# Tumor-Induced Osteomalacia: A Case Report

## TL;DR

A 55-year-old woman with hip pain and low phosphate levels was diagnosed with tumor-induced osteomalacia, a rare condition caused by a benign tumor, and successfully treated with surgery.

## Contribution

This case report highlights the successful diagnosis and treatment of tumor-induced osteomalacia through a systematic workup and surgical intervention.

## Key findings

- The patient had hypophosphatemia and a stress fracture, leading to further evaluation revealing a phosphate-wasting condition.
- Elevated fibroblast growth factor levels and a PET-CT scan identified a mesenchymal tumor in the tibia.
- Surgical removal of the tumor normalized phosphate levels without additional supplementation.

## Abstract

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome with a variable presentation. We present a case of a 55-year-old female who presented with pain in the bilateral hip region for the last two years. On routine biochemical evaluation, she was found to have hypophosphatemia with an X-ray of the bilateral hip region showing an acute stress fracture in the bilateral intertrochanteric region of the femur. An evaluation for the cause of hypophosphatemia revealed renal phosphate loss with low percentage tubular reabsorption of phosphate (% TRP) of 83% (reference range: 85-95%), with tubular maximum phosphate reabsorption per unit glomerular filtration rate (TmP/GFR) of 2.07 mg/dL (reference range: 2.5-4.5 mg/dL (0.67 mmol/L; range: 0.84-1.23 mmol/L)). Further evaluation revealed elevated levels of intact fibroblast growth factor, 445.7 pg/mL (reference range: 23-95 pg/mL). A 68-Gallium DOTA-1-Nal3-octreotide (DOTANOC) PET-CT revealed a focal increased tracer uptake with a lytic lesion at the lateral metaphyseal aspect of the proximal right tibia, suspicious of somatostatin receptor avid mesenchymal tumor, leading to the diagnosis of TIO. Definitive treatment with complete surgical excision of the tumor was done. Postoperatively, her phosphorus level was within the normal target range even without oral phosphate supplementation. While it is a rare condition, a proper and systemic workup can lead to timely diagnosis and management of this debilitating benign condition.