SUN-784 - Normophosphatemic Tumoral Calcinosis Associated with Osteoporosis: Evidence of Dysregulated Mineral Metabolism Beyond Serum Phosphate

Background
Tumoral calcinosis is a rare disorder characterized by periarticular calcium-phosphate deposition and is classically associated with hyperphosphatemia due to impaired Fibroblast Growth Factor-23 (FGF23) signaling. Normophosphatemic tumoral calcinosis (NTC) is a less recognized entity defined by normal serum calcium, phosphate, and preserved renal function. Emerging evidence suggests a potential familial etiology involving mutations in the sterile alpha motif domain-containing protein 9 (SAMD9), though systemic skeletal effects remain poorly understood. The relationship between tumoral calcinosis and bone mineral density has not been well described. We report a case of NTC associated with osteoporosis, suggesting a broader disorder of mineral metabolism.

Case Presentation
A 59-year-old female presented with a progressively enlarging mass in the left gluteal region. She had no history of chronic kidney disease, inflammatory arthritis, malignancy or fractures. Radiographic imaging demonstrated multilobulated periarticular calcified masses consistent with tumoral calcinosis. Laboratory evaluation revealed normal serum calcium, phosphate, thyroid function, parathyroid hormone, alkaline phosphatase, and renal function. Hematocrit and 25-hydroxyvitamin D levels have been historically normal. Menopause occurred at age 50; she has not habitually used tobacco or alcohol. FGF23 level was 35 pg/mL (reference ≤59 pg/mL). A 24-hour urine collection showed calcium excretion of 58 mg/day and phosphorus excretion of 840 mg/day with a total volume of 2.4 L. The mass was surgically excised, and pathology demonstrated an 18.5 × 17.0 × 4.7 cm specimen with diffuse calcifications and foreign body giant cell reaction, confirming tumoral calcinosis.
Dual-energy X-ray absorptiometry showed severe osteoporosis (lumbar spine T-score −4.1, left total hip −2.7, right total hip −1.8). Genetic testing for SAMD9 was ordered.

Conclusion
This case highlights a possible association between NTC and osteoporosis, an association not previously identified in our literature review. Potential mechanisms include altered local phosphate transport, impaired skeletal mineralization, or preferential ectopic calcium-phosphate deposition. Optimal management of osteoporosis in this setting remains unclear; however, given the severity of disease in this case, treatment with romosozumab was initiated. We further hypothesize that surgical resection of the lesion may improve overall mineral regulation and increase the likelihood of bone density improvement. This observation underscores the importance of routine bone health assessment in patients with tumoral calcinosis and warrants further investigation into the skeletal effects of surgical intervention in this rare disorder.

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