INTERSTITIAL MICROWAVE THERMAL THERAPY FOR PROSTATE CANCER: METHOD OF TREATMENT AND RESULTS OF A PHASE I/II TRIAL

Interstitial microwave thermal therapy for prostate cancer

Thermal therapy is used to kill tumors by heating them to temperatures >50 degrees C for an extended period of time. Cell death results from thermal coagulation. The energy sources available for this approach include radiofrequency electrodes, microwave antennas, laser fiberoptics, and ultrasound transducers. Each of these modalities has the potential to be delivered in a minimally invasive manner, and many theoretical and experimental investigations of these devices have been performed. This review describes current knowledge of interstitial microwave thermal therapy for prostate cancer. Examples are given from an ongoing trial in patients who have recurrent or persistent disease following radiation therapy. Future directions for pretreatment planning and real-time monitoring and control are discussed. These techniques have the potential to optimize treatments on a patient-specific basis and will be instrumental in planned future trials of this therapy as first line for prostate cancer.

Highly directional transurethral ultrasound applicators with rotational control for MRI-guided prostatic thermal therapy

Transurethral ultrasound applicators with highly directional energy deposition and rotational control were investigated for precise treatment of benign prostatic hyperplasia (BPH) and adenocarcinoma of the prostate (CaP). Two types of catheter-based applicators were fabricated, using either 90 degrees sectored tubular (3.5 mm OD x 10 mm) or planar transducers (3.5 mm x 10 mm). They were constructed to be MRI compatible, minimally invasive and allow for manual rotation of the transducer array within a 10 mm cooling balloon. In vivo evaluations of the applicators were performed in canine prostates (n = 3) using MRI guidance (0.5 T interventional magnet). MR temperature imaging (MRTI) utilizing the proton resonance frequency shift method was used to acquire multiple-slice temperature overlays in real time for monitoring and guiding the thermal treatments. Post-treatment T1-weighted contrast-enhanced imaging and triphenyl tetrazolium chloride stained tissue sections were used to define regions of tissue coagulation. Single sonications with the 90 degrees tubular applicator (9-15 W, 12 min, 8 MHz) produced coagulated zones covering an 80 degrees wedge of the prostate extending from 1-2 mm outside the urethra to the outer boundary of the gland (16 mm radial coagulation). Single sonications with the planar applicator (15-20 W, 10 min, approximately 8 MHz) generated thermal lesions of approximately 30 degrees extending to the prostate boundary. Multiple sequential sonications (sweeping) of a planar applicator (12 W with eight rotations of 30 degrees each) demonstrated controllable coagulation of a 270 degrees contiguous section of the prostate extending to the capsule boundary. The feasibility of using highly directional transurethral ultrasound applicators with rotational capabilities to selectively coagulate regions of the prostate while monitoring and controlling the treatments with MRTI was demonstrated in this study.

Feasibility of salvage interstitial microwave thermal therapy for prostate carcinoma following failed brachytherapy: studies in a tissue equivalent phantom

Thermal therapy is an experimental treatment to destroy solid tumours by heating them to temperatures ranging from 55 degrees C to 90 degrees C, inducing thermal coagulation and necrosis of the tumour. We are investigating the feasibility of interstitial microwave thermal therapy as a salvage treatment for prostate cancer patients with local recurrence following failed brachytherapy. Due to the electrical and thermal conductivity of the brachytherapy seeds, we hypothesized that the seeds could scatter the microwave energy and cause unpredictable heating. To investigate this, a 915 MHz helical antenna was inserted into a muscle-equivalent phantom with and without brachytherapy seeds. Following a 10 W, 5 s input to the antenna, the temperature rise was used to calculate absorbed power, also referred to as specific absorption rate (SAR). Plane wave models based on Maxwell's equations were also used to characterize the electromagnetic scattering effect of the seeds. In addition, the phantom was heated with 8 W for 5 min to quantify the effect of the seeds on the temperature distribution during extended heating. SAR measurements indicated that the seeds had no significant effect on the shape and size of the SAR pattern of the antenna. However, the plane wave simulations indicated that the seeds could scatter the microwave energy resulting in hot spots at the seed edges. Lack of experimental evidence of these hot spots was probably due to the complex polarization of the microwaves emitted by the helical antenna. Extended heating experiments also demonstrated that the seeds had no significant effect on the temperature distributions and rates of temperature rise measured in the phantom. The results indicate that brachytherapy seeds are not a technical impediment to interstitial microwave thermal therapy as a salvage treatment following failed brachytherapy.

Theoretical modelling, experimental studies and clinical simulations of urethral cooling catheters for use during prostate thermal therapy

Urethral cooling catheters are used to prevent thermal damage to the urethra during thermal therapy of the prostate. Quantification of a catheter's heat transfer characteristics is necessary for prediction of the catheter's influence on the temperature and thermal dose distribution in periurethral tissue. Two cooling catheters with different designs were examined: the Dornier Urowave catheter and a prototype device from BSD Medical Corp. A convection coefficient, h. was used to characterize the cooling ability of each catheter. The value of the convection coefficient (h = 330 W m(-2) C(-1) for the Dornier catheter. h = 160 W m(-2) C(-1) for the BSD device) was obtained by comparing temperatures measured in a tissue-equivalent phantom material to temperatures predicted by a finite element method simulation of the phantom experiments. The coefficient was found to be insensitive to the rate of coolant flow inside the catheter between 40 and 120 ml min(-1). The convection coefficient method for modelling urethral catheters was incorporated into simulations of microwave heating of the prostate. Results from these simulations indicate that the Dornier device is significantly more effective than the BSD catheter at cooling the tissue surrounding the urethra.

Models and measurements of light intensity changes during laser interstitial thermal therapy: implications for optical monitoring of the coagulation boundary location

We have developed a multi-region spherical Monte Carlo (MC) model to simulate the dynamic changes in light intensity measured during laser interstitial thermal therapy (LITT). Model predictions were validated experimentally in tissue-simulating albumen phantoms with well-characterized optical properties that vary dynamically with LITT in a way similar to tissue. For long treatments (2.5 W, approximately 1800 s), the transient light intensity changes demonstrated better qualitative agreement with a three-region MC model (with an inner layer of fully coagulated optical properties, a middle layer of partially coagulated properties and an outer region of native properties); for short treatments (4 W, approximately 240 s), better qualitative agreement was seen with a two-region MC model (with an inner layer of fully coagulated properties and outer region of native properties). These differences were attributed to differences in coagulation formation during low- and high-powered heating regimes, respectively. At the end of heating, a three-region coagulation zone was observed for both heating schemes. Quantitatively, final light intensity changes at the end of heating were compared with changes predicted by both two- and three-region MC for the same experimentally measured coagulation size and found to agree within approximately 30% for both models. The developed MC model helps lend insight into the nature of thermal coagulation events occurring for low and high power LITT irradiation schemes.