Introduction: Primary myofibroblastic sarcoma of the bone is a rare spindle cell tumour with, to the best of our knowledge, only eight cases reported in the available English language literature. The disease's rarity and its low-grade features make an accurate diagnosis difficult in most cases. The differential diagnoses of this unusual tumour include various benign entities as well as other sarcomas. Due to the difference in prognosis, a precise pathologic diagnosis is essential, which requires a combination of thorough morphologic examination, immunohistochemistry and electron microscopy wherever available.
Case presentation: We report the case of a 38-year-old Indian woman with a lytic lesion in her left femur. The tumour was associated with cortical destruction and soft tissue extension. A biopsy from the soft tissue component showed features suggestive of a low-grade malignant mesenchymal tumour. Excision of the tumour was performed and histopathological examination showed a low-grade spindle cell sarcoma with collagenous stroma. Expressions of vimentin and smooth muscle actin were also noted. Ultrastructural examination confirmed its myofibroblastic nature. A final diagnosis of low-grade myofibroblastic sarcoma of the left femur was thus rendered.
Conclusion: Low-grade myofibroblastic sarcoma is one of the rarer osseous spindle cell sarcomas depicting a favourable prognosis in the cases reported so far. Its diagnosis requires ancillary techniques like immunohistochemistry and electron microscopy. To the best of our knowledge, we report the ninth case in the literature and the first case from our subcontinent.