Purely extradural meningioma presenting as an incidental sacro-iliac joint mass

We report the case of an asymptomatic 32-year-old woman in whom a 38-mm right sacroiliac mass was incidentally discovered on computed tomography. Computed tomography demonstrated a well-defined enhancing lesion centered on the right sacral ala and sacroiliac joint with sclerotic margins, without cortical destruction or presacral extension. Magnetic resonance imaging showed a relatively homogeneous mass, iso- to hypointense on T1-weighted images, hyperintense on short tau inversion recovery sequences, and demonstrating intense homogeneous enhancement after gadolinium administration. Computed tomography-guided biopsy revealed a spindle-cell proliferation with diffuse somatostatin receptor 2A (SSTR2A), epithelial membrane antigen, and progesterone receptor expression, and a low Ki-67 index (1%), consistent with a World Health Organization grade 1 meningioma. One-year imaging follow-up demonstrated strict stability, and conservative management was maintained.