ORLANDO — New modalities for detecting prostate cancer metastases may help physicians target treatment more effectively to diseased tissues, speakers said at a symposium on prostate cancer sponsored by the American Society of Clinical Oncology.
Current staging of the lymph nodes of patients with intermediate- or high-risk prostate cancer involves noninvasive tests such as CT and MRI, or the gold standard procedure of pelvic lymphadenectomy, noted Mukesh Harisinghani, M.D., of Massachusetts General Hospital, Boston.
Noninvasive tests rely on the diameter of the short axis of the lymph nodes to determine their status. On MRI, oval nodes less than 10 mm in diameter or round nodes less than 8 mm in size are labeled benign, whereas larger nodes of similar shape are considered malignant. However, Dr. Harisinghani said, “Size criterion is very inaccurate in staging lymph nodes.”
During pelvic lymphadenectomy, surgeons generally sample the lower external iliac and obturator lymph nodes.
A study showed that when the dissection was extended to the higher external, internal, and common iliac lymph nodes as well as the presacral lymph nodes, the metastasis rate increased from 10% to 26% (J. Urol. 2002;167:1681–6).
But even an extended pelvic lymphadenectomy does not allow sampling of all nodes at the time of surgery, Dr. Harisinghani said at the symposium, cosponsored by the Society of Urologic Oncology and the American Society for Therapeutic Radiology and Oncology. Frozen section analysis of lymph node specimens also carries a false-negative rate of up to 30%, he added.
A new MRI technique involves the infusion of tiny, 20-nm-diameter, dextran-coated iron oxide spheres into the bloodstream. The spheres bind to macrophages in the lymph nodes. After 24 hours, nodal areas that did not take in the nanospheres indicate metastatic regions.
In 80 patients with advanced prostate cancer, the nanosphere imaging technique yielded significantly higher sensitivity (91% vs. 35%) and specificity (98% vs. 90%) as well as greater accuracy (97% vs. 76%) than conventional MRI alone in detecting metastases on a node-by-node basis (N. Engl. J. Med. 2003;348:2491–9). About 71% of the malignant nodes were below the size criterion, Dr. Harisinghani noted.
The imaging procedure allows surgeons to pinpoint positive nodes that they normally would not have dissected and to see exactly where they lie in relation to vessels, obturator nerves, and ureters. And by combining MRI data with data from CT scans, cancerous nodes can be targeted, thereby avoiding unnecessary radiation.
Clinicians also desire a test that complements anatomic imaging with CT and MRI or metabolic imaging with PET to determine if prostate cancer has spread to bone or soft tissue, Neil H. Bander, M.D, said in a separate presentation. Bone is the earliest and most common site of metastatic prostate cancer.
Second-generation monoclonal antibodies may offer the best hope for staging metastasizing or recurrent disease in prostate cancer patients, said Dr. Bander, professor of urologic oncology at Weill Medical College of Cornell University, New York.
Antibodies such as
In vivo imaging with J591 shows soft tissue and bone metastases of prostate cancer patients equally well. J591 imaging can detect bone metastases that go unseen in a standard bone scan, Dr. Bander noted.
Few clinicians still use imaging with the first-generation
Findings from clinical studies on Prostascint have shown that the antibody scan detects soft tissue metastases with greater accuracy than MRI or CT, but it does not detect bone metastases very well.