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A 51-year-old man presented with generalised bone pain. Initial evaluation revealed a low serum phosphorus level and elevated urinary phosphorus excretion. Aminoaciduria was normal. Standard imaging showed only minimal changes. The patient was treated with daily oral supplementation with phosphate (1 g/day) and 1,25-dihydroxyvitamin D3 (1 μg/day) to maintain euphosphataemia. In spite of the fact that this treatment was maintained for 3 years, there was no modification of renal phosphate clearance. A diagnosis of hypophosphataemic osteomalacia with renal phosphate wasting was proposed. Therefore, tumour-induced osteomalacia was suspected, triggering a diagnostic workup to find the primary tumour. These tumours are known to express somatostatin receptors, so whole body positron emission tomography (CT) imaging was performed after intravenous administration of 68Ga-DOTA-TOC (68Ga-DOTA-D-Phe1-Tyr3-pentreotide). A solitary intense hot spot was detected in soft tissue near the right femoral internal condyle. Based on this result, curative resection of the tumour was performed.
A 51-year-old man presented with generalised bone pain involving the spine, rib cage and talalgia. Initial evaluation revealed a low serum phosphorus level (1.8 mmol/l; normal range: 2.5–4.5 mmol/l) and elevated urinary phosphorus excretion (2833 mg/day; normal range: 400–1300 mg/day). Aminoaciduria was within the normal range.
The patient was treated with daily oral supplementation with phosphate (1 g/day) and 1,25-dihydroxyvitamin D3 (1 μg/day) to maintain the euphosphataemia. In spite of the fact that this treatment was maintained for 3 years, there was no modification of the renal phosphate clearance. The diagnosis of hypophosphataemic osteomalacia with renal phosphate wasting was proposed. Therefore, a tumour-induced osteomalacia was suspected, triggering a diagnostic workup to find the primary tumour.
Standard radiological imaging (planar radiographs of the hips, spine and pelvis) showed the presence of enthesopathy and minimal changes compatible with osteomalacia. Bone densitometry indicated lumbar osteopenia.
Positron emission tomography coupled with CT (PET-CT) imaging was performed 90 min after the administration of 67 MBq (1.8 mCi) 68Ga-DOTA-TOC (68Ga-DOTA-D-Phe1-Tyr3-pentreotide). Whole body PET-CT including the lower limbs within the field of view was performed. An intense hot spot was detected in the soft tissues near the right femoral internal condyle which coincided with a dense nodule on CT images measuring 20 × 14 × 15 mm (figure 1). PET-guided MRI confirmed this tumour which appeared intensely hypervascularised and contained haemosiderin deposits.
Surgical resection of the tumour was performed.
Histopathological examination showed a proliferation of small round tumour cells with a well-developed capillary network. Tumour cells with ovoid homogeneous nuclei were occasionally observed, but mitotic changes were not seen. The matrix contained smudgy blue-grey material (grungy calcifications), typical of tumour inducing osteomalacia.
Surgical resection of the mass resulted in the normalisation of serum phosphorous and calcitriol concentrations, and in the reversal of phosphaturia.
The generalised bone pain experienced by the patient only partially reduced after surgery, probably because of the long evolution of the osteomalacia until discovery of the causal tumour.
Weidner and Santa Cruz1 described the pathological features of 17 mesenchymal tumours that caused osteomalacia or rickets. The large number of histological diagnostic terms used to classify these tumours reflects their pathological diversity. At the time of the diagnostic workup, it was not possible to use immunohistochemistry to look for fibroblast growth factor 23 since a suitable technique was not available in our institution at that time; this was a limitation in the workup of this patient.
Nevertheless, locating the underlying tumour is essential because complete surgical removal resolves the disordered phosphocalcic metabolism. The majority of these tumours are small and often occur in the extremities (skin, muscles, bones) or around the head (paranasal sinuses). Due to the different possible locations of this tumour, whole body imaging including the lower limbs is required.
The introduction of radiolabelled somatostatin analogues for molecular imaging of neuroendocrine tumours was a major breakthrough in the detection and management of these neoplasms. Gamma camera imaging based on scintigraphy classically includes whole body planar and tomography acquisition (SPECT) performed 6, 24 and often 48 h after the intravenous administration of a tracer dose of radiolabelled somatostatin analogues, most often 111In-DTPA-D-Phe1-octreotide (OctreoScan, Covidien). Some reports confirm the successful visualisation of this type of tumour using this technique.4 5
Recently, the use of 68Ga-labelled DOTA-conjugated peptides for PET has become more common. This imaging technique has been reported in patients with suspected tumour-induced osteomalacia.6 PET is theoretically the preferred molecular imaging technique because it has a sensitivity (to detect molecular processes) 50 times higher than SPECT and 5 × 106 times higher than MRI spectroscopy.7 As regards radioprotection, 68Ga has a favourable half life (68 min) compared to 111In (67 h). Also, the small in-house 68Germanium/68Gallium generator can be used for a long time period, up to 1 year or more at a relatively low cost.8
Competing interests None.
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