Unusual Nodule on the Left Leg of an Elderly Woman—A Case Report: Challenge

We present the case of a 71-year-old woman with no significant medical history who developed a nodular, ulcerated lesion on her left leg, alongside bruise-like lesions on her extremities. Histologic examination revealed basal cell carcinoma with adenoid-cystic features colliding with blastic plasmacytoid dendritic cell neoplasm (BPDCN), a rare and aggressive hematologic malignancy. Immunohistochemical analysis confirmed BPDCN, with markers such as CD123, CD4, CD56, and TCL1, and a high proliferation index (Ki67 >90%). This represents the first reported case of a collision lesion comprising adenoid-cystic basal cell carcinoma and BPDCN. The findings highlight the need for comprehensive histopathologic evaluation in atypical cases to achieve an accurate diagnosis.