast irradiation (PBI) with stereotactic body radiation therapy for definitive local therapy of nonmetastatic breast cancer has not been established. Here we evaluate the maximal achievable coverage of the planning target volume suitable for PBI without violating organs-at-risk constraints.

Planning computed tomography scans of 22 patients with pulmonary or cardiac risk factors and left-sided disease in prone and supine position (sp) were obtained. Plans for PBI in 5 fractions were generated according to the Guidelines of the American Society for Radiation Oncology. Maximum tolerated dose (MTD) was defined when the dose reached any constraint of a neighboring organ based on recommendations of the American Association for Physics in Medicine.

Mean MTD was 45.9 ± 3.9 Gy (range, 38.8-53.9) in sp and 46.1 ±3.2 Gy (range, 37.3-53.9) in prone position (pp), respectively. The MTD was ≥44.3 Gy in sp and ≥44.8 Gy in pp in 95% of patients. Fat tissue was the dose limiting structure in 11 of 22 patients in sp and 15 of 22 in pp. D_max to the fat tissue reached 40.0 Gy (±3.3 Gy) in sp and pp. Skin was the dose limiting structure in 7 of 22 patients in sp and in 6 of 22 in pp. D_max to the skin was 30.5 Gy (±7.4 Gy) in sp and 31.0 Gy (±7.0 Gy) in pp (P = .8). Ribs were dose limiting in 4 of 22 patients in sp and in 1 of 22 in pp. D_max to the ribs was 31.4 Gy (±9.5 Gy) in sp and 21.4 Gy (±11.0 Gy) in pp (P < .01). D_max to the intraventricular artery was 3.4 Gy (±3.1 Gy) in sp and 7.5 Gy (±5.7 Gy) in pp (P < .01).

For definitive stereotactic body radiation therapy for early-stage breast cancer, we propose a dose escalation starting with 45 Gy in 5 fractions to be tested in a clinical trial. Prone position is advised for tumors close to the thoracic cage.