Psoas metastasis mimicking abscess in untreated muscle-invasive bladder cancer: A diagnostic pitfall.

Psoas involvement represents a rare clinical entity presenting significant diagnostic challenges due to nonspecific symptomatology. We report a 62-year-old male patient with muscle-invasive bladder carcinoma treated by transurethral resection of bladder tumor (TURBT) who declined radical cystectomy. Six months following initial surgical intervention, he presented with back pain, fever, and progressive clinical deterioration. Laboratory investigations demonstrated marked systemic inflammation. Computed tomography revealed recurrent bladder tumor with a 12-cm left psoas collection, initially interpreted as representing an abscess. Despite percutaneous drainage and broad-spectrum antibiotic therapy, clinical improvement remained limited. Magnetic resonance imaging demonstrated an infiltrative lesion within the psoas musculature, separated from the bladder by a preserved fat plane, consistent with skeletal muscle metastasis rather than direct tumor invasion or infectious abscess. This case illustrates the diagnostic difficulty in distinguishing abscess from malignant infiltration in advanced bladder cancer and emphasizes the critical role of magnetic resonance imaging when conventional treatment modalities fail to achieve expected clinical response.