# Relative Effects of Brachytherapy and Beam Radiation for DCIS on Subsequent Invasive Events

## TL;DR

This study compares brachytherapy and external-beam radiation for DCIS and finds that brachytherapy may increase the risk of invasive breast cancer over time.

## Contribution

The study provides new evidence on the long-term risks of brachytherapy compared to external-beam radiation for DCIS patients.

## Key findings

- Brachytherapy showed no significant difference in invasive cancer risk compared to no radiation at 10 years.
- Brachytherapy was associated with increased invasive cancer risk after 3 years compared to external-beam radiation.
- The study suggests brachytherapy may not be as effective as external-beam radiation in reducing invasive recurrence.

## Abstract

Background/Objectives: Patients with a diagnosis of ductal carcinoma in situ (DCIS) were poorly represented in the four trials that established the efficacy of partial relative to whole-breast irradiation. In contrast to invasive cancers, patients with DCIS are equally likely to have a subsequent ipsilateral invasive event in a different site of the breast from their initial DCIS lesion as they are at the same site. We aim to compare the efficacy of a type of partial-breast irradiation, brachytherapy, to external-beam radiation in the reduction of subsequent invasive cancers. Methods: Women diagnosed with a first breast cancer of unilateral DCIS treated with breast-conserving surgery without endocrine therapy were identified in SEER. Matching was performed 1:2 from patients receiving brachytherapy to patients receiving external-beam radiation or no radiation. External-beam radiation was assumed to be whole-breast radiation for the majority of patients in this cohort diagnosed from 2007 to 2011. Competing risks methods were used to estimate the cumulative incidence of invasive breast events at 10 years and subdistribution hazard ratios (sHRs) in adjusted models with time-varying treatment coefficients were calculated. Results: Among the 1392 matched patients who received brachytherapy or no radiation, the 10-year cumulative incidence of ipsilateral invasive cancer was 5.5% without radiation and 5.7% with brachytherapy (p = 0.92). Brachytherapy was associated with reduced risk of ipsilateral invasive events in the first 3 years (sHR = 0.19, p = 0.03) and non-significantly increased risk after 3 years (sHR = 1.66, p = 0.07). Among the 1392 matched patients who received brachytherapy or external-beam radiation, the 10-year cumulative incidence of ipsilateral invasive cancer was 5.7% with brachytherapy and 3.1% with external-beam radiation. In multivariate regression, brachytherapy was associated with unchanged risk in the first 3 years but increased risk after 3 years (sHR = 2.20, p = 0.009). Conclusions: Our results suggest that brachytherapy may be associated with a higher invasive recurrence risk for patients with DCIS treated without endocrine therapy, as it did not prevent more invasive ipsilateral events than no radiation, and provided reduced risk reduction in the ipsilateral breast relative to external-beam radiation. Further work is needed to identify if this is specific to brachytherapy or applies to all forms of partial-breast irradiation.