2685 - Effectiveness of Hypofractionated Radiotherapy in Microinvasive Breast Cancer

C. Hardy Abeloos¹, J. Gurewitz², J. Xiao³, C. Oh⁴, A. Solan⁵, O. Cahlon³, and N. K. Gerber³; ¹NYU Grossman School of Medicine, New York, NY, ²Department of Radiation Oncology, NYU Langone Health and Perlmutter Cancer Center, New York, NY, ³New York University Grossman School of Medicine, Department of Radiation Oncology, New York, NY, ⁴Biostatistics, Department of Population Health, NYU Langone Health, New York, NY, ⁵NYU Grossman School of Medicine, New York City, NY

Purpose/Objective(s): The natural history, prognosis, and optimal treatment for microinvasive (T1mi) breast cancer is controversial as it is unclear whether it should be classified with ductal carcinoma in situ (DCIS) or with invasive disease. A recently published secondary analysis from the Ontario Clinical Oncology Group (OCOG) trial comparing conventional fractionation and hypofractionation found a significantly higher 10-year local recurrence rate in patients with T1mi disease compared to patients with T1a-2 disease, suggesting that microinvasive disease may be more locally aggressive than even early stage invasive cancer. The goal of our study was to compare long term outcomes between patients with T1mi disease and T1a-2 disease after breast conserving surgery (BCS) and hypofractionated whole breast irradiation (WBI) and identify prognostic factors. Materials/

Methods: We included patients with T1mi-2 N0 breast cancers who received hypofractionated WBI between 2013-2019. Cox proportional hazard model was used to find independent prognostic variables associated with local recurrence (LR), distant metastasis free survival (DMFS) and overall survival (OS). Survival curves were analyzed by Kaplan-Meier method, and differences between survival rates were compared by using the log-rank test.
Results: At a median follow up of 5 years, 1,155 patients were identified and 56 (4.8%) had T1mi disease. Patients with T1mi disease were more likely to be hormone receptor negative and HER2 positive compared to those with T1-2 invasive disease (p < 0.05). The 5-year local recurrence rate was 5.3% in patients with T1mi disease and 1.2% in patients T1-2 disease (HR = 2.73; 95% CI: 0.43, 17.9; p = 0.09). On Cox regression analysis, younger age (HR 0.96, p = 0.034), positive margins (HR 4.93 p = 0.010) and the need for re-excision (HR 3.59, p = 0.003) were prognostic for LR. Microinvasive disease (HR 2.73, p = 0.10), lymphovascular invasion (HR 2.28, p = 0.13), estrogen receptor subtype (HR 0.55, p = 0.23), progesterone receptor subtype (HR 0.52, p = 0.12), HER2 status (HR 0.62 p = 0.52), the use of a boost (HR 2.78, p = 0.062), or the use of endocrine therapy (HR 0.69, p =0.43) were not significant for LR. Out of the 3 patients with microinvasive disease who developed a LR, two had DCIS < 2 mm from the margin and the third patient had 48 mm of associated DCIS with two re-excisions due to DCIS margins < 2 mm. Five year DMFS was 100% in patients with T1mi disease and 96.5% in patients with T1a-2 disease (p=0.16). Five year OS was 100% in patients with T1mi disease and 98% in patients with T1a-2 disease (p = 0.32).
Conclusion: Our study showed that patients with microinvasive disease treated with hypofractionated WBI were not at increased risk of local recurrence compared to those with T1a-2 disease after adjusting for covariates. Further work is needed to define the optimal margin width in micro-invasive disease as our study, and many others, confirms the association of positive margins and re-excisions with local recurrence.