Latest Information on Molecular Imaging for Prostate Cancer
The Prostate Cancer Research Institute (PCRI) publishes a periodical entitled Insights which is available by e mail or regular mail. I strongly advise readers to subscribe electronically. The February issue discussed several very important topics including molecular imaging. After initial surgery or radiation prostate cancer treatment, 40% of patients will experience a PSA relapse. Knowing the location of the recurrence is important since recurrence at or near pelvic lymph nodes may be amenable to additional curative focal therapy. Standard imaging techniques such as technetium bone scan, CT scans and MRI are usually unable to see tiny recurrent tumors. However, PET scans which work by exploiting various aspects of cancer metabolism can often visualize and locate these small tumors. Acetate and choline represent two building blocks of cancer cell membrane synthesis. Therefore, if these areas of the cancer cell are made radioactive with carbon-11 (C-11) acetate or C-11 choline or fluorine-18 (F-18) radioactive isotopes, these areas can be observed on a body scan. The article’s author, Dr. Fabio Almeida, Director of the Arizona Molecular Imaging Center, provides data on 373 patients with PSA recurrence who were scanned with C-11 acetate PET scan. Cancer detection rates correlated with the patient’s PSA level. PSA levels of 0.2 – 0.4 ng/mL, led to 50% cancer detection rate; a PSA of 0.41-1.0 ng/mL led to a 77% detection rate; while a PSA level greater than 1.1 produced a detection rate of over 90%.
Other scanning methods discussed in the article include PET scanning using a radioactive gallium-68 labelled prostate specific membrane antigen (PSMA) agent. PSMA is a cell surface transmembrane glycoprotein that is over-expressed in prostate cancer cells. Another method uses a synthetic, non-metabolized amino acid analog labelled with radioactive fluorine-18 (anti-18F-FACBC) which accumulates in prostate cancer cells due to over-expression of multiple amino acid transport systems. PET scan results using this label appear to be comparable to C-11 acetate or C-11 choline scans especially when the PSA level is greater than 1.0 ng/mL. Very small bony lesions can be best detected by PET/CT scanning using sodium fluoride radiolabelled with fluorine-18 (F18-NaF) which is readily absorbed into the matrix of bone and has very high affinity for bone metastatses. F18-NaF provides higher sensitivity and specificity than bone scans based on technetium-bone imaging. In conclusion, while multi-parametric MRI is becoming the imaging study of choice for initial prostate cancer diagnosis and targeted biopsy, PET scans such as those discussed above may be complimentary especially in higher-risk patients to rule out the presence of local and distant sites of metastatic disease. The PCRI article concludes with a comparison of the above radiolabels pertaining to their production in a cyclotron, their urinary excretion, their half-lives and specificities. The author states that while C-11 acetate and C-11 choline offer several positive characteristics, their short half-life requires more dedicated equipment. Of the probes, F18-FACBC seems to come closest to having the optimal characteristics.