"Signet Ring" sinus histiocytosis mimicking metastatic adenocarcinoma: report of two cases with immunohistochemical and ultrastructural study.

The axillary lymph nodes from two patients with invasive breast carcinoma showed "signet ring" histiocytosis with sinusoidal distribution of cells having prominent cytoplasmic vacuolization which simulated metastatic adenocarcinoma. Immunohistochemical studies confirmed the histiocytic nature of the signet ring cells and eliminated the possibility of metastatic adenocarcinoma. Electron microscopy in one case revealed lipid within the cytoplasmic vacuoles. The etiology of this reactive change is unknown; however, a history of previous surgery involving the chest wall may be causally related with disruption of adipose tissue. Prior to ipsilateral mastectomy both patients had undergone a contralateral mastectomy for mammary carcinoma. Signet ring sinus histiocytosis is a reactive sinusoidal proliferation that can mimic metastatic carcinoma, but close inspection will usually reveal the histiocytic nature of the cells.