Invasive micropapillary carcinoma (IMPC) is an uncommon subtype of breast cancer. Previous studies of this subtype demonstrated a higher propensity for lymph node metastases as compared with invasive ductal carcinoma (IDC). The purpose of the present study was to determine the clinical characteristics, outcomes, and propensity for lymph node metastasis of patients with IMPC of the breast recorded in the National Cancer Database (NCDB).

Records of patients with IMPC diagnosed between 2004 and 2014 were retrieved from the NCDB. Log-rank test was performed to evaluate associations of clinical characteristics with overall survival (OS). Cox proportional hazards model was used to determine variables associated with OS.

Overall, 2660 patients with IMPC met the selection criteria; the 5-year OS rate was 87.5% and 24.9% of patients had nodal involvement at presentation. Patients with ≥ 4 positive lymph nodes had shorter OS than node-negative patients, whereas patients with 1-3 positive nodes had similar OS to node-negative patients. Age < 65 years, receipt of radiotherapy, and estrogen receptor positivity were also associated with prolonged OS. The benefit of radiotherapy was limited to IMPC patients undergoing lumpectomy; there was no benefit for the patients undergoing mastectomy (regardless of nodal positivity/negativity).

Favorable prognostic factors of IMPC patients included age < 65 years, < 4 positive lymph nodes, receipt of radiotherapy, and estrogen receptor positivity. The results presented herein suggest a survival benefit associated with radiotherapy in IMPC treatment, though this may be limited to the patients treated with lumpectomy.