Evaluation and optimization of the electromagnetic performance of interstitial antennas for hyperthermia

Multivariate analysis of prognostic parameters using interstitial thermoradiotherapy (IHT-IRT): tumor and treatment variables predict outcome

PURPOSE

From January 1986 to October 1991, 90 patients with localized tumors were treated in a Phase I/II trial using low-dose 192Ir brachytherapy (IRT) plus interstitial 915 MHz microwave (MW) hyperthermia (IHT) and external beam radiotherapy (ERT). Tumors were classified as locally advanced primary (class 1: 27), recurrent (class 2: 40), metastatic (class 3: 10) and persistent (class 4: 13) lesions. The treatment sites included tumors of the head and neck (62), pelvis (26), and others (2). The mean cuboidal tumor volume was 63 cm3 (range: 8-288 cm3). Most recurrent and metastatic lesions (48) had received prior treatment including ERT.

METHODS AND MATERIALS

The treatment protocol prescribed two heating sessions (each 60 min) at 41-44 degrees C before and after IRT. One hundred sixty-one IHT sessions were evaluated. Invasive thermal data were recorded at an average of 18 sites throughout the implant volume. Several thermal variables were analyzed (e.g., averaged parameters: Tmaxav, Tmean, Tminav; index parameters: T10, T50, T90). The study was evaluated with a minimum follow up (FU) of one year. Median follow-up was 19 months.

RESULTS

At 3 months FU, a complete response (CR) was observed in 59 of 90 (66%) patients. At 12 months FU, local control (LC) was achieved in 54 of 84 (64%) evaluable patients. Ten patients developed a local and 14 a regional recurrence (REC) after achieving a CR and/or LC. At last FU, a total of 31 (34%) patients were still alive and 28 (31%) patients had relapse-free survival. For all 90 patients, the median overall survival was 20 months and the median relapse-free survival was 17 months. Overall and relapse-free survival was significantly longer for primary and persistent lesions as compared to recurrent and metastatic lesions (p = 0.002; p < 0.001). Totally 22 (24%) patients experienced acute or subacute side-effects (Grade 1: 12 patients; Grade 2: eight patients; Grade 3: two patients).

CONCLUSION

Univariate logistic regression analysis revealed significant dependencies of CR, LC and REC upon tumor parameters as well as radiation and thermal parameters. The overall and relapse free survival was associated with tumor and radiation parameters. The multivariate analysis revealed two independent predictors of CR: tumor volume and minimum tumor temperature variables. We conclude, that IHT-IRT is a safe and effective treatment. The results provide important implications for planning HT-RT studies and for defining quality assurance (QA) criteria and thermal performance standards in HT studies.

Three-dimensional theoretical temperature distributions produced by 915 MHz dipole antenna arrays with varying insertion depths in muscle tissue

Interstitial microwave antenna array hyperthermia (IMAAH) systems are currently being used in the treatment of cancer. The insertion depth of an interstitial microwave antenna, defined as the length of the antenna from the tip to the point of insertion in tissue, affects its ability to produce uniform power deposition patterns in tumor volumes. The effect of varying insertion depths on the ability of an IMAAH system to heat two theoretical tumor models was examined. Four dipole microwave antennas were implanted in a 2 x 2 cm array and driven at 915 MHz in muscle tissue. The explicit power deposition patterns were calculated for each insertion depth using known theory. The bioheat transfer equation was solved for the 3-dimensional steady-state temperature distributions in cylindrical and ellipsoidal tumor models using a finite element method. Homogeneous and nonhomogeneous blood flow models were considered. As a basis of comparison of the various temperature distributions, the volume of tumor heated to greater than or equal to 43 degrees C was calculated. Under the conditions of this study, the insertion depth was shown to have a significant effect on the ability of an IMAAH system to heat the tumor volumes. A sharp decrease in the percentage of tumor volume heated to greater than or equal to 43 degrees C was seen for insertion depths between 7.8 and 14.6 cm. At an insertion depth of 11.7 cm (3/4 lambda) there was virtually no heating of the tumor. Regions of elevated power occurred outside of the desired treatment volume, stressing the importance of adequate thermometry techniques and demonstrating the need for hyperthermia treatment planning prior to implantation of an antenna array. Plots of the power deposition patterns and the corresponding temperatures produced in the diagonal plane of the antenna arrays are present.

Experimental brain hyperthermia: techniques for heat delivery and thermometry

An experimental canine brain model was developed to assess the effects of hyperthermia for a range of time and temperature endpoints, delivered within a specified distance of an interstitial microwave antenna in normal brain. The target temperature location was defined radially at 5.0 or 7.5 mm from the microwave source at the longitudinal location of maximum heating along the antenna in the left cerebral cortex. Temperatures were measured with fiberoptic probes in a coronal plane at this location in an orthogonal catheter at 1.0 mm intervals. Six antennas were evaluated, including dipole, modified dipole, and four shorted helical antennas with coil lengths from 0.5 to 3.9 cm. Antenna performance evaluated in tissue equivalent phantom by adjusting frequency at a fixed insertion depth of 7.8 cm or adjusting insertion depth at 915 MHz showed dipoles to be much more sensitive to insertion depth and frequency change than helical antennas. Specific absorption rate (SAR) was measured in a brain/skull phantom and isoSAR contours were plotted. In vivo temperature studies were also used to evaluate antenna performance in large and small canine brain tissues. A helical antenna with a 2.0 cm coil length driven at 915 MHz was chosen for the beagle experiments because of tip heating characteristics, well-localized heating along the coil length, and heating pattern appropriate to the smaller beagle cranial vault. Verification of lesion dimensions in 3-D was obtained by orthogonal MRI scans and histology to document the desired heat effect, which was to obtain an imagable lesion with well-defined blood-brain-barrier breakdown and necrotic zones. The desired lesion size was between 1.5 to 2.5 cm diameter radially, in the coronal plane with the greatest diameter.