Dose Finding and Positioning for Definitive Stereotactic Ablative Radiotherapy of Breast Cancer

Abstract

Background: Partial breast irradiation (PBI) harbors the potential for definitive local tumor control. The optimal dose to the target and the maximal tolerated doses (MTD) of the surrounding tissues are unknown. In this cohort planning study, we assess the maximal dose to the target while keeping the dose to the surrounding organs at risk within their tolerance.Patients and Methods: Planning computed tomography scans of 22 patients with a history of pulmonary or cardiac risk factors and left-sided pT1c pN0/pN1a M0 breast cancer were treated with adjuvant RT in either position acccording to better dose distribution to lung or heart. Plans were calculated, generating a planning target volume (PTV) suitable for PBI in five fractions according to guidelines of the American Society for Radiation Oncology. Dose was escalated starting with 6 Gy daily. MTD was defined when the dose reached a constraint of a neighboring organ based on recommendations of the American Association for Physics in Medicine.Results: The mean MTD was 45.9 Gy ± 3.9 Gy (range 38.8 - 53.9) in supine and 46.1 Gy ± 3.2 Gy (range 37,3 - 53.9) in prone position, respectively (p>0.05). In 95% of patients, the MTD was ≥ 44.3 Gy in supine and ≥ 44.8 Gy in prone position, respectively. Fat tissue was the dose limiting structure for 11 of 22 patients in supine and 15 of 22 in prone position. Dmax to the fat tissue reached 40.0 Gy (±3.3 Gy) in both supine and prone position (p=0.3). Skin was the dose limiting structure in 7 of 22 patients in supine and in 6 of 22 in prone position. Dmax to the skin was 30.5 Gy (±7.4 Gy) in supine and 31.0 Gy (±7.0 Gy) in prone position (p= 0.8). Ribs were dose limiting in 4 of 22 patients in supine and in 1 of 22 in prone position. Dmax to the ribs was 31.4 Gy (±9.5 Gy) in supine and 21.4 Gy (±11.0 Gy) in prone position (p <0.01). Dmax to the intraventricular artery (IVA) was 3.4 Gy (±3.1 Gy) in supine and 7.5 Gy (±5.7 Gy) in prone position (p<0.01). Conclusion: 37.5 Gy in five fractions given to the planning target volume appears to be safe. Escalating the dose to the visible tumor to 45 Gy in five fractions is recommended for further testing in trials. Prone positioning is advised for patients with tumors close to the ribs.