Tumour-Induced Osteomalacia in Metastatic Prostate Carcinoma: The Rest of the Story

Dear Editor,

Recently, the authors reported the occurrence of a rare case of oncogenic osteomalacia in a 61-year-old man with metastatic carcinoma of the prostate in the Indian Journal of Endocrinology and Metabolism. The patient had presented with severe low back pain, proximal myopathy, and difficulty in ambulation. Investigations revealed a hypophosphatemic osteomalacia with a low fasting phosphate of 1.4 and 2.4 mmol/L (repeated) and an elevated FGF-23 of 587 RU/mL. He was initiated on neutral phosphate and calcitriol for relief of symptoms of osteomalacia. He had received radiotherapy to the lumbar vertebrae and pelvis, followed by bilateral orchidectomy, and was initiated on abiraterone with prednisolone.[1] We now wish to inform our readers of his status in the last review.

He followed up after a period of 11 months. He was being continued on abiraterone and prednisolone. At presentation, he reported significant improvement in his symptoms. On clinical examination, there was no proximal myopathy. He was ambulant without support. There were no fragility fractures. Given the resolution of symptoms of osteomalacia, he had stopped the neutral phosphate and calcitriol supplements. His investigations revealed normal phosphate (3.8 mg/dL) and a declining trend of PSA. His serial alkaline phosphatase declined from 551 U/L to 345 U/L and normalized to 80 U/L. Bone turnover markers showed a serial decline, and FGF 23 levels normalized to 89 RU/mL from a baseline value of 587 RU/mL (N: 21.6-91.0 RU/mL). A review with the oncologist was done, and it was planned to start him on docetaxel with continuation of abiraterone and prednisolone. He had received two doses of Zoledronic acid one year back. His clinical features and investigations were suggestive of the resolution of osteomalacia with control of underlying prostatic malignancy.

Tumor-induced osteomalacia (TIO) or oncogenic osteomalacia is a rare paraneoplastic condition resulting in hypophosphatemia secondary to elevated fibroblast growth factor (FGF 23), which is a phosphatonin.[2] The reports of hypophosphatemic osteomalacia associated with carcinoma prostate, an epithelial malignancy, have been limited as TIO is predominantly associated with mesenchymal tumors, and epithelial tumors have accounted for less than 5%. TIO was first described in prostatic malignancy in 1975.[3] In a meta-analysis of 34 patients with malignant TIO, adenocarcinoma of the prostate was the most frequently reported tumour (9 out of 34).[4] Khadilkar et al.[5] described a similar 63-year-old male with castrate resistant prostate carcinoma and tumor-induced osteomalacia who responded to chemotherapy, abiraterone, prednisolone, along with phosphate and vitamin D replacement.[5]

TIO-related bone pain and weakness can be mistaken for that associated with bony metastases.[3] In the later stages of malignancy, there might be refractoriness to conventional therapy and a favorable response to anti-tumor therapy. Early identification and management of the condition to prevent further complications due to osteomalacia, namely fractures, from contributing to the morbidity burden. In our patients, TIO was a paraneoplastic manifestation of metastatic carcinoma, which was diagnosed earlier owing to the characteristic symptom complex. The management of the neoplasm has resulted in biochemical and clinical improvement.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.