Apocrine Variant of Pleomorphic Lobular Carcinoma In Situ: Further Clinical, Histopathologic, Immunohistochemical, and Molecular Characterization of an Emerging Entity.

To date, the apocrine variant of lobular carcinoma in situ (AP-LCIS) has been cursorily described as a subtype of lobular carcinoma in situ (LCIS). We retrospectively reviewed 34 cases of AP-LCIS (including 23 associated with invasive lobular carcinoma) to fully characterize it. AP-LCIS typically presented with screen-detected calcifications in older women (mean age: 65 y) and was characterized by distended terminal duct lobular units with relatively large "pleomorphic" cells, central necrosis, and calcifications. AP-LCIS cells exhibited abundant eosinophilic occasionally granular cytoplasm, hyperchromatic nuclei, and prominent nucleoli. Synchronous classic and/or florid LCIS was identified in 24/34 (70%) AP-LCIS, and in 9/11 (82%) pure AP-LCIS. Most (68%) cases of AP-LCIS were estrogen receptor-positive (50% strongly), 35% were progesterone receptor-positive, 26% were human epidermal growth factor 2-positive, 18% demonstrated high-proliferation rate (Ki67: >15%), and 90% were androgen receptor-positive. Aurora kinase A, immunoreactive in 38% of AP-LCIS cases, was not significantly associated with recurrence, development of invasion, or nodal positivity (P>0.05). Compared with conventional (nonapocrine) pleomorphic lobular carcinoma in situ (P-LCIS), aurora kinase A was expressed in a significantly greater proportion of P-LCIS (100%). AP-LCIS and P-LCIS did not otherwise differ in clinicopathologic features. Next-generation sequencing utilizing the Oncomine Comprehensive Panel v2, performed on 27 AP-LCIS cases, showed no specific molecular findings. In a mean follow-up of 57 months, 2 (of 11, 18%) pure AP-LCIS cases recurred (2 both in situ and invasive) and none metastasized or proved fatal. AP-LCIS should be regarded as another high-grade LCIS similar to P-LCIS in many respects, and pending additional studies should be managed similarly.