IgG4-related disease of the pelvic floor musculature mimicking neoplasm: a case report with imaging-pathologic correlation.

IgG4-related disease (IgG4-RD) is an immune-mediated fibroinflammatory condition that can form tumefactive mass-like lesions and mimic malignancy across organ systems. We describe a rare presentation involving the deep pelvic floor musculature with imaging-pathologic correlation while underscoring diagnostic pitfalls. A 49-year-old man with a history of systemic lupus erythematosus, night sweats, and left lower quadrant abdominal pain underwent CT of the abdomen and pelvis, which demonstrated a right posterior pelvic sidewall mass and retroperitoneal lymphadenopathy. Pelvic MRI demonstrated an ill-defined intramuscular mass centered in the right iliococcygeus and pubococcygeus muscles. Relative to skeletal muscle, the lesion demonstrated iso- to low T1 signal intensity, intermediate T2 signal intensity, restricted diffusion, and moderate, diffuse contrast enhancement, raising concern for neoplasm or an inflammatory process. Image-guided biopsy revealed fibrotic soft tissue with severe lymphoplasmacytic infiltrate and increased IgG4-reactive plasma cells. The IgG4+/IgG+ plasma cell ratio was > 75%, consistent with IgG4-RD when integrated with the clinical, laboratory, imaging and pathologic findings. IgG4-related disease with mass-like involvement of the pelvic floor musculature has not, to the best of our knowledge, been previously reported in the English literature. This entity can present as an ill-defined, enhancing, diffusion-restricting intramuscular mass and therefore may mimic a neoplastic process.