Applied Radiology

CASE SUMMARY

A 37-year-old woman with a history of thyroid carcinoma, status post thyroidectomy and I-131 therapy, presented with multiple pulmonary nodules and mediastinal/hilar lymphadenopathy on a computed tomography (CT) scan of the chest. This patient was currently taking Synthroid (Abbott Laboratories, Abbott Park, IL). Laboratory data were unremarkable and physical examination revealed mild shortness of breath. A whole-body positron emission tomography (PET) scan was performed on a dedicated GE Advance PET scanner (GE Medical Systems, Waukesha, WI) approximately 1 hour after the injection of 14.8 mCi of 18 fluorine-fluorodeoxyglucose (FDG) (Figure 1).

DIAGNOSIS

Sarcoidosis

IMAGING FINDINGS

The whole-body FDG PET scan revealed FDG-avid lymph nodes in the hila and mediastinal regions. The pulmonary nodules noted on the CT scan were not metabolically active. Mediastinoscopy with lymph node biopsy was subsequently performed. Pathologic evaluation revealed findings compatible with sarcoidosis.

DISCUSSION

Positron emission tomography scanning has established itself as a standard tool in the evaluation and follow-up of oncology patients. Until recently, issues such as cost and extremely short isotope half-life had limited PET scanning primarily to research centers. Positron emission tomography scanning has exploded, and this modality has firmly rooted itself even within the private practice setting.

A vast experience with PET scanning is still evolving. As with any new technique, a learning curve is expected. An essential knowledge of normal excretion, uptake, and normal variations of distribution of radiotracer, as well as technical artifacts, is critical in the correct interpretation of PET images. ¹

FDG uptake can be seen in a number of benign lesions, including infectious and inflammatory processes, leading to false-positive findings. ² Benign lesions that have been found to take up FDG are chronic thyroidosis, Warthin's tumor of the parotid, and chronic sinusitis. ³ In the chest, false-positive findings can be seen with sarcoidosis, active tuberculosis, granuloma, thymoma, pneumonia, and abscess. ⁴ Less common etiologies causing false-positive findings include thymic hyperplasia, aspergillosis, histoplasmosis, rheumatoid disease, alveolitis, and radiation pneumonitis. ^4,5

Most patients with sarcoidosis are discovered incidentally by abnormal radiographic findings on chest X-ray. However, as in this case, incidental diagnosis on whole-body FDG PET scanning is being reported with increased frequency. The increased FDG uptake is thought to be related to the actively dividing macrophages, lymphocytes, and epithelial monocytes that comprise sarcoid granulomas. ⁶ It should be noted that the pattern of uptake in sarcoidosis can overlap that of lymphoma on a PET scan. Other clinical and radiographic findings will help differentiate these two entities.

Although Gallium citrate scanning has classically been used to diagnose and monitor disease activity in patients with sarcoidosis, recent use of FDG PET as an inflammatory marker has been investigated in patients with known sarcoidosis. Parenchymal uptake has also been reported in patients with sarcoidosis, which, along with the uptake in the associated lymphadenopathy, returned to normal after steroid therapy. ⁶ This may lead to a role for FDG PET scanning in monitoring patients with sarcoidosis.

CONCLUSION

FDG PET scanning has been shown to be an effective tool in the physician's diagnostic arsenal and is playing an increasingly important role in the follow-up of oncology patients. The radiologist must become familiar with the multitude of potential false-positive findings in order to accurately interpret PET images.