Primary desmoplastic cutaneous leiomyosarcoma associated with high MIB-1 labeling index: a teaching case giving rise to diagnostic difficulties on a small biopsy specimen.

A case of primary desmoplastic cutaneous leiomyosarcoma is reported. A flat and elevated tan plaque, measuring 30 mm × 20 mm, was noticed in the left back of a 74-year-old Japanese male 6 months before the resection. The biopsy specimen showed an overgrowth of desmoplastic fibrocollagenous stroma, focally admixed with a less cellular proliferation of spindle cells having mildly pleomorphic nuclei, but no mitotic figures, arranged in small clusters or appearing as individual cells. Based on these features, we interpreted it as a benign keloid-like lesion. A local resection was done, and gross examination revealed a poorly demarcated grayish tumor lesion, replacing the entire dermis and extending into the subcutis. Microscopic findings demonstrated a sparsely cellular proliferation of atypical spindle cells having cigar-shaped or multi-nucleated pleomorphic nuclei and abundant eosinophilic cytoplasm with few mitotic hot spots, arranged in interlacing bundles, alternating with scattered tumor cells within an abundant desmoplastic stroma. Immunohistochemically, these atypical cells were positive for α-smooth muscle actin, HHF-35, desmin, and caldesmon, and MIB-1 labeling index was greater than 10%. Therefore, we finally made a diagnosis of desmoplastic leiomyosarcoma as a very rare variant of cutaneous leiomyosarcoma. We should be aware that owing to its characteristic features, pathologists might misinterpret it as benign when examining only small or inadequate specimens. It is thus suggested that a large panel of antibodies including smooth muscle cell markers and MIB-1 in immunohistochemistry are useful and adjunctive diagnostic aids for recognizing malignancy, especially in diagnostically difficult cases such as ours.