MON-508 Misdiagnosed Bone Metastasis: A Rare Case of Polyostotic Bone Lesions

Background The most common cause of diffuse bone lesions in adults is metastatic disease. We present a case of a rare cause of polyostotic bone disease associated with low bone mineral density in a postmenopausal woman. Case description 53-year-old female presented with back pain. She had no significant past medical history besides a chronic rash on her back and chest that was triggered by direct sunlight. She denied flushing, diarrhea or syncope. On exam, she had tenderness in the lumbar spine and dermatographism. Plain radiographs revealed multiple sclerotic lesions in the ribs, lumbar spine and pelvis, and notably a 2.5cm lesion at L5 vertebral body concerning for metastatic disease. Her blood cell counts, liver enzymes, glomerular filtration rate, lactate dehydrogenase, alkaline phosphatase, PTH intact, Phosphorus, Calcium and vitamin D were within normal limits. Serum and urine protein electrophoresis did not show monoclonal gammopathy. Serum tryptase level was elevated at 57 ng/mL (<11ng/mL). Bone marrow biopsy revealed multiple aggregates of atypical spindled mast cells that showed aberrant expression of CD25. KIT point mutation at codon 816 was identified. Systemic Mastocytosis (SM) was diagnosed. This was considered indolent variant due to lack of organopathy or dysmyelopoiesis. Initiation of histamine receptor blockers led to improvement of symptoms. Initial bone density scan interpretation was challenging due to mastocyte infiltration in regions of interest. The only regions not affected were L1 and the left femoral neck. Osteoporosis was diagnosed due to a T-score under -2.5 at these sites. The patient was less than one year post-menopause and likely at the peak of her bone loss. She was started on alendronate. Discussion The skeleton is one of the most frequent sites of involvement in SM. Patients can present with diffuse bone pain or pathologic fractures leading to imaging studies that reveal both osteolytic and osteosclerotic lesions. Widespread metastatic disease may be initially misdiagnosed, such as in our patient, leading to medical errors. SM can be identified by an elevated tryptase level and confirmed with bone marrow biopsy. Mast cells can exert a direct stimulatory effect on osteoblast and osteoclasts. The predominance of either process would direct bone involvement towards bone sclerosis or bone loss. There is a high prevalence of fractures and osteoporosis in SM. Available studies identify osteoporosis prevalence around 18-37%. Whether patients with focal or diffuse osteosclerosis have more or less osteoporosis or risk of fractures has not been determined due to absence of large longitudinal studies. DXA diagnosis of osteoporosis may be inaccurate due to patchy distribution of mastocyte cell infiltrates or presence of fractures. Small studies on the effect of bisphosphonates have been published with evidence of greater BMD gain with zoledronic acid.