Clinicopathological factors associated with HER2 FISH positivity in HER2 IHC 2 + breast cancer: a multicenter study

HER2 immunohistochemistry (IHC) 2 + breast cancer represents a clinically challenging subgroup requiring confirmatory fluorescence in situ hybridization (FISH) testing. This multicenter retrospective study included 284 patients with HER2 IHC 2 + breast cancer from a regional cohort to evaluate the FISH positivity rate and identify associated clinicopathological factors. Overall, 52 patients (18.3%) were FISH-positive. Compared with FISH-negative patients, the FISH-positive group showed significantly higher Ki-67 levels, lower PR expression, and a higher proportion of histological grade III tumors. Multivariable analysis identified histological grade III as the only clinicopathological characteristic independently associated with FISH positivity (OR = 2.43, 95% CI: 1.26-4.71, P = 0.008). The multivariable model demonstrated moderate discriminatory ability, with an area under the curve of 0.703 (95% CI: 0.628-0.777). Exploratory categorical analysis showed higher FISH positivity rates in patients with Ki-67 ≥ 30% and histological grade III tumors, with a stepwise increase in FISH positivity according to the number of these risk factors.These findings suggest that histological grade and Ki-67 may provide supportive information for characterizing the likelihood of HER2 gene amplification in HER2 IHC 2 + breast cancer. However, these routinely available clinicopathological features should not replace guideline-recommended confirmatory FISH testing.