By James Williams
In the past decade, the molecular imaging (MI) community saw only limited introduction of new radiotracers for positron emission tomography (PET) imaging.
Now, however, we look with great anticipation to a new generation of tracers that are increasingly demonstrating value in the assessment of recurrent prostate cancer — the second most- common form of cancer in American men (just behind skin cancer), with roughly 180,000 new cases diagnosed each year in the United States. Even more compelling is the future potential of targeted radionuclide therapy to provide a true theranostic approach, where diagnostic and therapeutic capabilities combine into one agent to treat post-recurrent prostate cancer patients. To fully appreciate the mounting enthusiasm over these new radiotracers, we must first consider MI’s role thus far in prostate cancer diagnostics.
Ultrasound-guided biopsy is the current gold standard for the diagnosis of primary prostate cancer, with multi-parametric magnetic resonance imaging (MRI) gaining support as an accurate tool for primary diagnosis and staging. MI currently comes into play when evaluating potential metastatic disease: A bone scan with a gamma camera can determine if and how widely the disease has spread. Another test is commonly used with PET/CT or single-photon emission computed tomography/CT (SPECT/CT) to identify pelvic lymph node involvement.
New tracers are enabling PET/CT to become a central tool in assessing recurrent prostate cancer. Early on, the radiotracer 11CCholine was generally regarded as having greater value for detecting recurrent disease in patients with an elevated prostate-specific antigen (PSA) level. However, 11C-Choline has a short radioactive half-life and is difficult to distribute. For these reasons, its use is extremely limited. The next generation of tracers suggests great potential and could enable prostate cancer imaging capabilities beyond current methods. With a longer radioactive half-life, new tracers also offer broader distribution potential and increased patient access. In the short term, the combination of new tracers and precise diagnostics via imaging of biological processes presents physicians with the opportunity to be more selective with treatment.
Looking forward, we see value in a number of tracers, including those that target prostate-specific membrane antigen (PSMA), a surface protein that appears on both healthy prostate cells and, in much greater concentration, on prostate cancer cells. These tracers, which are already used extensively in Europe, support early visualization of small recurrent or metastatic lesions in patients with an elevated PSA level and who exhibit no other symptoms after treatment. Although the present application of this subset of new tracers is in recurrence, evaluation of primary disease is an emerging topic of investigation.
These new tracers also play a key role in the aforementioned theranostic approach to prostate cancer. By binding a therapeutic radionuclide to a ligand — the same as or similar to the diagnostic ligand — physicians can deliver a targeted therapeutic dose to the patient to identify and destroy prostate cancer cells. And with recent technological developments enabling automated and reproducible quantification, SPECT/CT could be used to measure actual dose distributions in patients.
The cost of this theranostic approach appears to be modest, and it has the potential for broad application. Germany has pioneered this therapy for the past half-decade, with its use spreading to countries such as the Netherlands, Austria, Turkey and Australia. Early clinical trials are expected in the not-too-distant future in the U.S. Siemens supports research to improve these theranostic techniques, recognizing the considerable potential of personalized patient care. The millions of men worldwide who have been diagnosed with prostate cancer deserve nothing less.
About the author: James Williams, Ph.D., is head of Siemens Healthineers’ Molecular Imaging business.