The science of imaging prostate cancer has advanced to a point where multiparametric MRI (mpMRI) is now recognized as “the most promising and mature clinical imaging tool for diagnosis of prostate cancer.”[i] Not only urologists and radiologists, but patients themselves use mpMRI to detect and characterize suspected prostate cancer prior to biopsy. In addition, real time in-bore MRI targeted biopsy offers a more precise, accurate alternative to the risks of conventional TRUS biopsies. Thus, mpMRI can confirm the need for a biopsy plus provide essential information to guide treatment decisions.
Prostate cancer is considered a highly curable disease, especially if diagnosed and treated early. Treatments can be whole-gland (radical) approaches including surgery, all forms of radiation, cryotherapy and HIFU, or subradical tumor destruction (thermal ablation by extreme heat or cold) including partial gland ablation, hemiablation (half of the gland) and focal ablation which can be applied with cryo, HIFU, or laser. When patients are properly staged and matched with appropriate treatment, they have the greatest likelihood of a successful outcome. Still, all treatments carry the risk of recurrence, and the first sign that the cancer has come back is almost always an elevated or rising PSA.
Just as mpMRI excels at detecting primary (first-time) prostate cancer, it likewise is the imaging of choice for early identification of recurrence after treatment. Each of the functional sequences (T2 weighted, diffusion weighted, dynamic contrast enhanced, and spectroscopy) contributes specific information that distinguishes tumor tissue from healthy prostate tissue. When used in combination of at least two sequences (generally T2 and diffusion weighted together are considered the work horse of mpMRI) and read by an experienced interpreter, mpMRI is able to establish whether or not the recurrence is still localized. This fact will determine subsequent treatment strategies.
An excellent article by Oppenheimer et al. (2016) provides information about the use of mpMRI to identify recurrence after each type of treatment[ii]. The following is a summary of the article’s main points:
- Radical prostatectomy – When the prostate is surgically removed, the section of urethra (urine passage) through the prostate is also removed so the urethra must be reconnected with the bladder outlet. This is called the anastomosis, and it is there that most localized recurrence is found (though it can also be in the seminal vesicle bed, bladder wall or rectal wall). Because scar tissue can be misleading on prostate imaging, dynamic contrast enhanced MRI should be included in the scan sequences since it reveals tumor blood vessel activity that would not be present with scarring alone. In addition, close attention must be paid to diffusion weighted imaging.
- External beam radiation – Recurrence is most commonly found at the site of the original tumor. Dynamic contrast enhancement may be useful to help distinguish radiation-damaged tissue from tumor. However, recent research suggests that a combination of T2 and diffusion weighted imaging is very good at defining recurrence, and has the advantage of not using the gadolinium-based contrast agents with patients whose kidney function is not robust.
- Brachytherapy – As with beam radiation, the global tissue damage to the gland obscures prostate zone anatomy, making interpretation of T2 and diffusion weighted images more difficult. Spectroscopy is also of limited value because of the presence of the seeds and tissue distortions. Therefore, dynamic contrast enhanced imaging should be included in the scan.
- Focal ablation – There is not much literature available, but published papers seem to point to dynamic contrast as a way to differentiate tumor activity from other artifacts related to the zone of ablation. However, the combination of T2 weighted and diffusion weighted imaging may be more specific for tumors.
Our experience at the Sperling Prostate Center suggests that the knowledge and skill of the MRI technician and the reader play a key role in the quality of the images and the interpretation of them. Detecting recurrence using at least three of the four MRI functional parameters is a superior method for pinpointing tumor activity and for guiding biopsy and treatment decisions.
[ii] Oppenheimer D, Weinberg E, Hollenberg G, Meyers S. Multiparametric magnetic resonance imaging of recurrent prostate cancer. J Clin Imaging Sci. 2016;6:18.