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.
Degree of confidence
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 positives/negatives
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.
