Intravascular Fasciitis Mimicking Deep Vein Thrombosis with Pulmonary Embolism in May-Thurner Syndrome: First Reported Case with Literature Review.

Intravascular fasciitis (IVF) is a rare, benign myofibroblastic proliferation involving arteries or veins that can mimic deep vein thrombosis (DVT). We report the first known case of IVF arising in the setting of May-Thurner syndrome (MTS), a congenital venous anomaly. A 15-year-old male presented with leg pain, swelling, and pulmonary embolism. Imaging showed iliofemoral "thrombosis" and MTS. Mechanical thrombectomy was performed. Histopathology revealed a spindle cell lesion with myxoid stroma, mitoses, focal necrosis, and organizing thrombi. Immunohistochemistry demonstrated smooth muscle actin positivity and desmin negativity. USP6 rearrangement was detected by fluorescence in situ hybridization (FISH), confirming IVF. This case illustrates the clinical and radiologic overlap between IVF and DVT and highlights the importance of histopathologic and molecular evaluation. Unlike prior reports, IVF was successfully managed with an endovascular approach.