Biopsying the primary lesions is technically difficult since it is located in the cerebellum. Studies of PET have focused on the population with a biopsy proven source of metastases. For example, in one study of biopsy proven cancer of unknown origin, a survey on clinical usefulness of PET (response rate 83%) suggested that PET positively contributed to diagnostic understanding in 29 of 52 evaluable cases. It concluded that late in the diagnostic trajectory, approximately four patients need to be scanned by PET in order to find one primary tumour. However, in addition to direct demonstration of unknown primaries, there appears to be a positive effect on the diagnostic work-up of these patients of a similar magnitude.
A 1999 study found that PET identified focal hypermetabolic abnormalities in 19 of 22 intracranial metastases, 2 hypometabolic lesions, and 1 renal cell tumor embolism that hemorrhaged (hypometabolic lesion). It also identified 82% of extracranial primary tumor sites, of which 55% were found only on PET and not on conventional diagnostic tests. Patients did not have a biopsy proven diagnosis in this study. Another study concluded: ” Screening the patients with suspected metastatic brain tumors using whole-body (18)F-FDG PET could be helpful in differentiating metastatic brain tumor from primary brain tumor and in detecting the primary lesion.”
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