PET/CT-positive brown tumour - a potentially misleading finding in the evaluation of a patient for malignant primary tumour or metastases

Abstract

Brown tumours are the end stage of primary or secondary hyperparathy-roidism. Clinically, brown tumours most often manifest as slowly growing, painful masses. These tumours can behave aggressively and be destructive. We report a patient with high accumulation of fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) in brown tumours as the potential cause of false-positive results in the evaluation of a patient for malignant primary tumour or metastases. Case report A 49-year-old man came to the hospital with a long-term history of painful right lower leg after trauma. The initial examination was unre-markable. X-ray of the right tibia demonstrated a cystic lesion in the middle third. Magnetic resonance imaging (MRI) demonstrated a T1-hypointense mass lesion with significant enhancement in the subcortical medulla with scalloping of the internal cortical rim. The T2-weighted images demonstrated a hypointense aspect of the centre of the lesion with some hyperintensity around the lesion. In the differential diagnosis chondromyxoid fibroma and osteitis fibrosa cystica were considered (Figs 1 a & b). Tc-99m methylene diphosphate (MDP) bone scintigraphy revealed abnormal uptake in the right tibial diaphysis, the proximal femur bilaterally , the sacrum and the right acetabulum. The multifocality on bone scan raised the suspicion of malignancy. The patient was referred for biopsy of the lesion in the right leg and a whole body positron emission tomography/computed tomography (PET-CT) scan with 18F-FDG was performed. The CT scan revealed multifocal osteolytic lesions in both femora, the right acetabulum, both iliac bones, the right acromioclavicular joint and the left pubic bone (Fig. 2).A nodular structure inferior to the left lobe of the thyroid gland was discovered. The PET scan showed increased FDG uptake in the osteolytic lesions (Fig. 2). There was no increased FDG-uptake in the nodular structure inferior to the left lobe of the thyroid gland. The PET-CT results were in concordance with the pre-assumption of metastatic bone lesions. The biopsy results however revealed no signs of malignancy. In view of the lesion inferior to the left lobe of the thyroid, the patient was subsequently evaluated for hyperparathyroidism. His serum calcium level was 3.21 mmol/l (normal range: 2.10-2.55 mmol/l). His alkaline phosphatase concentration was 241 IU/l (normal range: 40-150 IU/l), and his parathyroid hormone level was 682 pg/ml (normal range: 12-72 pg/ml). Tc-99m methoxy isobutyl isonitrile (MIBI) parathyroid scintigraphy revealed a parathyroid adenoma inferior to the left thyroid lobe (Fig. 3) and ultrasonography showed a 6 × 9 mm nodule of the inferior left parathyroid gland. Parathyroidectomy was undertaken and histopathological examination revealed the presence of a parathyroid adenoma. Postoperatively, plasma parathyroid hormone and calcium levels normalised. Based on these results, the osteolytic FDG avid bone lesions were classified as multiple brown tumours caused by primary hyperparathyroidism. Discussion This case demonstrates that increased FDG uptake in clinically occult brown tumours can be misleading in the evaluation of a patient for malig-nancy. Kuwahara et al. 1 have recently demonstrated a case of increased FDG uptake in brown tumours with primary hyperparathyroidism. Brown tumours of bone or osteoclastomata are highly vascular, lytic bone lesions representing a reparative cellular process rather than a neo-plastic process usually seen in patients with hyperparathyroidism with hypercalcaemia. 2,3 These tumours occur less frequently than in the past, because of earlier detection of hypercalcaemia. 3,4 Fig. 1. Axial T1 (a) and axial postcontrast T1-weighted (b) MR image with fat suppression demonstrates a hypointense mass lesion at the corticomedul-lary junction of the tibial diaphysis. There is scalloping of the internal cortical rim without penetration of the cortex. Significant contrast enhancement in the mass lesion and some reactive enhancement in the adjacent perios-teum and soft tissues is noted.