Follicular dendritic cell sarcoma of the neck: report of two cases complicated by pulmonary metastases.

BACKGROUND

Follicular dendritic cell (FDC) sarcoma is an uncommon neoplasm occurring primarily in lymph nodes but also in extranodal sites. A correct diagnosis can be difficult to make, especially in the latter sites.

METHODS

Two patients with FDC sarcoma of the cervical soft tissues that metastasized to the lungs are reported. Both were initially misdiagnosed as having CASTLE (carcinoma showing a thymus-like element). Additional immunohistochemical stains were performed.

RESULTS

The primary tumors showed jigsaw puzzle-like lobulation resembling thymic epithelial tumor and consisted of spindly cells arranged in fascicles, whorls, and a storiform pattern. The spindly cells had indistinct cell borders, vesicular nuclei, and distinct nucleoli. Perivascular spaces were present. Lymphocytes were sprinkled throughout the tumor in one case but were sparse in the other. The metastatic deposits in the lungs appeared 27 and 2 years, respectively, after the initial presentation and were histologically similar to the original tumors. The FDC nature of the primary and metastatic tumors was confirmed by positive staining with CD21/CD35 cocktail and CD23 and by negative staining for cytokeratin. In one case, in direct continuity with the main tumor, there was a lobulated lesion composed of small lymphocytes punctuated by large cells with vesicular nuclei, histologically reminiscent of thymoma. The large cells were shown by immunohistochemistry to represent FDCs forming complex interconnecting meshworks. It is unclear whether this contiguous mass represents a precursor lesion or an unusual-looking component of the neoplasm.

CONCLUSIONS

FDC sarcoma can look deceptively like a thymic epithelial tumor histologically. A correct diagnosis requires a high index of suspicion and immunohistochemical evaluation. The tumor shows a propensity to metastasize to the lungs, which can be delayed until more than 20 years after initial presentation.