A metastatic renal carcinoid tumor presenting as breast mass: a diagnostic dilemma.

We present clinicopathological and cytological findings of a well-defined breast mass in a patient with history of primary renal carcinoid tumor. Fine-needle aspiration (FNA) cytology showed monotonous tumor cells with plasmacytoid appearance arranged singly and in small clusters. Occasional tumor cells were arranged in acinar architecture resembling glandular differentiation. Tumor cells showed fine speckled chromatin. The unusual location for metastasis of this rare type of carcinoid tumor and overlapping cytological features with primary mammary carcinoma led to an erroneous preliminary cytological diagnosis of primary breast carcinoma with plasmacytoid features. Tumor cells in the corresponding cell block showed strong diffuse positivity for synapthophysin and pan-cytokeratin with weak focal positivity for chromogranin markers. These patterns of immunostaining were similar to the original renal carcinoid tumor. To the best of our knowledge, a few cases of carcinoid tumor metastatic to the breast have been reported in the literature and more than half of these cases were initially misdiagnosed as primary breast carcinoma causing unnecessary surgical treatment. This is a first reported case of metastatic renal carcinoid tumor into breast diagnosed with FNA biopsy. This report highlights the cytological features of well-differentiated neuroendocrine tumor (carcinoid tumor) and its potential diagnostic pitfalls.