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PROSTASCINT SCAN: 27% Positive Predictive Value for Detecting Prostate
Cancer Outside the Prostate Bed in Men with Rising PSA Following Primary
Therapy.
(May 2007)
The question is
frequently raised in tumor board discussions as to the effectiveness of
a ProstaScint scan in detecting metastatic disease. This issue is
addressed in a Loyola University study, "Long-term follow-up of 111 In-capromab
pendetide (ProstaScint) scan as pretreatment assessment in patients who
undergo salvage radiotherapy for rising prostate-specific antigen after
radical prostatectomy for prostate cancer" (Int J Rad Oncol Bio Phys,
Mar 1, 2007).
The test's radiolabled
tracer targets to prostate cancer in soft tissues by recognizing an
epitope in the transmembrane prostate-specific membrane protein (PSMA),
which is "highly expressed in malignant prostate tissue." The study was
designed to "evaluate long-term failure patterns" associated with the
use of this test as staging prior to salvage radiotherapy. Although
other studies have reported differing outcomes for accuracy of the
ProstaScint, the authors point out that their study involves the largest
cohort with the longest median follow-up for evaluation of the test.
The 4-year biochemical
outcome after salvage radiotherapy to the prostate bed (median dose 66.6
Gy; range 63-70.2) was assessed for twenty patients whose post-RP PSA
levels were >0.2 ng/mL, and who, at failure, showed no evidence of
metastatic disease on CT and isotope bone scanning. The median follow-up
was 41 months from salvage RT; the median Pre-RT PSA was 0.4 ng/mL.
Their findings: The 4-year bRFS for patients with negative scans was
53%; for scans positive in the prostate bed only, 45%; or for scans
positive elsewhere, 74%. There was no significance in these different
results. (p=0.51)."
One study observation
was confirmatory to current salvage RT strategy: "... a pre-RT PSA level
of less than 1 ng/mL was the only factor predictive for improved bRFS."
Study conclusion: "Although the capromab pendetide scan revealed
regional or distant uptake in approximately one-third of the patients,
the bRFS in this group did not differ from those whose scans showed no
or local uptake only," i.e. "a positive ProstaScint scan beyond the
prostate bed had no effect on the 4-year bRFS."
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