Bolus use during post-mastectomy radiotherapy (PMRT) doubles the risk of grade 2 and 3 skin toxicity. Despite its unknown benefit, bolus is often prescribed during PMRT for patients without skin involvement.

For women with breast cancer receiving photon 3D conformal RT, bolus was used routinely for chest walls, but was omitted for breast reconstructions by about half of radiation oncologists from 2007-2011. Eligible patients had newly diagnosed invasive breast cancers without skin involvement (pT1-4a, any-N, M0) treated with adjuvant or neoadjuvant RT. For the bolus and no-bolus groups, we compared the cumulative incidence of local recurrence (LR) and locoregional recurrence (LRR) with distant recurrence and death as competing risks and breast cancer mortality (BCM). Multivariable analysis of LR and BCM included stage, subtype, lymphovascular invasion, grade, margin status, beam energy, bolus use, hormone therapy, chemotherapy, and reconstruction.

Systemic therapy was used for 98% of the 1,887 patients. The bolus group had 1,569 patients and the no-bolus group had 318 patients. Bolus was used in 51% (281/550) of patients treated with reconstruction and 96% (1,288/1,337) of patients treated without reconstruction. The 10-year outcomes [95% confidence interval] in patients treated with and without bolus were, respectively: LR 1.9% [1.3, 2.7] versus 0.9% [0.3, 2.6], LRR 3.1% [2.3, 4.0] versus 3.2% [1.6, 5.5], and BCM 20.9% [18.3, 23.7] versus 18.3% [13.9, 23.2]. On multivariable analysis, bolus use was not associated with better LR (HR = 1.44 [0.33, 6.39]) or BCM (HR = 0.81 [0.54, 1.22]).

For patients treated with mastectomy, radiotherapy, and modern systemic therapy, the cumulative incidence of local recurrence was low, with or without bolus. As bolus use increases toxicity and does not reduce LR or BCM, it should no longer be used routinely for patients without skin involvement who receive systemic therapy.