Other Common Clinical Problem Areas

MR and biopsy are the techniques of choice for the diagnosis of primary brain and spinal cord tumors. In some patients,18FDG PET can provide additional important information about grade of tumor which can have significant diagnostic and therapeutic implications (18). In some patients,18FDG PET can complement MRI for delineating the full extent of disease at the primary site. In select patients, where there is uncertainty on anatomical imaging and also a relative contra-indication to biopsy, it can be useful for distinguishing between benign and malignant lesions (19). The major shortcoming of 18FDG PET in brain application is the intrinsic high background activity of the cerebral cortex and other gray matter structures and lower anatomical resolution compared to MRI. This limits its use as a method for detecting low-grade tumors, and it is not an adequate screening modality for the detection of intracranial metastatic disease by itself.

18FDG PET cannot replace CT but instead is complementary to CT in the evaluation of pancreatic lesions particularly when malignancy is being suspected and where CT has failed to identify a discrete mass or in patients in whom biopsy is not definitive. There are a small number of studies in the literature that suggest that 18FDG PET can be of value for characterizing pancreatic masses (20) when there is a need to distinguish between chronic pancreatitis and malignancy. False positive lesions include acute pancreatitis and cholangitis, and false negative findings can occur in patients who are diabetic and hyperglycemic at the time of 18FDG PET scan, in well differentiated neuroendocrine as well as some cystic neoplasms.

Initial enthusiasm for using 18FDG PET for characterizing thyroid and parotid gland lesions has not been substantiated by subsequent results. Both malignant and benign lesions in these glands can be equally avid for 18FDG (21,22). Conventional methods of assessment remain the mainstay for characterizing and staging thyroid and parotid gland malignancies.18FDG PET also appears to be of limited use for detecting sarcomatous change in neurofibromas, as neurofibromas can themselves be avid for 18FDG (23).18FDG PET is recommended for localizing the site of the primary tumor in patients presenting with squamous cell lymph node metastases in the neck without an obvious primary tumor in the mucosa of the aerodigestive tract (Fig. 3).

Figure 3 Squamous carcinoma of unknown primary origin in the head and neck region. This patient presented with a left level III squamous cell carcinoma lymph node metastasis. No primary tumor was evident on CT, MRI and EUA which included multiple biopsies and bilateral tonsillectomies. (A) Fused FDG PET-CT through the neck. FDG PET scan confirmed high uptake in the level III node. (B) Fused FDG PET also shows increased uptake in the left ton-sillar fossa inferiorly, indicating the site of the primary tumor which was confirmed by subsequent repeat EUA and biopsies. (See color insert.)

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