Answer: Papillary carcinoma

Histology: This tumor has a rounded, multi nodular architecture, arguing against invasive carcinoma. The surrounding breast tissue is histologically unremarkable. A papillary process distorts the ducts that form these nodules. The papillae are lined by hyperchromatic stratified spindle cells, which focally form a cribriform pattern. Myoepithelial cells are not apparent on the H&E section, and their absence is confirmed by immunostaining with actin

Discussion: A papilloma should demonstrate an intact myoepithelial layer, and lacks the hyperchromatic nuclei and cribriform growth patterns typically seen within papillary carcinoma. Some use the term atypical papilloma when areas of ductal carcinoma in situ colonize approximately one third of a papilloma. Our lesion is homogeneous throughout which excludes this potential diagnosis. Invasive carcinoma is characterized by stromal desmoplasia which is absent in the current lesion.

This case represents a variant of intraductal papillary carcinoma known as the stratified spindle cell type, which is easily mistaken for a benign process. The key is to note the lesion’s overall “blueness,” caused by the hyperchromasia of the lesional nuclei. The focal presence of cribriform architecture helps establish a diagnosis that is based primarily on the atypical hyperchromatic cytology of the tumor cells.