Prostate cancer imaging looks beyond bones scans

San Francisco-The traditional bone scan remains an important tool for evaluating prostate cancer patients with a rising PSA. Next to PSA testing itself, metastable technetium-99 bone scans currently are the second most common test in men with rising PSA following definitive therapy, said Richard Wahl, MD, director of nuclear medicine at Johns Hopkins University School of Medicine, Baltimore. But as useful as bone scans can be, they have limited utility.

In newly diagnosed prostate cancer patients, the frequency of positive bone scans is low if the PSA is below 10.0 to 20.0 ng/mL. Scans can be more useful if PSA becomes detectable after radical prostatectomy or other definitive therapy, but results can be difficult to interpret if the post-treatment PSA level is low.

"If the patient's PSA is less than 2.0 ng/mL, the yield will be quite low for positive bone scans," Dr. Wahl cautioned attendees at the Prostate Cancer Symposium here. "The highest PSAs give the highest bone scan yields."

Both SPECT and SPECT/CT fusion techniques offer three-dimensional imaging, as the camera rotates around the patient. Traditional bone scans provide a planar image.

"One can do quite well with short acquisition times with the rotating camera," Dr. Wahl said. "New techniques will allow us to do even better."

One of the more promising new techniques is (18)F-fluoride PET/CT using sodium fluoride. A recent prospective study found that a planar bone scan missed 81 of 156 lesions (52%) of those identified by (18)F-fluoride PET (J Nucl Med 2006; 47:287-97). The planar bone scan also missed 62% of the lesions identified by SPECT. PET/CT was more sensitive and more specific than bone scan and was more specific than PET alone (p<.001 for both).

Another promising technique is gamma camera imaging using (111)In-capromab pendetide (ProstaScint) after radical prostatectomy in men with rising PSA. Immunoscintigraphy showed the antibody to be localized primarily in the prostatic fossa (34% of cases), abdominal lymph nodes (23%), and pelvic lymph nodes (22%). Early findings suggest that this method can be useful in determining the extent of disease in men with rising PSA after radical prostatectomy.

Other imaging advances

Fluoro-2-deoxy-D-glucose (FDG), a tracer for the early steps of glucose metabolism, may also be useful in PET imaging. A study reported in Clinical Cancer Research (2005; 11:2785-808) compared magnetic resonance imaging, bone scan, and computed tomography with FDG-PET between 1997 and 2003. Results of FDG-PET imaging showed a true positive in 28 of 91 patients (31%), including isolated disease in the prostate bed and metastatic disease with and without simultaneous disease in the prostate bed. The other imaging techniques showed isolated local recurrence or metastatic disease in only two patients.

Applications of PET using C11 choline, C11 acetate, and fluorocholine analogs also are promising. C11 acetate is listed in the US Pharmacopeia, but none of these techniques has been approved by the FDA. A report in the Journal of Nuclear Medicine (2003; 44:556-808) found that C11 acetate identified recurrent disease in 30% of patients compared with 14% of (18)F-FDG PET.

In a separate study, similar results were reported for C11 choline (J Urol 2003; 169:1337-40).