Prostascinttm

This is a monoclonal antibody (originally known as CYT-356) to the prostatic surface membrane antigen. It is labeled with In-111. The approved name is capro-mab pendetide.

This antigen is thought to occur only at the surface of "metabolically" prostate cells. Therefore, under normal conditions, localization should only occur in the prostate. Any accumulation outside the prostate must logically be in metastatic cancer deposits. The acquisition of the data is not simple. Because it is a large molecule, localization is slow, and imaging is performed at 72 or 96 hours, with repeat images 24 or 48 hours later, if the findings are uncertain. The analysis of the images is also complex, as there is considerable blood pool activity. Typically, this requires acquisition of a separate set of images of the blood pool using Tc-99m-red blood cells.

In a series of 181 studies in patients with a rising prostate-specific antigen (PSA) after radical prostatectomy, focal accumulation was seen localized to the prostatic fossa in 34% of the cases, to abdominal lymph nodes in 23%, and to pelvic nodes in 22%. Forty-two percent of these patients had localization outside the prostatic fossa. Half of the localizations in the prostatic fossa were confirmed by biopsy (3).

Obviously, it would be of value if this method could be used for initial staging of high-risk prostate cancer.

When compared with surgical lymphadenectomy specimens in a series of 31 patients, the sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and accuracy of the scan were 94%, 42%, 53%, 92%, and 65%, respectively, when analyzed by surgical site and 100%, 33%, 62%, 100%, and 68%, respectively, analyzed by patient (4).

In another study of 160 patients, in 152 evaluable patients, the sensitivity was 72%, the specificity 62%, the PPV 62%, and the NPV 72%. In the same patients, the sensitivity of CT and MRI was 4% and 15%, respectively (5).

In a series of 198 patients with clinical high-risk T2 or T3 disease, 39% had pelvic lymph node deposits on pathology. The Prostascint™ study had a PPV of 67% (6).

The same group have also reported (7), on a series of studies in high-risk patients prior to pelvic lymph node dissection, a PPV of 62%, an NPV of 72%, sensitivity of 62%, and a specificity of 72%; for prostatic bed recurrence, using needle biopsy as the reference, the figures were PPV of 50%, NPV of 70%, sensitivity of 49%, and specificity of 71%.

However, in a group of 22 patients with less extensive disease—lower PSA values and lower Gleason scores—who had bilateral pelvic and obturator node resections at the time of surgery, the accuracy was lower (8). Nine of the 88 node basins were positive on the Prostascint™ study, but only one (11%) was histo-logically involved. Of 79 scan-negative node groups, five were involved, giving a sensitivity of 17%, a specificity of 90%, NPV of 94%, and PPV of 11% (8).

It is this problem of identifying the accuracy of localization that is at the heart of trying to assess the value of this agent. There is some indirect evidence of accuracy of localization. In a group of 32 patients with prostate cancer who had a ProstascintTM study and then had salvage radiotherapy to the pelvis, sixteen out of 23 (70%) men with a normal scan outside the prostatic fossa achieved a durable complete response, compared to two of nine (22%) who had a positive scan outside the prostatic fossa and pelvis (9).

A potentially useful approach to improving accuracy has recently been reported (10). These authors used fusion of the Prostascint™ volume dataset with CT and MR datasets. Fifty-eight patients were studied. Seventy-four of 161 sites of reported uptake on the ProstascintTM study were found to be negative after fusion, corresponding to normal tissue, such as bowel, vessel, or bone marrow. In two patients, nodal disease was identified on the fused data only. Twenty-five patients previously thought to have nodal disease appeared to have only local disease after fusion. Obviously one problem is that there is no external verification of the results, but the numbers are compatible with the reported accuracy in other studies. This also reflects the experience in improved accuracy from fusion of datasets in PET (see below). These authors used CT and MRI datasets acquired separately, but it is of interest that the major manufacturers of nuclear medicine equipment are now producing gamma cameras with built-in CT systems.

For the moment, it appears that Prostascint™ is of moderate value in localizing recurrence but is not sufficiently reliable for initial staging prior to curative therapy.

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