Mucinous tubular and spindle cell carcinoma with papillae: A report of 5 cases and comparison with 18 cases of papillary renal cell carcinoma.

As its descriptive name indicates, mucinous tubular and spindle cell carcinoma (MTSCC) is composed of tubules, spindle cells, and extracellular mucinous stroma. Papillary architecture in MTSCC is regarded as infrequent finding and often is described as papillation or pseudopapillary appearance since bona fide papillary structures with fibrovascular cores are not seen. In this study, we report five cases of MTSCC with papillary formation and compare those with 18 cases of former type 1 papillary renal cell carcinoma (PRCC). Chromosomal microarray analysis was performed to confirm the diagnosis. All 5 MTSCC tumors exhibited at least focal papillary formation. However, the fibrovascular cores were generally mucinous with scant cellularity and vessels. In addition, psammoma bodies were observed in two, and foamy macrophages were seen in four cases of MTSCC. All PRCC cases exhibited classical papillary architecture without bland spindled tumor cells. Interestingly, focal mucinous stroma was observed in 7 PRCC (39 %). Foamy macrophages were identified in 15 (83 %), and psammoma bodies in 5 PRCC cases (28 %). The MTSCC had the typical monosomy of multiple chromosomes. However, the trisomy of 7, 17, and loss of Y typically found in PRCC were not observed in any of the 5 MTSCC. In summary, MTSCC and PRCC share many morphological features, including papillary formation, foamy macrophages, psammoma bodies, and mucinous stroma which should be emphasized. These shared features make distinguishing MTSCC from PRCC difficult in a small core biopsy or fine needle aspiration (FNA) specimen. Chromosomal microarray or FISH can be helpful in problematic cases.