To determine the invasive recurrence (IR) risk among patients with small, node-negative human epidermal growth factor receptor 2 (HER2) -positive breast cancer.

Among 16,975 consecutive patients with invasive breast cancer diagnosed from January 1, 2000, to December 31, 2006, in a large, integrated health care system, we identified a cohort of 234 patients with HER2-positive T1aN0M0 or T1bN0M0 (T1abN0M0) disease with a median follow-up of 5.8 years. Kaplan-Meier methods were used to estimate the percentage of patients who were free of invasive recurrence (recurrence-free interval [RFI]) at 5 years for both distant (DRFI) and local (LRFI) recurrences.

Of 15 IRs, 47% were locoregional only. Among T1ab patients not treated with adjuvant trastuzumab or chemotherapy (n = 171), the 5-year invasive DRFI was 98.2% (95% CI, 94.5% to 99.4%); it was 99.0% (95% CI, 93.0% to 99.9%) for T1a patients, and 97.0% (95% CI, 88.6% to 99.2%) for T1b patients. Locoregional plus distant 5-year invasive RFI was 97.0% (95% CI, 90.9% to 99.0%) for T1a and 91.9% (95% CI, 81.5% to 96.6%) for T1b patients; it was 89.4% (95% CI, 70.6% to 96.5%) for T1b tumors reported at 1.0 cm. T1b tumors reported at 1.0 cm accounted for 24% of the T1ab cohort, 61% of the cohort total tumor volume, and 75% of distant recurrences. Invasive RFI for T1b 1.0 cm tumors was lower than that for T1a tumors: 84.5% versus 97.4% (P = .009).

The distant IR risk of T1a HER2-positive breast cancer appears quite low. The distant IR risk in T1b patients, particularly those with 1.0-cm tumors, is higher. Potential risk differences for T1a and T1b, including the 1.0-cm tumors, should be considered when making treatment decisions.