Retroperitoneal Cystic Malignant Fibrous Histiocytoma Mimicking a Psoas Abscess.

Primary neoplasms in the psoas muscle including schwannoma and soft tissue sarcoma with secondary cystic degeneration are rare entities. They are difficult to distinguish from psoas abscess purely based on radiological findings. Malignant fibrous histiocytoma (MFH) in the retroperitoneum is an uncommon entity in contrast to liposarcoma and leiomyosarcoma. Psoas abscess is a common infection in the retroperitoneum, especially in regions where tuberculosis is endemic. In the current case, the patient presented with gradually progressive lower abdominal pain and raised erythrocyte sedimentation rate (ESR), lymphocyte count and sputum positive for acid fast bacilli. There was a presence of previous history of skeletal tuberculosis. Imaging revealed well-defined multilocular cystic lesion involving the left psoas muscle which along with the clinical scenario suggested psoas abscess. However, post-operative biopsy showed the lesion to be a MFH with extensive cystic degeneration. To the best of our knowledge, cystic MFH mimicking an abscess has been previously reported only once in an oncology literature.