Radionuclide bone scanning after the injection of a technetium-99m (99m Tc) tracer is the standard method for assessing potential bone metastases from prostate cancer. With diffuse bone metastases, a "superscan" may be seen; this superscan demonstrates high uptake throughout the skeleton, with poor or absent renal excretion of the tracer. Evidence-based guidelines for the use of radionuclide bone scans in patients with serum PSA levels greater than 10ng/mL have been devised.
Bone scans have a high sensitivity but low specificity for metastatic prostate cancer. Isotopic bone scans revealing metastatic prostate cancer are shown below.
Isotopic bone scans show multiple areas of increased tracer activity from metastatic prostate cancer.
Isotopic bone scans. Diffuse metastases demonstrate a superscan appearance. Note that no renal excretion of radioactive tracer is demonstrated.
Positron emission tomography (PET) scanning with fluorodeoxyglucose (FDG) may have a role in the detection of lymph node metastases from prostate cancer, particularly in patients with relapsed disease after primary treatment. Localized disease within the prostate and pelvic lymph nodes can be difficult to image because of the proximity of bladder activity. Currently, the sensitivity of FDG-PET for detection of recurrence after radical prostatectomy is less than 50%.
Carbon-11 (11 C) acetate and 11 C choline have shown promise as alternatives to FDG in prostate cancer, but they are still under assessment and are less readily available than FDG. Retrospective image fusion of11 C-acetate PET scanning with CT scanning and MRI is technically feasible and appears to be a promising technique.
The use of immunoscintigraphy to assess prostate cancer is under investigation. This method uses radiotracer-labeled antibodies to acid phosphatase and to PSA. Initial studies used iodine-131–labeled antiprostatic acid phosphatase antibody, and subsequent studies have used indium-111 (111 In )–labeled antibody. The use of labeled anticarcinoembryonic antigen (anti-CEA) antibodies is being investigated.
The most commonly used monoclonal antibody (mAb) is capromab pendetide (ProstaScint; Cytogen, Princeton, NJ), which is indium-111 (111 In)–labeled mAb 7E11-C5.3 (CYT-356, which recognizes an intracellular epitope of prostate-specific membrane antigen [PSMA]). This immunoscintigraphic technique is approved for imaging soft-tissue metastases from prostate cancer but not for bone metastases.
FDG-PET has a reported sensitivity of approximately 50% for the detection of skeletal prostatic metastases. In general, FDG-PET has an excellent detection rate for lytic skeletal metastases, but it has a poor detection rate for sclerotic metastases. Disease that localizes within the prostate and pelvic lymph nodes can be difficult to image because of the proximity of bladder activity. The sensitivity of FDG-PET for detecting disease recurrence after radical prostatectomy is currently less than 50%.11 C-acetate and11 C-choline imaging have shown promise as alternatives to FDG-PET imaging in prostate cancer, but these are less readily available than FDG-PET techniques.
In a review of 631 scans, the sensitivity and specificity of capromab pendetide for lymph node metastases were 62% and 72%, respectively; for prostatic fossa recurrence, they were 49% and 71%, respectively. Two roles of capromab pendetide imaging may be advocated: evaluation of newly diagnosed high-grade prostate cancer before definitive treatment and assessment of men with rising PSA levels after definitive treatment (radiotherapy or radical surgery). The fusion of capromab pendetide images with CT or MRI scans can provide details of prostate cancer localization and improve the low spatial resolution of the capromab pendetide images.
False-positive bone scan findings may be the result of increased uptake on bone scans not caused by a skeletal abnormality. Artifacts may result from the presence of tracer at the injection site, scars from recent operations, and sweat in the axillae. Physiologic variants that cause false-positive findings may include calcification of cartilage, an inferior angle of the scapula, and bladder diverticulum. Increased tracer uptake on bone scan may be demonstrated as a result of metastatic disease, joint disease, fracture, Paget disease, osteomyelitis, or surgery.