Computed tomography and/or FDG-PET (or FDG-PET/CT) may be used to search for nodal disease after treatment, especially in aggressive and/or hormone-refractory disease; however, as there have been no randomized clinical trials comparing the efficacy of FDG-PET/CT to that of CT in this setting, evidence-based recommendations cannot be made. Because a staging lymphadenectomy is usually performed during radical prostatectomy, assessment for nodal disease with cross-sectional imaging in surgical patients is focused on common iliac and retroperitoneal nodes; in patients treated with radiation therapy, cross-sectional imaging of the pelvis is standard. Limited data suggest that, unlike CT, FDG-PET can also demonstrate tumor in the prostate bed .
As with conventional imaging modalities, the detection of metastases with FDG-PET increases with higher PSA levels . In a study of 91 patients with PSA relapse after radical prostatectomy, FDG-PET detected locally recurrent or systemic cancer in only 28 patients (31%) ; it appeared to be most useful in patients with PSA > 2.4 ng/ml or a PSA doubling time >1.3 ng/ml per year. Still, nearly all disease sites detected by CT or bone scanning were also detected by FDG-PET. The authors therefore suggested that the combination of FDG-PET (to detect systemic disease) and body MRI (to detect local recurrence) might provide a valuable imaging algorithm in appropriately selected patients with PSA relapse .
Preliminary results suggest that PET with 11 C-choline or 11 C-acetate may allow better detection of nodal disease than FDG-PET . The rapid 10-min uptake and plateau of agents labeled with 11 C within prostate cancer allows whole-body PET/CT imaging (with decay correction) and results in less interference from the bladder . However, because 11 C has a short half-life of approximately 20 min, its use requires a local cyclotron and is therefore not a widely available option.
Another tracer under development is 11 C-methionine, which differentiates tumor from normal tissue due to elevated protein synthesis . Patients who receive 11 C-methionine and FDG scans on the same day may demonstrate metastases that are positive by both tracers, or that are positive by 11 C-methionine only or FDG only . These differences appear to indicate changes that occur in tumor biology as the tumor adapts to specific sanctuary sites. The use of PET with multiple radiotracers that answer different questions is likely to play an important role in the future of metabolic prostate cancer imaging.
Monoclonal antibody imaging with the prostate-specific membrane antigen (PSMA) antibody 111 In-capromab pendetide (also known as ProstaScint®) is another modality used to detect soft-tissue metastases in high-risk patients who are being considered for local salvage therapy. In studies comparing capromab to CT and MRI, capromab proved capable of detecting some small soft-tissue metastases (5-10 mm in size) that could not be detected by the other two modalities. However, the overall sensitivity of capromab for soft-tissue metastases was only 50%-62%; it missed a fair number of metastases that were detected by CT or MRI and it yielded false-positive or false-negative results in approximately one-third of patients studied .
Furthermore, capromab demonstrated low sensitivity for detecting bone metastases, which usually occur before lymph node metastases . For these reasons, its use is controversial. A new antibody, J591, is currently undergoing experimental investigation and appears more promising for the detection of both bone and soft-tissue metastases . Further refinements in the procedures for performing and interpreting capromab scans may also increase its accuracy in advanced prostate cancer.
Hedvig Hricak and Peter T. Scardino
Prostate Cancer, eds. Hedvig Hricak and Peter T. Scardino. Published by Cambridge University Press.
© Cambridge University Press 2009.
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