Background: Pleomorphic lobular carcinoma in situ (PLCIS) is an uncommon high-grade in situ lesion that shares morphologic features of both classic lobular and ductal carcinoma in situ. Data on the natural history of pure PLCIS are limited, and no evidence-based consensus guidelines for management exist. Methods: From our prospectively maintained institutional pathology and breast surgery databases, we identified all patients with a diagnosis of PLCIS on core needle biopsy (CNB) or excisional biopsy from 2004 to 2017. Patient, tumor, treatment, and outcome data were abstracted to analyze upstage rates and treatment outcomes. Results: We identified 18 patients with pure PLCIS: 15 diagnosed on CNB, 2 diagnosed at operation for atypia on CNB, and 1 diagnosed after excisional biopsy without preceding CNB. Of the 15 patients with PLCIS on CNB, 3 (20%) were upgraded to invasive cancer on surgical excision. Overall, 7 patients were treated with mastectomy (all margin-negative) and 11 with lumpectomy (one with a focally positive margin). Eight patients received adjuvant therapy: six endocrine therapy, one radiation therapy, and one received both. Among patients with a final diagnosis of PLCIS, two ipsilateral recurrences were observed at follow-up: one invasive lobular carcinoma at 87 months and one PLCIS at 16 months postoperatively. Conclusion: PLCIS on CNB mandates surgical resection as 20% of patients may be upgraded to invasive cancer, and outcomes following pathologic margin-negative surgical resection were excellent with only one invasive recurrence observed. Larger-scale investigation with longer follow-up is needed to define a role for adjuvant therapy and to develop evidence-based treatment guidelines.
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