Inflammatory myofibroblastic tumor of the psoas muscle presenting as a retroperitoneal mass: a case report

Inflammatory myofibroblastic tumor (IMT) is a rare mesenchymal neoplasm of intermediate biological potential characterized by myofibroblastic spindle-cell proliferation and a variable inflammatory infiltrate. Although IMT may arise in a wide range of anatomical locations, primary involvement of the psoas muscle is exceptionally uncommon and may present significant diagnostic challenges due to its nonspecific clinical and radiological features.

A 29-year-old man presented with progressive left flank and left iliac fossa pain radiating to the back and left lower limb. Contrast-enhanced computed tomography (CT) revealed a 5.8 × 4.7 × 6.6 cm hypervascular intramuscular mass within the left psoas muscle, highly suspicious for a malignant retroperitoneal soft-tissue neoplasm. CT-guided core needle biopsy was inconclusive. Due to persistent symptoms and diagnostic uncertainty, the patient underwent complete surgical excision of the lesion. Histopathological examination demonstrated a spindle-cell proliferation with a prominent inflammatory infiltrate, while immunohistochemistry showed positivity for vimentin and smooth muscle actin with focal weak ALK-1 expression and a low Ki-67 proliferation index, confirming the diagnosis of IMT. Surgical margins were negative. The postoperative course was uneventful, and the patient remained free of recurrence at 5-month follow-up.

Primary IMT of the psoas muscle is extremely rare and may closely mimic malignant retroperitoneal tumors on clinical and radiological evaluation. Preoperative diagnosis is often difficult, and image-guided biopsy may be non-diagnostic. Definitive diagnosis relies on histopathological and immunohistochemical assessment, while complete surgical excision remains the treatment of choice for localized disease.

IMT should be considered in the differential diagnosis of retroperitoneal and intramuscular soft-tissue masses, particularly in young adults. Complete surgical resection provides both definitive diagnosis and effective treatment, with favorable short-term outcomes when negative margins are achieved.