The accuracy of temperature measurement from within an interstitial microwave antenna

Implications of using thermocouple thermometry in 27 MHz capacitively coupled interstitial hyperthermia

The 27 MHz Multi Electrode Current Source (MECS) interstitial hyperthermia system uses segmented electrodes, 10-20 mm long, to steer the 3D power deposition. This power control at a scale of 1-2 cm requires detailed and accurate temperature feedback data. To this end seven-point thermocouples are integrated into the probes. The aim of this work was to evaluate the feasibility and reliability of integrated thermometry in the 27 MHz MECS system, with special attention to the interference between electrode and thermometry and its effect on system performance. We investigated the impact of a seven-sensor thermocouple probe (outer diameter 150 microns) on the apparent impedance and power output of a 20 mm dual electrode (O.D. 1.5 mm) in a polyethylene catheter in a muscle equivalent medium (sigma 1 = 0.6 S m-1). The cross coupling between electrode and thermocouple was found to be small (1-2 pF) and to cause no problems in the dual-electrode mode, and only minimal problems in the single-electrode mode. Power loss into the thermometry system can be prevented using simple filters. The temperature readings are reliable and representative of the actual tissue temperature around the electrode. Self-heating effects, occurring in some catheter materials, are eliminated by sampling the temperature after a short power-off interval. We conclude that integrated thermocouple thermometry is compatible with 27 MHz capacitively coupled interstitial hyperthermia. The performance of the system is not affected and the temperatures measured are a reliable indication of the maximum tissue temperatures.

A comparison of transurethral and transrectal microwave hyperthermia in poor surgical risk benign prostatic hyperplasia patients

From 1987 to 1989, 36 poor surgical risk patients with benign prostatic hyperplasia (BPH) were treated with transrectal or transurethral microwave hyperthermia. Most of the 36 patients treated (85%) had severe signs and symptoms of urinary outflow obstruction. Of the patients 22 (61%) underwent transrectal and 14 (39%) underwent transurethral hyperthermia. Followup ranged from 10 to 28 months (mean 19 months) for the transrectal hyperthermia group and 7 to 16 months (mean 10 months) for the transurethral hyperthermia group. The patients were given 6 hyperthermia sessions of 30 minutes each with the temperature controlled on the rectal or urethral surface at 45C. Hyperthermia was well tolerated with mild acute toxicity and no late complications were observed. In the important subjective and objective parameters, major improvement was noted more frequently in the 14 transurethral than in the 22 transrectal hyperthermia treated patients (p less than 0.05). The Food and Drug Administration severity score, prostate volume, post-voiding residual volume and urethral flow showed substantial improvement in 79, 86, 79 and 79%, respectively, of the 14 transurethral hyperthermia treated patients compared to 41, 45, 82 and 82%, respectively, for the 22 transrectal hyperthermia treated patients. A prospective randomized trial comparing transrectal and transurethral hyperthermia is required to define the role of each treatment mode in patients with BPH.

Heating characteristics of a helical microwave applicator for transurethral hyperthermia of benign prostatic hyperplasia

A new applicator for intraurethral hyperthermic treatment of benign prostatic hyperplasia is described. The applicator uses an insulated helical antenna wound on the outer surface of a silicone urological (Foley) balloon catheter. The balloon catheter assures rapid and reproducible localization of the antenna in the prostatic urethra. Two small cannulae are fixed to the exterior surface of the applicator. One holds a temperature control sensor at a fixed location, the other is used to map temperature along the applicator. Two-dimensional SAR and steady-state temperature distributions measured in a plane tangent to the applicator in a tissue-equivalent phantom are presented, as well as longitudinal temperature distributions measured in situ at the applicator-urethral interface. Prostatic temperatures were also measured intraoperatively. The applicator appears to be capable of elevating temperature to greater than 42 degrees C in a cylindrically symmetric volume of about 4 cm length and about 0.5 cm radial penetration surrounding the antenna. The heating characteristics of this applicator are similar to an earlier design that employed an array of three dipoles. The helical applicator is narrower, more flexible and simpler to use than the earlier design.

Transurethral microwave hyperthermia for benign prostatic hyperplasia: preliminary clinical and pathological results

Transurethral microwave hyperthermia is a new conservative treatment modality for benign prostatic hyperplasia. We treated 15 patients with 915 MHz. microwaves delivered transurethrally by a helical applicator. Of the patients 12 showed substantial objective and subjective improvement of obstructive outflow parameters. Significant improvement in objective study parameters included increased mean flow rate (p less than 0.00021), decreased mean residual volume (p less than 0.00001) and decreased mean prostatic volume (p less than 0.0077). Analysis of patterns of failure showed chronic bladder atony, prostate asymmetry and middle lobe configuration as important factors that could explain the failure of hyperthermia in 3 patients. Toxicity was mild, consisting of bladder spasms, perineal pain, dysuria and hematuria. Hyperthermia-induced pathological changes in prostatic tissues, causing periurethral shrinking and secondary dilatation of the prostatic urethra, are described. The reported clinical results of this phase I study are preliminary due to the short followup. A phase II study to optimize transurethral hyperthermia currently is underway. A phase III study is to be phased in comparing hyperthermia with transurethral resection of the prostate.

Transurethral hyperthermia for benign prostatic hyperplasia: preliminary clinical results

A total of 21 patients with biopsy proved benign prostatic hyperplasia underwent treatment on a pilot protocol involving intracavitary transurethral radiating microwave (630 or 915 MHz.) antenna hyperthermia. Acute and subacute toxicity was mild and consisted primarily of bladder spasm (26% of the patients), hematuria (23%) and dysuria (9%), none of which significantly limited the achievement of desired temperatures during the treatment sessions. No chronic treatment-related morbidity or mortality was observed. Detailed thermal mapping, performed along the course of the prostatic urethra, recorded temperatures of 43C or more at greater than 75% of the loci. Highly significant increases in urine flow rate, decrease in post-void residual urine capacity and decrease in frequency of nocturia were observed. A marginally significant decrease in prostate volume was noted and, with a median followup of 12.5 months, only 3 patients have required subsequent prostatic resection. Transurethral hyperthermia represents a safe and promising outpatient approach to treatment of benign prostatic hyperplasia, particularly for patients who are not candidates for conventional surgical approaches because of medical or personal reasons. Further studies with the goal of optimizing the technique appear to be warranted, although long-term results would be best evaluated with prospective phase 3 trials.

Microwave applicator for transurethral hyperthermia of benign prostatic hyperplasia

An applicator for heating the prostate gland using a transurethral approach is described. This technique uses three microwave antennas and a thermometry sensor attached to the outer surface of a balloon (Foley) type urological catheter. Each microwave antenna also includes a built-in thermistor to control temperature and balance power. The balloon catheter assures rapid and reproducible localization of the antennas in the prostatic urethra. The two-dimensional SAR and steady-state temperature distributions surrounding the applicator in tissue equivalent phantom are reported. Longitudinal temperature distributions measured in situ at the applicator-urethral interface and the longitudinal and radial temperature distributions measured in normal canine prostate are presented and discussed. The technique appears to be capable of elevating temperature to greater than 42 degrees C in a cylindrically symmetrical volume up to 5 cm length and 0.5 cm radial penetration surrounding the applicator.

A technique for combining microwave hyperthermia with intraluminal brachytherapy of the oesophagus

A technique for combining microwave hyperthermia with 192Ir brachytherapy for the treatment of oesophageal carcinoma is described. This approach uses an intraluminal afterloading applicator and up to six microwave antennae to deliver both hyperthermia and brachytherapy, with minimal modification of the existing procedure for brachytherapy alone. Each microwave antenna includes a built-in thermistor to control temperature and balance power. Longitudinal temperature distributions were measured in situ from within the applicator, and at the applicator-tissue interface in vivo. Two-dimensional SAR and steady-state temperature distributions measured in muscle-equivalent phantom are presented and discussed. The technique appears to be capable of elevating tissue temperature to greater than 42 degrees C in a radially symmetric volume of length greater than 5 cm, with radial penetration of 0.5 cm. The clinical technique is relatively simple and well tolerated.