Interpretation

Since FDG-PET is a function of glucose metabolism, any organ with a higher metabolic rate will demonstrate greater tracer activity under normal circumstances (e.g., brain, intestines, liver, heart, etc.). Tracer uptake by malignant lesions will also depend on the rate of glycolysis. Although most tumors will demonstrate an inherently higher metabolic rate, some tumors (e.g., prostatic cancers) may have decreased proliferative activity and therefore not be apparent on PET images. The lower limit of the size of lesion detectable by PET scanning is 5 mm in diameter but is likely to improve in the future with improved resolution scanners.

Testicular tumors

Results of PET in the management of testicular tumors show the most promise compared to other renal tract cancers.

• Primary disease: PET has a superior performance profile than conventional imaging. CT scan has been reported to have false-negative and false-positive rates of as high as 59% and 25%, respectively, in low-stage disease. By contrast, a number of studies have demonstrated a higher specificity (87-100%), higher sensitivity (70-100%), and better negative predictive value for PET compared to CT scanning. If metastatic disease at presentation is obvious after conventional imaging, then PET has a rather limited role. It may, however, be of value in the assessment of patients with stage II disease and could potentially alter management. Furthermore, emerging evidence may suggest a role for PET in predicting response to chemotherapy, but more data is required

• Recurrent disease: patients presenting with raised tumor markers following previous therapy can represent a diagnostic dilemma, especially if conventional imaging is negative. PET scan performs better than CT in this instance and is more likely to detect recurrent disease earlier than CT or MRI. Some authors have advocated the use of PET as first line investigation in patients with suspected recurrence

• Residual tumor: PET can play a significant role in the treatment of patients with residual mass following radiotherapy or chemotherapy for seminoma. CT cannot confidently discriminate between malignant and fibrotic/necrotic tissue in up to 50% of cases. The positive predictive value of PET in such patients is between 80% and 96%. A negative PET scan is indicative of non-cancerous tissue in 88-90% of cases. Thus it is in this category of patients that PET has been found to be most useful

Renal cell cancer (RCC)

• Primary disease: even though the normal kidney will demonstrate FDG uptake, PET performs as well as CT in the characterization of a renal lesion as benign or malignant. Some renal cell cancers may be PET negative and therefore may be missed. Overall, PET is more likely to detect unsuspected metastatic disease compared to CT and this may hugely influence treatment options. Nevertheless, PET is not routinely indicated prior to radical nephrectomy at present

• Recurrent disease: PET can confidently distinguish between malignant and benign disease in patients with a previous history of RCC, presenting with a subsequent indeterminate lesion, in 75-100% of cases. Lymph node involvement and local or distant spread can be effectively identified by PET

Bladder cancer

Because of the urinary excretion of FDG, visualization of the bladder is difficult, and PET is not currently indicated in local staging of bladder cancer. Accuracy of lymph node detection is marginally better than with CT or MRI scan and further work is required in this subset of patients.

Prostate cancer

Slow glycolytic rates (resulting in poor FDG uptake), low-volume tumors, and suboptimal visualization of the prostate area (due to tracer accumulation in the bladder) have meant that PET has a rather limited role in investigating localized prostate cancer. It may, however, be useful in the detection of metastatic lymph nodes, recurrent distant disease, and to monitor treatment response (e.g., to androgen deprivation). Emerging new tracers, including "C-methionine (amino acid tracer), "C-choline, and "C-acetate (both lipid based tracers) have shown promising results compared to FDG, but sparse data and their relatively short half-lives have made routine clinical use difficult.

PET is inferior to conventional bone scintigraphy in the detection of skeletal metastases but can be used to provide additional information if the latter is equivocal.

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