Invasive lobular carcinoma of the breast; clinicopathologic profile and response to neoadjuvant chemotherapy over a 15-year period.

Invasive lobular carcinoma (ILC) accounts for 5-15% of invasive breast cancers. Typical ILC is oestrogen receptor (ER) positive and human epidermal growth factor receptor 2 (HER2) negative. Atypical biomarker profiles (ER- and HER2+, ER+ and HER2+ or triple negative) appear to differ from typical ILCs. This study compared subtypes of ILC in terms of clinical and pathological parameters, and response to neoadjuvant chemotherapy (NACT) according to biomarker profile. All patients with ILC treated in a single centre from January 2005 to December 2020 were identified from a prospectively maintained database. Clinicopathologic and outcome data was collected and analysed according to tumour biomarker profile. A total of 582 patients with ILC were treated. Typical ILC was observed in 89.2% (n = 519) and atypical in 10.8% (n = 63). Atypical ILCs were of a higher grade (35% grade 3 vs 9.6% grade 3, p < 0.001). A larger proportion of atypical ILC received NACT (31.7% vs 6.9% p < 0.001). Atypical ILCs showed a greater response to NACT (mean RCB (Residual Cancer Burden Score) 2.46 vs mean RCB 3.41, p = 0.0365), and higher pathological complete response rates (15% vs 0% p = 0.017). Despite this, overall 5-year disease-free survival (DFS) was higher in patients with typical ILC (91% vs 83%, p = 0.001). Atypical ILCs have distinct characteristics. They are more frequently of a higher grade and demonstrate a superior response to NACT. Despite the latter, atypical ILCs have a worse 5-year DFS which should be taken into consideration in terms of prognostication and may assist patient selection for NACT.

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