Keywords
FDG PET/CT scan - oncogenic osteomalacia - tumor-induced osteomalacia
Introduction
Oncogenic osteomalacia is a paraneoplastic syndrome which involves hypophosphatemia,
phosphaturia, and osteomalacia. It is commonly associated with benign connective tissue
mesenchymal tumors and features mimic that of X-linked or autosomal dominant hypophosphatemia
rickets. Tumor resection is curable; however, since these lesions are small and can
be located anywhere in the body, they pose great difficulty in detection by routine
anatomic imaging. Since most of these tumors demonstrate fluorodeoxyglucose (FDG)
avidity, whole-body FDG positron emission tomography-computed tomography (PET-CT)
scan can be useful in diagnosis, as seen in our report.
Case Report
A 47-year-old woman presented with generalized weakness and pain in multiple joints
for 2 years. She was referred for99m Tc-MDP bone scan [Figure 1], which was showed increased tracer uptake in multiple joints, likely favoring metabolic
bone disease/osteomalacia. Her laboratory tests revealed normal calcium; however,
she had hypophosphatemia (1.2 mg/dl).
Figure 1 Bone scan image of patient suggestive of increased tracer uptake in multiple joints,
likely favoring metabolic bone disease/osteomalacia
Subsequently, on clinical suspicion of occult malignancy, she was referred for whole-body
18F-FDG PET-CT scan. The maximum intensity projection image [Figure 2c] showed fairly physiological distribution with faint increased uptake in the right
proximal thigh. The axial image and coronal images [Figure 2a] and [Figure 2b] revealed low-grade FDG uptake in soft tissue nodular lesion in the proximal right
thigh, just lateral to femoral vessels in anteromedial aspect of infratrochantric
region. Biopsy of the lesion confirmed diagnosis of mesenchymal tumor, which was then
surgically excised. Thereafter, the patient showed dramatic improvement within a month.
Figure 2 Fluorodeoxyglucose positron emission tomography-computed tomography scan images of
patient, axial (a) and coronal images (b) showing tracer uptake in revealed low-grade
fluorodeoxyglucose uptake in soft tissue nodular lesion in proximal right thigh, just
lateral to femoral vessels and maximum intensity projection (c) image of the scan
Discussion
Oncogenic osteomalacia involves abnormal phosphate metabolism due to phosphaturic
hormone fibroblast growth factor 23.[1] Biochemical findings are of hypophosphatemia due to renal phosphate wasting. Patients
usually present with debilitated state, generalized weakness, body pain, and recurrent
fractures. They are mostly associated with benign mixed connective tissue mesenchymal
tumor and surgical excision is treatment of choice. Since they are small and varied
in location, conventional imaging modalities such as CT scan and magnetic resonance
imaging are often less useful in detecting the occult lesion.[2] The various locations reported includes middle cranial fossa,[3] skull bone, and tibia.[4] Increased somatostatin expression has been seen in these tumors, and few reports
of indium-111 octreotide scan for localization have been published,[5] and now, gallium-68 has also been used for lesion localization;[6] however, in occasional case reports, somatostatin expression was not seen.[7]
Due to generalized symptoms, small tumors, varied locations, and slow growth rate,
anatomical imaging is not much helpful and there is significant delay between onset
of symptoms and diagnosis. In literature, cases have been reported for detecting occult
mesenchymal lesion using FDG PET-CT scan.[8] As seen in our case, there was a delay in diagnosis as initial investigations failed
to locate the lesions. Whole-body FDG PET-CT scan was useful in diagnosis, thus leading
to appropriate treatment. Since mesenchymal tumors are low-grade FDG avid and whole-body
FDG PET scan can evaluate entire body (including lower limbs, as these tumors are
usually in extremities), this investigation is helpful to localize the occult lesion,
thus helping in management. Thus, early use of FDG PET-CT scan in high clinical suspicion
of oncogenic osteomalacia would be worthwhile to arrive at diagnosis earlier and help
in excision of occult lesion.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms.
In the form the patient(s) has/have given his/her/their consent for his/her/their
images and other clinical information to be reported in the journal. The patients
understand that their names and initials will not be published and due efforts will
be made to conceal their identity, but anonymity cannot be guaranteed.
