MON-LB69 Calcitriol-Mediated Hypercalcemia in a Patient With Metastatic Gastrointestinal Stromal Tumor

Abstract

Background: Hypercalcemia is a common complication of advanced malignancy, affecting up to 30% of cancer patients through various mechanisms (1). Hypercalcemia has rarely been described in gastrointestinal stromal tumors (GIST), with fewer than ten case reports as of 2018 (1,2). We describe a case of calcitriol-mediated hypercalcemia in a patient with GIST. Clinical Case: An 80-year-old woman with a history of metastatic GIST and nivolumab-induced type 1 diabetes and thyroiditis presented with dramatic progression of metastatic peritoneal disease and new severe hypercalcemia with acute kidney injury. On hospital admission, calcium (Ca) was 15.1 mg/dL (8.6-10.3 mg/dL), ionized Ca was 1.98 mmol/L (1.09-1.29 mmol/L), and creatinine was 2.56 mg/dL (0.6-1.3 mg/dL, baseline 1.8 mg/dL). She was treated with IV fluids and 45 mg of IV pamidronate with initial Ca improvement to 10.7 mg/dL over the next 48 hours. Additional workup showed that 25-hydroxyvitamin D was 18 ng/dL (20-50 ng/dL), PTH was 9 pg/mL (11-51 pg/mL), PTHrP was 3.1 pmol/L (0.0-3.4 pmol/L), and calcitriol was elevated to 172 pg/mL (19.9-79.3 pg/mL). Prior chest/abdomen/pelvis CT scans had not shown bony metastases or granulomas. After stopping IV fluids, Ca rose to 12.2 mg/dL the next day. Prednisone 20 mg daily was started which stabilized Ca levels and lowered calcitriol to 17.4 pg/mL after two weeks. She also began a new regimen of cabozantinib. Prednisone was tapered to 10 mg daily and she continues to maintain normal Ca levels with the addition of home health IV fluids three times a week. Conclusion: GIST tumors are a rare cause of hypercalcemia of malignancy. Although hypercalcemia of malignancy is most often due to tumor-secreted PTHrP or bony metastases, a small percentage of cases are mediated by excess calcitriol production. There is a growing number of case reports, including this case, to suggest that calcitriol-mediated hypercalcemia is the most common cause of hypercalcemia in GIST tumors (2-4). Glucocorticoids may be used to decrease calcitriol production and help maintain eucalcemia. Definitive therapy for hypercalcemia in these patients involves decreasing tumor burden by treatment of the underlying malignancy (3).