Real-Time Magnetic Resonance Imaging–Guided Focal Laser (Ablation) in Patients with Low-Risk Prostate Cancer

Excerpts below:

Abstract

The tumor was identified on MRI. A laser fiber was delivered via a catheter inserted through a perineal template and guided to the target with MRI. The tissue temperature was monitored during laser ablation by MRI thermometry. Accumulated thermal damage was calculated in real time. Immediate post-procedure contrast-enhanced MRI confirmed devascularization of the target. No adverse events were noted. MRI-guided focal laser (ablation) of low-risk PCa is feasible and may offer a good balance between cancer control and side effects.

Once the catheter reached its target, the metal trocar was replaced by an optical fiber with a 1-cm-long cylindrically diffusing tip attached to a 980-nm diode laser (Visualase Inc, Houston, TX, USA).

During laser ablation, temperature was measured simultaneously on five 3-mm-thick image slices that covered the target volume (Fig. 5a). The thermometry scan was repeated every 6 s.

Fig. 5 (a) Tissue temperature map measured by magnetic resonance thermometry (echo planar imaging with multiphase; field of view: 25 × 25 cm; matrix 256 × 256; number of excitations: 1; repetition time: 545 ms; echo time: 20 ms; flip angle: 20°; slice thickness: 3 mm) during laser ablation; (b) a map of the tissue volume exceeding the threshold damage for coagulation was superimposed on the anatomical image, with the pink line measuring the maximum diameter of the ablated tissue.

The MRI thermometry software (Visualase, Inc, Houston, TX, USA) allowed us to monitor temperature at specific points in the tissue. The temperature at those points was used as a feedback to control the laser. During the laser heating, the temperatures at the border of the rectal wall and urethra were monitored and maintained at safe levels by shutting down the laser automatically when the temperature at these critical points exceeded 45 °C. Thermal damage was calculated using an Arrhenius formula. Temperature and damage maps were superimposed onto anatomic images (Fig. 5b). Once the desired volume of tissue destruction was achieved, laser power was stopped.

The patients were discharged home within 3 h. MR scans performed 2 wk post-procedure showed no evidence of complications with preservation of rectum and neurovascular bundles. No adverse effects were noted at ≤1 mo after the procedure. Six-month follow-up biopsies are pending.

Outpatient MRI-guided FLT as used in the present study allows for visualization of the tumor; real-time guidance of the thermal device to the target; monitoring and control of the zone of ablation and surrounding tissue during the procedure; and the ability to immediately confirm the success of the (ablation) and, if necessary, immediately repeat (ablation). The required skills are common to other minimally invasive procedures, resulting in a short learning curve for the surgeon. Refinement of this outpatient procedure may result in an inexpensive, minimally invasive alternative to current active therapies. Further trials will be necessary to define the safety and oncologic efficacy of this therapy, but our early results are promising.

Acknowledgment

The authors would like to thank Drs. Ashok Gowda and Roger McNichols of Visualase Inc., Houston, Texas, USA, for generously supplying the laser and magnetic resonance thermometry system and for their technical support of this study.