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Mastocytosis is a disorder characterized by abnormal mast cell proliferation, leading to cutaneous mastocytosis (CM) or systemic mastocytosis (SM) when internal organs are affected. While adult mastocytosis is associated with osteoporosis, data on bone mineral density (BMD) in children are lacking. To our knowledge, this is one of the first studies investigating BMD in pediatric mastocytosis. We conducted a prospective cross-sectional study including clinical and laboratory data from a database (2004-2025), covering mastocystosis subtype, serum tryptase, and c-kit mutation status. BMD was measured in children aged 4-18 years using dual-energy X-ray absorptiometry (DEXA) at the lumbar spine and total body. 76 children were included (73 CM, 3 SM [ISM]). Low BMD (Z-score ≤ -2) was observed in 6.8% of CM patients, significantly more prevalent than in the general pediatric population (p = 0.0014). However, all normalized spontaneously during follow-up. None had osteoporosis. ISM patients had significantly higher tryptase levels but normal BMD. BMD was not correlated with disease specific characteristics, bone pain or fractures.

Our findings suggest that bone involvement is not a major concern in pediatric mastocytosis. DEXA scans could potentially be reserved for children with suspected systemic involvement. Future longitudinal studies are necessary to further elucidate bone involvement in these patients.

• Mastocytosis is a rare disorder, associated with osteoporosis and fractures in adults. Data on bone density in the pediatric population are lacking.

• Our results suggest that bone involvement is not a major concern in pediatric mastocytosis.