A high mitotic score in breast cancer after neoadjuvant chemotherapy is predictive of outcome and associated with a distinct morphology.

Aims: Neoadjuvant chemotherapy (NAC) is frequently used for the treatment of breast cancer. We sought to analyse the clinical, morphological and immunohistochemical features of tumours from patients who did not achieve pathological complete response following NAC.

Methods And Results: We identified stage I-III post-NAC breast cancers from surgical resections (2000-2016) with evaluable residual invasive carcinoma [ypT1a(m) or greater and ≥15% tumour cellularity]. One hundred and forty-three tumours from 142 patients were included. On univariable analysis, a high (score 3) post-NAC mitotic score (as compared with 1 or 2) was significantly associated with invasive ductal carcinoma (IDC) subtype (P = 0.023), high grade, pushing borders with zones of necrosis, hormone receptor and triple-negative status, lack of hormonal therapy, higher cellularity (P < 0.001), and a higher percentage of tumour-infiltrating lymphocytes (P = 0.016). Multivariable analysis showed a high post-NAC mitotic score to be significantly associated with recurrence, distant metastasis, and shortened survival (hazard ratios of 5.73, 4.49, and 3.68, respectively). High post-NAC mitotic score tumours (n = 32) were IDC and had a high Ki67 proliferation index (median, 55%). Of these, 24 (75%) had pushing borders with zones of necrosis; 19 (79.2%) of these had necrosis on preoperative imaging, and 24 (75%), 15 (46.9%) and four (12.5%) lacked androgen receptor, GATA-3 and cytokeratin 18 expression, respectively.

Conclusions: High post-NAC mitotic score breast cancers cause high morbidity and mortality, frequently have pushing borders and zones of necrosis, and may show loss of common 'breast cancer markers'. Our findings support that necrosis in pretreatment studies and post-NAC mitotic score should be routinely reported, as they offer significant additional prognostic information to guide management.