Renal Peripelvic Extramedullary Haematopoiesis in Myelofibrosis: A Rare Imaging Pitfall Assessed by Multimodality Nuclear Imaging.

Extramedullary haematopoiesis (EMH) refers to haematopoietic proliferation outside the bone marrow, most often arising as a compensatory response to ineffective marrow function in chronic anaemias and myeloid neoplasms, particularly myelofibrosis and other myeloproliferative neoplasms. While the liver and spleen are typical sites, renal involvement remains particularly uncommon and may mimic infiltrative malignancy or infection on cross-sectional imaging. We report a 35-year-old woman with biopsy-proven grade 2 myelofibrosis who presented with constitutional symptoms, namely asthenia, progressive weight loss, and intermittent fever, in the setting of pancytopenia. Contrast-enhanced CT demonstrated bilateral thoracic paravertebral and presacral soft-tissue masses, with left peripelvic/pelvicalyceal infiltration, raising concern for infiltrative malignancy or infection. [18F]-FDG-PET/CT showed low-grade uptake in the paravertebral and presacral lesions, while the renal lesion remained indeterminate because of adjacent urinary tracer activity. Given the haemorrhagic risk of renal biopsy in a cytopenic patient, [99mTc]-sulphur colloid scintigraphy with SPECT/CT was performed and demonstrated concordant tracer uptake in all lesions, supporting multifocal EMH. After disease-directed treatment, follow-up CT at 12 months showed marked regression of the renal and other EMH lesions. This case highlights renal peripelvic EMH as a rare imaging pitfall and underscores the value of multimodality imaging when biopsy is high risk.