6463 A Case of Tumour-Induced Osteomalacia Which Remitted After Treatment of Metastatic Prostate Cancer

Abstract Disclosure: J. Chua: None. M. Chuah: None. A case Tumour-induced osteomalacia (TIO) which remitted after treatment of metastatic prostate cancer. Dr Chua Jia Min, Dr Matthew Chuah Bingfeng Abstract Case report: A 69-year-old gentleman with a known history of bladder cancer presented to the emergency department with fatigue, lower back pain and bilateral lower limb weakness worsening over the past one month. MRI and bone scan revealed extensive osteoblastic bone metastases across the skeletal system. This gentleman was referred to the Endocrinology service for the evaluation of high PTH. His corrected calcium was normal at 2.17mmol/L (2.09-2.46mmol/L) and iPTH was raised at 8.74pmol/L (1.60-6.90pmol/L). He was noted to have severe hypophosphatemia 0.34mmol/L (0.94-1.50mmol/L). Bicarbonate levels were normal, there was no glycosuria or proteinuria. There was no history of IV ferrous carboxymaltose use. Further investigations were consistent with urinary phosphate loss - TRP was 0.398 and TMP-GFR was 0.199. ALP was elevated at 282ug/L (39-99ug/L). 25-hydroxyvitamin D was low at 12 ug/L, which accounted for the high PTH levels (secondary hyperparathyroidism). FGF-23 levels returned high at 1293 RU/ml (<180 RU/ml). BMD showed osteoporosis with T-score of -2.6 at the left femoral neck. He was subsequently diagnosed with metastatic prostate adenocarcinoma, based on histology from a bone biopsy of an affected vertebrae. PSA was markedly raised at 606ug/L. A diagnosis of TIO secondary to metastatic prostate cancer was made. The patient was started on PO sodium-phosphate solution 4ml TDS (approximately 3mmol/kg/day of elemental phosphorus), calcitriol 0.25mcg BD and vitamin D was replaced. After phosphate was replete, the patient's weakness and fatigue improved. He was subsequently started on androgen deprivation therapy with leuprorelin acetate injection once every 3 months and apalutamide daily. SC denosumab 60mg Q6monthly was also commenced in view of extensive bone metastases. Repeat imaging showed response to treatment with decrease in the size of the prostate and bone lesions. His PSA levels are now undetectable and TMP-GFR has normalised. His oral phosphate and calcitriol doses were gradually reduced and subsequently stopped. His calcium and phosphate levels remain within normal limits on no supplementation, suggesting remission of his TIO. PTH levels normalised after vitamin D repletion. Learning points TIO is a rare paraneoplastic syndrome usually associated with mesenchymal tumours, and treatment involves resection of the causative tumour. Cases of TIO secondary to prostate cancer (an epithelial cancer) are exceedingly rare. Despite metastatic disease, this patient responded exceedingly well to his cancer therapy. Suppression of the tumour cells with androgen deprivation therapy resulted in the lack of phosphotonin production and remission of his TIO. Presentation: 6/1/2024