The Advantages of MRI Guided Focal Treatments
By: Dan Sperling, MD
Image-guided focal ablation techniques for treating nonaggressive prostate cancer are rapidly increasing and gaining acceptance. A recent article published by the American Journal of Roentgenology reviewed the progress in MRI-guided focal ablation (tumor destruction) by surveying the published literature prior to January, 2012. [i] The article points to the advantages of MRI over other types of imaging.
Up to a third of prostate cancer patients with a unifocal lesion are potentially eligible for a targeted treatment; taking the theory of “index lesion” into account, the actual numbers would be even higher. In all such cases where tumors are no higher than Gleason 3+3, radical treatment exposes patients to the unnecessary risks of urinary and sexual side effects that result from over-treatment. Focal treatments greatly reduce, and even eliminate, such risks.
Because of the nonsurgical, minimally invasive nature of today’s thermal ablation approaches (such as focal laser ablation) image guidance is essential. Currently, such guidance is done using ultrasound, computerized tomography (CT) and magnetic resonance imaging (MRI). Of these three, “MRI—especially multiparametric MRI—is the most sensitive and specific imaging technique for prostate cancer.”[ii] The article summarizes reported data on MRI guidance in the following treatments:
There is not much literature on MRI-guided cryosurgery (freezing) of prostate cancer. Although it has been demonstrated to be feasible, the volume of the iceball during treatment imaging did not match the post-treatment zone of ablation-induced tissue destruction. The use of a contrast agent during imaging gives more accurately predictable destruction. Temperature mapping during cryo is not possible below 0° C, whereas the required lethal temperature is -40° C. Finally, the cryoprobes may produce some imaging artifacts that can obscure image fidelity.
MRI, especially on a powerful 3T magnet, seems ideally suited for the heat-based destruction of focal laser ablation. According to the article, it is a “promising technique for accurate focal targeting of prostate cancer because of the inherent high spatial and temporal resolution of MRI.”[iii] Temperature mapping is possible, assuring that the desired 60° temperature is reached. Because the laser fibers are made of a material that does not distort the electromagnetic field, there are no artifacts to distort images. However, at the time of the article (October, 2012) there were very few human cases reported in the literature. Now, in addition to non-clinical trial data reported on a series of patients (see https://sperlingprostatecenter.com/2014-journal-article-focal-laser-ablation/), there are more journal publications including emerging clinical trials.
HIFU (High Intensity Focused Ultrasound)
Both animal and human studies have demonstrated that MRI-guided prostate HIFU is feasible. The authors caution that HIFU can be a time-consuming procedure if done as a radical treatment, so they hypothesize that MRI-guided HIFU would best be used as a focal therapy. However, any movement of the patient or the prostate itself can compromise the accuracy of the targeting the tumor.
Other treatments that are not offered in the U.S. but are used in Europe and Asia to treat prostate cancer are commonly done using ultrasound guidance, but can be done under MRI. These include radiofrequency ablation, microwave ablation, and brachytherapy. Of all of the treatments described, the authors consider MRI-guided focal laser ablation to be “the most sophisticated focal therapy treatment option in the prostate.”[iv] Its advantages include efficiency, sharply defined ablation zone, no image distortion due to laser fibers, few if any side effects, very rapid recovery, and repeatability. According to the authors, it is also easy to integrate in clinical practice. While MRI-guided HIFU appears promising, especially because it is noninvasive, the issues of long treatment time and possible patient movement place it lower in desirability at this time.
The article wraps up by affirming the importance of accurate diagnosis and patient selection, and identifying the need for ongoing research and data publication. The conclusion of the authors is that focal laser ablation remains the most feasible of the MRI-guided focal treatments for prostate cancer.
[i] Bomers J, Sedelaar J, Barentsz J, Futterer J. MRI-guided interventions for the treatment of prostate cancer. AJR 2012 (October);199:714-20.
[ii] Ibid. p. 714.
[iii] Ibid. p. 716.
[iv] Ibid. p. 717.