Metatarsal leiomyosarcoma masquerading as acute osteomyelitis – A diagnostic trap unveiled by vigilant clinical, radiologic and pathologic analysis

• The present article represents the first reported case of metatarsal leiomyosarcoma.
• The present and the only other reported case of foot leiomyosarcoma show immunohistochemical staining pattern consistent with vascular smooth muscle derivation.
• Application of h-caldesmon and calponin staining in bone biopsy specimen is helpful in the differentiation of smooth muscle tumor from acute or chronic osteomyelitis with reactive fibroblastic proliferation.

Due to overlapping clinical and radiological features, the differentiation between osteomyelitis and bone tumor can be challenging. A 48-year-old lady presented with intermittent left foot pain for a few months. Plain radiographs showed an osteolytic lesion affecting the proximal diaphysis of the left fourth metatarsal bone, with thinning and irregularities of the cortex and focal periosteal reaction. Ultrasonography revealed diffuse subcutaneous edema in the dorsum of the left foot, cortical irregularities along the mid-shaft of the left fourth metatarsal bone, and surrounding periosteal collection. Computed tomography showed medullary expansion along the shaft and base of the left fourth metatarsal bone with cortical irregularities and defects suggestive of cloaca, and focal mild periosteal new bone formation. The clinico-radiologic diagnosis was acute osteomyelitis with periosteal collection. During open biopsy, the finding of intramedullary fleshy tissue in the absence of significant inflammatory edema and purulent discharge, and subsequent negative culture result prompted a review of the histologic slides which was initially reported as benign fibroblastic tissue proliferation. Careful analysis of the histomorphology disclosed a spindle cell sarcoma for which ray amputation of the fourth and fifth metatarsal was performed. The final diagnosis was grade 1 leiomyosarcoma and the patient remained well 33 months after the operation.