Aggressive Systemic Mastocytosis Presenting as Diffuse Osteoblastic Bone Disease: A Diagnostic Pitfall Mimicking Metastatic Cancer.

Background: Systemic mastocytosis is a rare clonal disorder characterised by the proliferation and accumulation of mast cells in one or more extracutaneous organs. Its clinical presentation is highly heterogeneous and aggressive variants may mimic metastatic malignancy, leading to significant diagnostic delay. Case description: We report the case of an 83-year-old woman presenting with fatigue, weight loss, low-grade evening fever and night sweats over two months. Laboratory evaluation showed pancytopenia, mild inflammatory response, and normal hepatic and renal function. Imaging studies revealed diffuse osteoblastic lesions throughout the axial skeleton, initially interpreted as bone metastases from an unknown primary tumour. Further imaging demonstrated hepatosplenomegaly and prominent periportal lymph nodes. Bone biopsy of an iliac lesion revealed replacement of marrow by sheets of epithelioid mast cells (CD117+, tryptase, CD68+). Serum tryptase was elevated (56.3 ng/ml), fulfilling World Health Organization (WHO) diagnostic criteria for systemic mastocytosis. The patient was diagnosed with aggressive systemic mastocytosis and started cladribine therapy with antimicrobial prophylaxis, with progressive improvement of constitutional symptoms and she remains under follow-up in the haematology department. Discussion: Aggressive systemic mastocytosis presenting with diffuse osteoblastic bone lesions is extremely uncommon and may closely mimic metastatic carcinoma, particularly of breast or prostate origin. Diagnosis requires a high index of suspicion and confirmation through histopathology, immunophenotyping and serum tryptase measurement. KIT mutations, especially D816V, are frequent but were absent in this case. Conclusion: This case highlights the importance of considering systemic mastocytosis in the differential diagnosis of osteoblastic bone disease, particularly in elderly patients with constitutional symptoms and unexplained cytopenias. Early recognition allows timely initiation of appropriate therapy and avoids unnecessary oncology investigations. LEARNING POINTS: Aggressive mastocytosis can rarely present with diffuse osteoblastic bone lesions mimicking metastatic disease.Elevated serum tryptase and histopathological confirmation are essential for diagnosis, even when KIT mutations are absent.Early recognition prevents unnecessary oncologic investigations and allows timely initiation of cytoreductive therapy.