Breast Tangent Beam Energy and Local Control after Breast-Conserving Treatment

Abstract

The use of high-energy photons for breast tangent beams during adjuvant breast radiotherapy lowers skin dose and breast dose inhomogeneity, thereby reducing acute and late toxicity. However, sparing of skin and superficial tissues with higher-energy beams may increase the risk of local recurrence, so some cancer centers use beam spoilers and bolus to increase the skin dose. The null hypothesis of this study was that the use of higher breast tangent beam energies would not affect the incidence of local recurrence after breast-conserving treatment. This population-based study included newly diagnosed invasive breast cancers (pT1-4a, any-N, M0) treated with breast-conserving surgery and adjuvant whole breast radiotherapy without use of bolus or beam spoilers. Patients with skin involvement (cT4b, c, d or pT4b, c, d) and those with previous or synchronous breast cancers were excluded. The primary endpoint was the cumulative incidence of local recurrence using regional recurrence, distant recurrence and death as competing risks. In this population, local recurrence events are actively sought until 15 years after diagnosis with annual queries to the primary practitioners of all breast cancer survivors. A multivariable analysis was conducted with beam energy (6 MV versus >6 MV), age, T-stage, nodal status, lymphovascular invasion, grade, margin status, extensive intraductal component, ER, PR, and HER2, as well as use of boost radiotherapy, hormone therapy and chemotherapy. The cohort consisted of 10,340 women diagnosed from 2002 to 2011, 7,374 treated with 6 MV tangents, 2,966 treated with >6 MV tangents and 1,319 treated with >10 MV tangents. The median follow-up by the reverse Kaplan-Meier method was 10.1 (95%CI: 9.9- 10.2) years. The 10-year cumulative incidence of local recurrence was 3.2% (95%CI: 2.7 – 3.7%) with 6 MV tangents, 3.4% (95%CI: 2.7 – 4.2%) with >6 MV tangents and 2.4% (95%CI: 1.7 – 3.4%) with >10 MV tangents. There were no significant differences in the distribution of patient or tumor characteristics (age, T-stage, N-stage, ER, PR, HER2, LVI, margin status) or treatment characteristics (hormone therapy, chemotherapy) between the 6 MV and >6MV energy cohorts in the multivariable analysis. The multivariable analysis demonstrated that a higher incidence of local recurrence was significantly related to the presence of higher-grade histology and lymphovascular invasion, as well as less use of hormone therapy and chemotherapy, but not to beam energy (HR = 1.00, 95%CI: 0.95 – 1.07), age, ER, PR, HER2, T-stage, nodal status, margin status, or boost usage. The use of higher-energy photons for breast tangent radiotherapy was not associated with an increased risk of 10-year local recurrence. Radiation oncologists can be reassured by this large, retrospective study that no special measures are required when high-energy tangent beams are used to reduce the toxicity of radiotherapy for breast cancers that do not involve the skin.