Extraskeletal osteosarcoma with partial spontaneous regression.

Resection for a mass in the proximal thigh was performed on a 57-year-old woman, the diagnosis of which was extraskeletal osteosarcoma. In pathological findings, tumor cells gradually decreased from the central area of the mass composed of spindle cell sarcoma and were replaced by fibrocollagenous tissue with no sarcoma cells. CD8(+), T-cell-restricted intracellular antigen-1 (TIA-1)(+), granzyme B(+) T lymphocytes appeared to infiltrate the mass lesion, so that we hypothesize that the immunological system was likely to be involved via T lymphocytes in triggering spontaneous regression in this case. One of the most unusual phenomena in cancer biology is the spontaneous regression of a tumor. Here, we report on the first case of extraskeletal osteosarcoma characterized by partial spontaneous regression of the primary lesion.