SAT-500 Osteoblastic Metastasis in Prostate Cancer: A Sink Trap for Calcium in a Patient with Concomitant Secondary Hyperparathyroidism and Metastatic Prostate Cancer

Introduction Hypocalcemia in chronic kidney disease with secondary hyperparathyroidism is usually mild and seldom requires aggressive and protracted replacement. We report a case of refractory hypocalcemia in a patient with chronic kidney disease with incidentally detected bone metastatic prostate cancer. Clinical case A 62-year-old male presented with acute worsening of his mild chronic kidney disease. A CT scan done to evaluate this worsening of kidney function revealed incidental metastatic prostate cancer. Tc-99m whole body bone scan showed extensive bony metastases. Serum uncorrected calcium was 1.57 (2.09 - 2.46) mmol/L, albumin was 32 (40 - 51) g/L. Serum phosphate was 3.16 (0.94 - 1.50) mmol/L and iPTH was 38.1 (0.9 - 6.2) pmol/L. He was treated with intravenous calcium gluconate and was discharged on oral calcium acetate. He was subsequently commenced on prednisolone, anti-androgen agents and gonadotropin releasing hormone (GnRH) agonist for the cancer. He was readmitted seven months later for acute urinary retention with worsening of bilateral hydroureteronephrosis. His serum uncorrected calcium was now 1.49 mmol/L, albumin was 35 g/L, phosphate was 1.25 mmol/L, iPTH was 32.1 pmol/L and ALP was 253 (39 - 99) U/L. His eGFR was 35ml/min. Despite being treated with multiple boluses of intravenous calcium gluconate daily and oral calcium carbonate 5 grams and calcitriol 0.5 micrograms respectively three times daily, his calcium level never normalized. Patient was eventually transferred to hospice care and demised. Conclusion Osteoblastic bone metastases commonly found in patients with prostate cancer is very uncommonly associated with hypocalcemia. The increased osteoblastic activity acts as a sink trap causing increased uptake and utilization of calcium leading to hypocalcemia. This patient had Stage 3 CKD which is usually not associated with severe hypocalcemia. However, normalization of calcium level was not achieved despite high doses of intravenous and oral calcium supplementation. The incidentally detected bone metastatic prostate cancer is believed to have been responsible for his refractory hypocalcemia. The rare possibility of such malignancies with osteoblastic metastasis should be considered in the differential of protracted and severe hypocalcemia.