George M. Samaras

Measurement of Radio Frequency Permittivity of Biological Tissues with an Open-Ended Coaxial Line: Part II - Experimental Results

The permittivity of several reference liquids and selected biological tissues in vivo was measured in the frequency range from 0.01 to 1 GHz. Open-ended coaxial line sensors and computer-controlled network analyzer systems, described in a companion paper, were used. The results were analyzed and compared with the estimated uncertainties. The described method proved to be convenient, fast, and relatively accurate for in vivo measurements.

Interstitial microwave hyperthermia for brain tumors

The technical feasibility and clinical safety of interstitial microwave hyperthermia was evaluated in six patients with glioblastoma and malignant astrocytoma. Prior to entry into the study, each patient had received surgery, radiation and nitrosourea chemotherapy. All patients were implanted at open craniotomy with a flexible microwave radiator/sensor (o.d. 1.5 mm) and transcutaneously connected to a 2450 MHz microwave generator. Intraoperative thermal field plots and cooling curves were obtained with the aid of non-perturbing probes (o.d. 1.2 mm) perpendicularly driven into the tumor at fixed radial distances from the central antenna. In comparison to similar measurements carried out in normal feline brains, human gliomas were unable to efficiently dissipate heat as demonstrated by doubling of the effective diameter of the thermal field to 4 cm and by prolongation of the decay time in all cooling curves. Patients were also implanted with subarachnoid ICP monitors over the contralateral hemisphere. Two postoperative treatments were given at 45°C for 60 min on the night of surgery and 48 hr later. No patient was aware of power on/ power off, there were no permanent neurologic sequelae and there were no significant changes in the ICP. Power was manually controlled with visual feedback in the first three patients and automatically controlled by a computer-based system in the final three patients. Four of the six patients have lived 18 months after implantation and two of these have negative CT scans at 18 and 27 months since recurrence. It appears that interstitial microwave hyperthermia is both feasible and safe within the intracranial cavity and that combined interstitial irradiation and hyperthermia deserves clinical study.

Techniques for Uniform and Replicable Microwave Hyperthermia of a Model Mouse Carcinoma

Two techniques for localized 2450-MHz hyperthermia of experimental mouse cancers are described. In the far-field approach superficial tumors are encapsulated in 5-cm mold-formed spheres of semi-solid phantom material, then placed in an anechoic chamber on an equipower surface. In the applicator approach, tissues are immersed in a temperature-controlled tissue-equivalent liquid bolus, and are irradiated by time-multiplexed parallel-opposed beams. Both techniques feature microwave bolusing for improved coupling and tumor heating uniformity.

Removable high intensity iridium-192 brain implants

A preclinical evaluation of the technical details and dosimetry for temporary high intensity192Ir brain implants is presented. The canine brain was used for this quality assurance study in which direct in vivo dose measurements were done by thermoluminescent dosimetry (TLD rods). Precise and reproducible positioning of the TLD rods and 192Ir ribbons were assured by simple accessories which can be utilized in the clinical situation. The neurosurgical procedure for this non-routine interstitial implant of the brain, suitability of type and size of afterloading cannulas and facility for firmly anchoring them to the scalp, and comparison of measured doses with computer-predicted values are details assured by the canine study. Agreement between the in vivo determination and computer-generated doses was consistently in the range 2–5%. Data derived from this preclinical evaluation are currently used in both stereotactic and non-stereotactic brain implants at our institution.Details are presented for the implant procedure, dose measurements and brachytherapy planning for multiple ribbons. The latter incorporates direct interaction on computed tomography (CT) images for a hypothetical patient case.

Correction of microwave‐induced thermistor sensor errors

Accurate and reliable thermometry is essential in the development of microwave-induced hyperthermal cancer therapy. While temperature measurements in strong electromagnetic fields usually require special sensors, this does not hold true for interstitial radiator/sensor systems. Miniature thermistors (with metallic leads) bonded to invasive microwave applicators exhibit a sensor error linearly related to the radiators transmitted power. This relationship permits thermistor sensor error correction and temperature measurements to within +/- 0.1 degrees C or better. The instrumental methods and empirical validation are presented.

Neurosurgery and clinical engineering.

Modern technology has profoundly altered the clinical practice of neurosurgery. For a wide variety of conditions, patients are being implanted with active and passive devices or treated with advanced microsurgical instrumentation. After surgery, such patients are sent to modern intensive-care units employing the latest advances in patient monitoring and computer technology. We contend that the responsibilities of the Clinical Engineer extend beyond simple installation and maintenance of equipment and systems. It is essential that he take part in the continuing education of non-technical personnel who must make use of the equipment in ways that are meaningful in the care of the patient and to the progress of clinical science. This point is illustrated by our experience with a neurosurgical intensive-care unit. It is also the thesis of this paper that the design and maintenance of increasingly sophisticated biomedical systems will benefit from the use of an interdisciplinary approach at the very inception of a project. This approach is illustrated by our current development of a multibeam microwave hyperthermia system for possible use in the treatment of brain tumors.