An Increased Bone Mineral Density As an Adverse Prognostic Factor in Patients with Systemic Mastocytosis

Systemic mastocytosis (SM) is characterized by expansion of clonal mast cells that infiltrate various organ systems. The extent of organ infiltration and subsequent organ damage distinguishes between indolent SM (ISM) and advanced SM (AdvSM). ISM patients usually present with a low mast cell burden and have a nearly normal life expectancy while AdvSM patients have a high disease burden, multiple organ damage and poor prognosis. In ISM patients, measurement of the bone mineral density (BMD) frequently reveals osteopenia and osteoporosis (lumbar spine BMD T-score of ≤ −2.5 standard deviation [SD]). In contrast, the association between increased BMD and osteosclerosis, respectively, and the various SM subtypes is unclear. We therefore sought to evaluate the BMD in 61 patients (ISM, n = 29, 48%; AdvSM, n = 32, 52%) and correlated the prevalence of osteoporosis, increased BMD and osteosclerosis with clinical parameters and prognosis. All patients were scanned on the same 16 row CT Scanner (SOMATOM Emotion 16, Siemens Healthcare Sector, Forchheim, Germany). The results were expressed as T-score (SD below the mean of young healthy adults) and as Z-score (SD below the age- and gender-matched mean reference value). According to established WHO criteria, osteoporosis was defined as a lumbar spine BMD T-score of ≤ −2.5 SD. Increased BMD and osteosclerosis were defined as Z-score > 1 SD and > 2 SD, respectively. Osteoporosis was detected in 11/29 (38%) patients with ISM and 2/32 (6%) patients with AdvSM (p = 0.004). Bone marrow mast cell infiltration (median 10% versus 20%, p=0.035) and serum tryptase levels (median 31 µg/L versus 58 µg/L, p = 0.047) were significantly lower in ISM patients with osteoporosis as compared to those without osteoporosis (n=18, 62%). No significant differences were seen regarding age, gender, blood counts, and overall survival. An elevated BMD was detected in 1/29 (3%) patient with ISM and 24/32 (75%) patients with AdvSM (p < 0.001) while osteosclerosis was only detected in AdvSM patients (16/32, 50%). AdvSM patients with increased BMD were older (median 77 years versus 68 years, p = 0.0043), had lower platelet counts (median 111 x 10⁹/L versus 238 x 10⁹/L, p = 0.041) and higher levels of bone marrow mast cell infiltration (50% versus 10%, p=0.002), serum tryptase (median 262 µg/L versus 62 µg/L, p = 0.003) and alkaline phosphatase (238 U/L versus 74 U/L, p < 0.0001). In consequence, prognosis of AdvSM patients with increased BMD was significantly inferior as compared to those without increased BMD (median overall survival 3.6 years versus not reached, p = 0.031). In conclusion, i) osteoporosis is a common feature in ISM but not in AdvSM patients, ii) an increased BMD is frequent in AdvSM but not in ISM patients, iii) osteosclerosis is restricted to AdvSM patients, and iv) an elevated BMD is associated with a more aggressive phenotype and inferior survival (Figure A-B).

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