Outcomes for Patients With Micro-Invasive or Small HER2+ Breast Cancer in Northern Sydney Local Health District.

[AIM] This study aimed to review the outcomes for patients with microinvasive and small HER2+ breast cancers, compare the clinicopathological features, and explore the patterns of care.

[METHODS] In this single-center retrospective study, patients with HER2+, pT1mic-bN0M0 disease who underwent surgery in Northern Sydney Local Health District between 2006 and 2022 were selected. The primary outcome was invasive disease-free survival (iDFS), and secondary outcome was overall survival (OS).

[RESULTS] Of the 32 eligible patients, those with pT1b disease had higher histological grades and Ki-67 indices, with a higher rate of trastuzumab administration. One recurrence and one death were observed in the pT1a group. The 5-year iDFS and OS were 93% for the pT1mic/a group and 100% for the pT1b group (p = 0.29). Sixty-six percent of patients received adjuvant trastuzumab. Trastuzumab prescription was associated with larger tumor sizes (p = 0.008) and higher histologic grades (p = 0.023).

[CONCLUSIONS] Patients with microinvasive or small HER2+ node-negative breast cancer experience favorable outcomes. The pT1b group had higher histological grades and proliferative indices. Patients with pT1b disease or higher tumor grades were more likely to receive trastuzumab. While trastuzumab is recommended for pT1b disease with strong evidence, its use in pT1a disease should be balanced against its unclear benefits and known toxicities.