# Breast Events After Treatment of Ductal Carcinoma In Situ in Women: A Population‐Based Study

## TL;DR

This study examines factors that influence the risk of breast events after treatment for ductal carcinoma in situ, focusing on recurrence and progression to invasive cancer.

## Contribution

The study identifies specific risk factors for breast events after DCIS treatment in a population-based cohort.

## Key findings

- Women under 45 and those with DCIS larger than 20 mm had a higher risk of invasive breast cancer recurrence.
- Mastectomy and additional radiation therapy reduced the risk of invasive breast cancer recurrence.
- Surgical margin less than 2 mm and treatment at private facilities were linked to higher risks of breast events.

## Abstract

Despite favourable survival prognosis, the main concern for ductal carcinoma in situ (DCIS) is local recurrence, especially progression to invasive cancer. This study identified factors associated with breast events following DCIS treatment.

Women with unilateral DCIS between 2000 and 2022 were identified from New Zealand Breast Cancer Foundation National Register. The primary endpoint was cumulative incidence of invasive breast cancer, ipsilateral (iIBC) or contralateral (iCBC). Secondary endpoints included ipsilateral breast event (IBE), in situ or invasive, and any breast event (IBE or CBC). Fine‐Gray models were used to identify the associated factors and estimate subdistribution hazard ratios (sHRs).

Among 5740 patients followed for a median duration of 4.8 years, the 5‐ and 10‐year cumulative risks were 3.0% (95% CI, 2.4%, 3.5%) and 6.6% (95% CI, 5.7%, 7.6%) for iIBC, and 2.7% (95% CI, 2.2%, 3.3%) and 6.3% (95% CI, 5.4%, 7.3%) for iCBC, respectively. A higher risk of iIBC was observed in women aged under 45 at diagnosis (sHR, 1.81; 95% CI: 1.18, 2.79) or had DCIS size > 20 mm (sHR, 1.42; 1.05, 1.93), and a lower risk in those who received additional RT after BCS (HR: 0.61; 0.44, 0.84) or mastectomy (sHR, 0.21; 0.13, 0.32). Similar associations were observed for IBE and any breast event, for which surgical margin < 2 mm was also associated with a higher risk. Having surgery at a private facility, where higher‐risk patients were likely to be treated, was associated with a higher risk of iCBC.

DCIS size, surgical approach, and age at diagnosis influenced the risk of breast events after DCIS, which may be considered in efforts to improve treatment strategies for higher‐risk women.