RF dosimetry: a comparison between power absorption of female and male numerical models from 0.1 to 4 ghz.

Realistic numerical models of human subjects and their surrounding environment represent the basic points of radiofrequency (RF) electromagnetic dosimetry. This also involves differentiating the human models in men and women, possibly with different body shapes and postures. In this context, the aims of this paper are, firstly, to propose a female dielectric anatomical model (fDAM) and, secondly, to compare the power absorption distributions of a male and a female model from 0.1 to 4 GHz. For realizing the fDAM, a magnetic resonance imaging tomographer to acquire images and a recent technique which avoids the discrete segmentation of body tissues into different types have been used. Simulations have been performed with the FDTD method by using a novel filtering-based subgridding algorithm. The latter is applied here for the first time to dosimetry, allowing an abrupt mesh refinement by a factor of up to 7. The results show that the whole-body-averaged specific absorption rate (WBA-SAR) of the female model is higher than that of the male counterpart, mainly because of a thicker subcutaneous fat layer. In contrast, the maximum averaged SAR over 1 g (1gA-SAR) and 10 g (10gA-SAR) does not depend on gender, because it occurs in regions where no subcutaneous fat layer is present.

Development of numerical phantoms by MRI for RF electromagnetic dosimetry: a female model.

Numerical human models for electromagnetic dosimetry are commonly obtained by segmentation of CT or MRI images and complex permittivity values are ascribed to each issue according to literature values. The aim of this study is to provide an alternative semi-automatic method by which non-segmented images, obtained by a MRI tomographer, can be automatically related to the complex permittivity values through two frequency dependent transfer functions. In this way permittivity and conductivity vary with continuity--even in the same tissue--reflecting the intrinsic realistic spatial dispersion of such parameters. A female human model impinged by a plane wave is tested using finite-difference time-domain algorithm and the results of the total body and layer-averaged specific absorption rate are reported.

A semi-automatic method for developing an anthropomorphic numerical model of dielectric anatomy by MRI.

Complex permittivity values have a dominant role in the overall consideration of interaction between radiofrequency electromagnetic fields and living matter, and in related applications such as electromagnetic dosimetry. There are still some concerns about the accuracy of published data and about their variability due to the heterogeneous nature of biological tissues. The aim of this study is to provide an alternative semi-automatic method by which numerical dielectric human models for dosimetric studies can be obtained. Magnetic resonance imaging (MRI) tomography was used to acquire images. A new technique was employed to correct nonuniformities in the images and frequency-dependent transfer functions to correlate image intensity with complex permittivity were used. The proposed method provides frequency-dependent models in which permittivity and conductivity vary with continuity--even in the same tissue--reflecting the intrinsic realistic spatial dispersion of such parameters. The human model is tested with an FDTD (finite difference time domain) algorithm at different frequencies; the results of layer-averaged and whole-body-averaged SAR (specific absorption rate) are compared with published work, and reasonable agreement has been found. Due to the short time needed to obtain a whole body model, this semi-automatic method may be suitable for efficient study of various conditions that can determine large differences in the SAR distribution, such as body shape, posture, fat-to-muscle ratio, height and weight.

Numerical and experimental analysis of e-field scattering from the ground for base station antennae.

Based on the numerical determination of the complete irradiation volume of a commercial RBS antenna--performed using the FDTD method and the Kirchhoff integral formula for near to far field transformation--open site estimations of the electric field are made and compared with experimentally measured values. To describe the actual behaviour of the radiation field, the inherently complex phasic nature of plane waves is taken into account, together with their two independent states of polarisation. This information is contained in the radiation pattern previously deduced. Moreover, a reflected contribution from flat ground is introduced, along with the line-of-sight ray. Amplitude and phase of the reflected wave are calculated using Fresnel formulae for stratified ground and two polarisation states, i.e. normal and parallel to the plane of incidence. Good agreement with measured values is achieved only by using such assumptions.

Prognostic significance of cervical lymph nodes density evaluated by contrasted computer tomography in head and neck squamous cell carcinoma treated with hyperthermia and radiotherapy.

INTRODUCTION: A correlation between node hypodensity assessed by means of computer tomography (CT) and resistance to both chemotherapy and radiation therapy (XRT) in advanced head and neck tumours has been suggested in the literature. The outcome of a retrospective series of 50 patients with head and neck squamous cell carcinoma (SCC) and cervical nodes metastases treated with combined hyperthermia (HT) and definitive XRT was reviewed to investigate if node density confirmed its prognostic value. MATERIALS AND METHOD: For all patients, a pre-treatment contrasted CT scan performed at the Institution between 1987-1993 was available. The density of the largest node (> 2cm) was compared to that of adjacent nuchal muscles. Nodes with hypodense areas present in more than one third of the total volume were considered necrotic nodes. RESULTS: The patients were divided in two groups (with and without nodal necrosis), well balanced in terms of potential prognostic factors. No significant difference in overall nodal response rate, local control and survival was found in the two groups of patients. CONCLUSION: Nodal density assessed by contrasted CT scan in the series did not result in a significant prognostic factor in patients with SCC node metastases treated with HT and XRT. It is suggested that HT could act as a radiosensitizer in the treatment of hypodense (at CT scan) metastatic nodes overcoming the radioresistance of necrotic, presumably hypoxic nodal metastases.

Hyperthermia and radiotherapy for inoperable squamous cell carcinoma metastatic to cervical lymph nodes from an unknown primary site.

INTRODUCTION: Neck node metastases from an unknown primary carcinoma represent an infrequent but challenging problem for oncologists. The management of such patients is controversial, but radiotherapy alone or as part of a multimodal approach is often indicated. Patients with inoperable lesions usually receive radiotherapy alone at palliative doses. In an attempt to increase local control in patients with locally advanced neck disease from an unknown primary carcinoma, local hyperthermia was combined with definitive radiotherapy. MATERIAL AND METHODS: Between 1982 and 1993, radiotherapy and local microwave hyperthermia were used to treat 15 patients with metastatic neck nodes from an unknown primary site. The patients had previously undergone only biopsy or fine needle biopsy, and showed no signs of metastases beyond the clavicle. Radiation to the nodes and the potentially primary sites in the head and neck was delivered by a 6 MV linear accelerator or a Cobalt 60 unit, to a total dose of 57.50-74.40 Gy (median 70 Gy). Hyperthermia was added using a BSD 1000 unit at an operating frequency of 280-300 MHz for 2-7 sessions (mean 3.1; median 2) at a desired minimum temperature of 42.5 degrees C. Two patients also received i.v. cisplatin 20 mg/m2/week as a radiosensitizer. RESULTS: Nine patients achieved a complete, and four a partial response for an overall response rate of 86.5%. Acute and late toxicity was mild: four patients experienced pain during hyperthermia, two moist cutaneous desquamation, and one cutaneous necrosis. The actuarial probability of maintaining local control at 5 years is 64.5% and the actuarial overall survival 29%. Five patients developed distant metastases and died of disease, two experienced nodal recurrence and two died of other unrelated causes. CONCLUSION: The addition of local microwave hyperthermia to radiotherapy in the treatment of metastatic squamous cell carcinoma of the neck in patients with an unknown primary site leads to good local control with moderate toxicity. No definite conclusions are possible because of the small number of patients involved in this phase II trial.

An automated method for mapping human tissue permittivities by MRI in hyperthermia treatment planning.

This paper presents an automatic method to obtain tissue complex permittivity values to be used as input data in the computer modelling for hyperthermia treatment planning. Magnetic resonance (MR) images were acquired and the tissue water content was calculated from the signal intensity of the image pixels. The tissue water content was converted into complex permittivity values by monotonic functions based on mixture theory. To obtain a water content map by MR imaging a gradient-echo pulse sequence was used and an experimental procedure was set up to correct for relaxation and radiofrequency field inhomogeneity effects on signal intensity. Two approaches were followed to assign the permittivity values to fat-rich tissues: (i) fat-rich tissue localization by a segmentation procedure followed by assignment of tabulated permittivity values; (ii) water content evaluation by chemical shift imaging followed by permittivity calculation. Tests were performed on phantoms of known water content to establish the reliability of the proposed method. MRI data were acquired and processed pixel-by-pixel according to the outlined procedure. The signal intensity in the phantom images correlated well with water content. Experiments were performed on volunteers' healthy tissue. In particular two anatomical structures were chosen to calculate permittivity maps: the head and the thigh. The water content and electric permittivity values were obtained from the MRI data and compared to others in the literature. A good agreement was found for muscle, cerebrospinal fluid (CSF) and white and grey matter. The advantages of the reported method are discussed in the light of possible application in hyperthermia treatment planning.

Quantitative colorimetric analysis of liquid crystal films (LCF) for phantom dosimetry in microwave hyperthermia.

A fully quantitative analysis of liquid crystal film (LCF) color patterns, in phantom thermal dosimetry for microwave hyperthermia, is presented. An accurate determination of absorption rate density (ARD) is achieved by color image computer processing. This work is proven to be an improvement upon the semi-quantitative or qualitative descriptions of LCF colors performed essentially by visual analysis of photographs. Temperature-induced chromatic distributions are acquired as R, G, B (red, green, blue) signals by a CCD camera connected to a PC frame grabber board. These data, stored into three 512 x 512 memory buffers, are then converted to H, S, I (hue, saturation, intensity) colorimetric system. Provided a suitable calibration of the LCF, the H quantity can be transformed to temperature using a monotonic relationship. In this way, a temperature accuracy lower than 0.2 degrees C and a spatial resolution less than 1 mm are obtained. A sequence of thermal maps can be acquired and stored on disk at a maximum rate of 1 image/2 s, and then the ARD is calculated at each pixel of the map using the least squares method.

Absorption rate density (ARD) computation in microwave hyperthermia by the finite-difference time-domain method.

A mathematical model has been developed, which is able to predict power distributions in biological tissues during microwave hyperthermia delivered by waveguide applicators. The numerical solutions of Maxwell's equations was obtained by the finite-difference time-domain (FDTD) technique. Two improvements with respect to the standard implementation of FDTD were introduced: a separation between the source and load calculations (based on the Schelkunoff equivalence principle) and a simple routine that automatically recognises the steady state. Two commercially available applicators, a dual-ridged and a side-loaded waveguide, were modelled using their theoretical aperture fields. The absorption rate density (ARD) distributions delivered by these applicators were measured through phantom thermal dosimetry and compared with the patterns estimated by the simulation.

Intracavitary hyperthermia: construction and heat patterns of individualized vaginal prototype applicators.

Patients who have inoperable or previously irradiated cervical recurrences, especially if very confined and alone, pose a challenging problem to the clinician. Low-dose irradiation and hyperthermia seems to be a promising method for salvage treatments in previously treated patients. In order to heat the vaginal apex, individualized prototype applicators utilizing commercial microwave antennas were constructed and tested in muscle equivalent phantom. The technical construction of vaginal applicators followed Pierquin's procedure for gynaecological brachy-therapy moulds: (1) a true negative vaginal print in alginate was made; (2) a positive mould in plaster of Paris was then constructed; (3) a negative vaginal print was produced in plaster of Paris and (4) heat moulding of a 0.1 cm thick sheet of cellulose acetate on the plaster mould was accomplished. Interstitial microwave antennas were inserted within the applicator and kept in place using silk wires or Teflon rings. The external part of the applicator was thermally sealed and protected with silicone. This was particularly useful in cases where an inner cooling system was required. Heat patterns in tissue equivalent phantom materials were determined using non-perturbing probes and infrared thermography. An illustrative clinical case is described.

Absorbed power distributions from single or multiple waveguide applicators during microwave hyperthermia.

A theoretical model has been developed to calculate the power distributions in biological-like tissues in direct contact with waveguide applicators. A numerical solution of the coupled integral equations is performed by means of their transformation into matrix equations by using the method of moments, where the biological tissue and the applicator are described by two independent matrices. The model permits an accurate evaluation of the electric field at the aperture. The application of this method for the evaluation of arrays is also discussed. Power contour plots produced by applicators routinely used in clinics have been computed and compared with experimental results obtained with tissue-equivalent phantoms for three typical bodies: homogeneous, stratified and inhomogeneous.