Other Malignancies

18FDG PET is valuable for the evaluation of recurrent head and neck tumors (67). High sensitivity and specificity have been recorded for the use of 18FDG PET in this circumstance. However, there are a number of normal variants for the uptake of 18FDG in the head and neck that present potential pitfalls with false positive interpretation, particularly when normal symmetrical uptake is lost. For example, lymphatic tissue in Waldeyer's ring often shows moderate uptake of 18FDG. Apparent increased uptake is seen on the untreated side after tonsillectomy, soft palatal resection, and also radiotherapy to the head and neck, and this should not be mistaken for pathology. It is in this circumstance, that careful correlation with the clinical history and also co-registered PET and MRI or CT imaging is especially helpful (9). False positive uptake has been observed within the early period after radiotherapy due to the inherent inflammatory reaction, but this usually subsides by four to six months (67). Other tumors where 18FDG PET has proven a useful tool in detecting recurrence include lung cancer, lymphoma, and testicular tumors (20).

Detection of recurrent brain tumors is a challenging problem for CT/MRI as it can be difficult to differentiate tumor from post-treatment gliosis. The use of 18FDG PET alone in this situation may not be optimal as it may be difficult to detect recurrent tumor against the background of normal cortical activity. The degree of 18FDG uptake correlates well with the grade of recurrence and may be helpful in directing

Figure 5 Pre-surgical evaluation: a patient with relapse of rectal cancer and an isolated liver metastasis on conventional assessment. Resection of the liver metastasis was planned. (A) FDG PET-CT confirmed the presence of the liver metastasis. It also showed unexpected disease within normal sized nodes in the retroperitonium. (B) FDG PET-CT showed that there was residual active disease within fibrosis in the pelvis. (C) Additionally, a deposit within erector spinae muscle is seen.The patient's management was changed from surgery to palliative treatment. (See color insert.)

Figure 5 Pre-surgical evaluation: a patient with relapse of rectal cancer and an isolated liver metastasis on conventional assessment. Resection of the liver metastasis was planned. (A) FDG PET-CT confirmed the presence of the liver metastasis. It also showed unexpected disease within normal sized nodes in the retroperitonium. (B) FDG PET-CT showed that there was residual active disease within fibrosis in the pelvis. (C) Additionally, a deposit within erector spinae muscle is seen.The patient's management was changed from surgery to palliative treatment. (See color insert.)

biopsies to the most active area of tumor. Some investigators advocate the use of nC-methionine PET in addition to 18FDG. The relatively low uptake of nC-methio-nine into normal brain cortex allows better definition of tumor extent than with 18FDG alone (68).

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