Re: Microwave focal therapy of prostate cancer: A non-clinical study and exploratory clinical trial

The authors conducted an animal model study and a phase 2 clinical trial to evaluate the safety and efficacy of focal therapy using microwave tissue coagulation (MTC) to treat prostate cancer.[1] The results indicated that focal therapy using microwave ablation is relatively safe and may be an option for lesion-targeted therapy for prostate cancer. In a canine prostate model, the authors demonstrated that the thermocoagulative effect of MTC can successfully cause coagulative necrosis on histological evaluation of healthy prostate tissue. This was evaluated by removing the prostate and the surrounding tissue from the animal model a few hours after treatment with MTC. The authors found that the extent of coagulation necrosis and cell death produced by MTC was proportional to both the energy output and irradiation time. The authors then demonstrated that intraoperative ultrasonographic measurements were correlated with pathological measurements of the ablated tissue. Although this study demonstrated the potential utility of MTC for the treatment of prostate cancer, the study had some notable limitations. First, microwave ablation causes heating by forcing water molecules in the tissues to oscillate out of phase with the applied fields; thus, some of the electromagnetic energy is absorbed and converted to heat.[2] Given that prostate cancer manifests with an altered glandular structure on histopathological examination, and studies have shown that the perfusion of blood to prostate cancer lesions varies between normal prostate parenchyma and cancer,[2] prostate cancer may have higher energy requirements to obtain a similar ablative temperature. In addition, the kill zone of MTC may be smaller than that of the ablated zone on real-time ultrasonographic findings. Second, in terms of the clinical study, the authors did not perform prostate biopsies in 3 of the 5 patients. An obvious concern is that residual cancer remains in the prostate gland after treatment. Although some clinicians choose to only perform for-cause biopsies after focal therapy, all patients who were enrolled in this trial should have undergone a scheduled prostate biopsy to determine the oncological effect of MTC. A recent consensus statement suggested performing prostate biopsy between 6 and 12 months after focal therapy, as most experts believe that prostate-specific antigen testing and magnetic resonance imaging are not sufficient to determine oncological success after ablation.[3] Lastly, most pathologists are not likely to encounter specimens obtained immediately after focal therapy, because patients are more likely to undergo needle biopsies 6–12 months after focal ablation. It is certainly possible that the extend of the ablation zone and the changes from MTC may not be visualized immediately after ablation. Interestingly, biopsies obtained up to 8 months after high-intensity focused ultrasound may show changes of coagulative necrosis.[4] This finding may be similar in patients undergoing MTC; hence longer-term biopsy data is needed to determine whether histological samples can adequately be characterized after MTC. Considering that animal model studies are extremely challenging and expensive to conduct, professor Ukimura and his team should be commended for successfully demonstrating that MTC may be a safe and efficacious treatment for prostate cancer. However, many questions must be answered for transperineal microwave ablation of the prostate to be adopted for mainstream usage in clinical care.