Dual Tracer Imaging Resolving a Clinical Dilemma in Tumour-Induced Osteomalacia

The patient presented initially with bony pains, low backache, and difficulty in walking for 10 years to the endocrinology department of our institution in 2017 and was found to have severe hypophosphatemia and elevated levels of fibroblast growth factor 23 (FGF23). Patient subsequently underwent ⁶⁸Ga-DOTANOC and 18F-Fluorodeoxyglucose positron emission tomography-computed tomography (¹⁸F-FDG PET/CT) scans for localisation of the culprit lesion, which were negative at the time. The patient was lost to follow-up and presented with recurrent symptoms after 7 years. Biochemical parameters were suggestive of hypophosphatemia (0.9 mg/dl) and elevated levels of FGF23 (479 pg/ml) with normal calcium, Vitamin D, and iPTH levels. The patient again underwent a ⁶⁸Ga-DOTANOC PET/CT scan for the detection of the culprit lesion and also reported persistent pain at the site of a previous root canal treatment in the lower jaw. ⁶⁸Ga-DOTANOC and ¹⁸F-FDG PET/CT scans were done. ⁶⁸Ga-DOTANOC scan demonstrated a DOTANOC avid (SUVmax 4.7) lytic-expansile lesion in relation to the right canine/premolar in the lower mandibular alveolus [Fig 1]. The patient had recently undergone dental intervention at the same site, which posed a diagnostic dilemma; therefore, an ¹⁸F-FDG PET/CT scan was done to rule out infection/inflammatory aetiology. ¹⁸F-FDG PET/CT scan showed no abnormal FDG uptake (SUVmax 2) at the same site; hence, given the DOTANOC avidity in the absence of FDG uptake, and consistent with the clinical and biochemical profile, the mandibular lesion was identified as the culprit tumour responsible for the paraneoplastic syndrome. The patient underwent a wide local excision of the lesion, and histopathology confirmed the diagnosis of a phosphaturic mesenchymal tumour. Postoperatively, the patient showed biochemical improvement in serum phosphate and FGF23 levels. TIO is a rare paraneoplastic syndrome typically associated with phosphaturic mesenchymal tumours that secrete FGF23, leading to renal phosphate wasting and impaired bone mineralisation.^[1,2] TIO is a debilitating condition with surgery being the curative treatment; however, localisation of the culprit lesion is a diagnostic challenge due to small and slow-growing tumours.^[3,4] Functional imaging is necessary for making the definitive diagnosis, with ⁶⁸GaDOTANOC outperforming ¹⁸F-FDG PET/CT scans for the detection of the culprit lesion.^[5] This case underscores the utility of combining targeted imaging modalities, especially with a diagnostic challenge, as a powerful synergistic tool in the detection of the culprit lesion.

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Fig 1: Patient presented with long-standing recurrent symptoms of tumour-induced osteomalacia for which positron emission tomography-computed tomography (PET/CT) studies were done. 68Ga-DOTANOC PET/CT images (A) MIP, (B) fused axial PET/CT and (C) axial computed tomography (CT) revealed a DOTANOC-avid lytic-expansile lesion in relation to the right canine/premolar in the lower mandibular alveolus (blue arrow in B). (D) 18F Fluorodeoxyglucose (18F FDG) PET/CT images (fused axial PET/CT) and (E) axial CT showed no abnormal FDG uptake at the corresponding site, favouring a noninflammatory aetiology (green arrow in D)

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