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

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). References: (1) Stewart AF. Clinical practice. Hypercalcemia associated with cancer. N Engl J Med. 2005;352(4):373-9. (2) Hart T, Sinitsky D, Shamsiddinova A, Rohatgi A. Refractory hypercalcaemia secondary to localised gastrointestinal stromal tumour. Ann R Coll Surg Engl. 2018;100(6):e136-e138. (3) Hygum K, Wulff CN, Harsløf T, et al. Hypercalcemia in metastatic GIST caused by systemic elevated calcitriol: a case report and review of the literature. BMC Cancer. 2015;15:788. (4) Barbaryan A, Bailuc S, Poddutoori P, et al. Gastrointestinal Stromal Tumor Induced Hypercalcemia. Case Rep Oncol Med.2017;4972017.