A pre-clinical phantom comparison of tissue harmonic and brightness mode imaging for application in ultrasound guided prostate brachytherapy.

PURPOSE: The current practice of prostate brachytherapy utilizes the brightness (B) mode ultrasound imaging for volume definition and needle guidance. However, tissue harmonic (H) mode available with new scanners has shown the improved image quality. The aim of this study was to perform a pre-clinical phantom evaluation of harmonic imaging as an alternative to B mode in prostate brachytherapy. METHODS: Performance characteristics viz. dead zone, depth of penetration, geometric accuracy, spatial resolution, tissue to clutter ratio (TCR) and signal to noise ratio (SNR), were compared between two modes using an in-house phantom. Images were acquired under the same settings except the gain; which is higher for the H mode than that of B mode. A qualitative comparison between two modes was also performed using commercial CIRS053 phantom. RESULTS: Dead zone, depth of penetration and geometric accuracy were respectively <1 mm, >8 cm and <1% for both modes. Relative TCR, SNR and the spatial resolution were improved in H mode compared with B mode. Images with CIRS053 phantom in H mode demonstrate sharper boundaries for prostate and urethra, freedom from background clutter, and better identification of the brachytherapy needles. CONCLUSIONS: This study indicates the superiority of H over B mode, in terms of spatial resolution, relative contrast, and overall image quality. Thus H mode has the potential benefit in prostate brachytherapy. This study provides the basis to move forward to investigate whether the superior image quality observed in the laboratory can be translated into a higher treatment quality for the patient.

Independent corroboration of monitor unit calculations performed by a 3D computerized planning system.

The checking of monitor unit calculations is recognized as a vital component of quality assurance in radiotherapy. Using straightforward but detailed computer-based verification calculations it is possible to achieve a precision of 1% when compared with a three-dimensional (3D) treatment planning system monitor unit calculation. The method is sufficiently sensitive to identify significant errors and is consistent with current recommendations on the magnitude of uncertainties in clinical dosimetry. Moreover, the approach is accurate in the sense of being highly consistent with the validated 3D treatment planning system's calculations.

The investigation and rectification of field placement errors in the delivery of complex head and neck fields.

PURPOSE: To evaluate a novel technique for resolving field placement errors into their components and to quantify the improvement in accuracy potentially achievable by translation and rotation of the radiation beam. METHODS AND MATERIALS: One hundred and eighty-five films (both simulator and portal) from seventeen patients receiving radiotherapy to the head and neck region were analyzed in pairs. The computer based comparisons of complex fields with curved edges employed the intersections of perpendiculars from two reference points with the field periphery to define field match points. Field placement errors were resolved into those due to patient motion within the immobilization shell and those due to incorrect beam position, orientation, or shape. RESULTS: The median and the 95 percentile of the distribution of differences between prescribed (simulator) fields and treated (portal) fields referenced to the patients anatomy were 4.4 mm and 8.9 mm, respectively. The analysis suggests that with appropriate translation and rotation of the beam with respect to the immobilization shell these figures could be reduced to 3.1 mm and 8.2 mm, respectively, confirming the large contribution of patient motion within the shell to field placement accuracy. Comparisons between treated fields indicated smaller variability during treatment than between simulation and treatment. CONCLUSION: The perpendicular intersection method described here was found appropriate for the identification of field match points. The distributions of field placement errors were similar to those in a published study of straight edged fields. Translation and rotation of the applied field with respect to the immobilization shell would generally result in only a small improvement in field placement accuracy.

On the field-size dependence of relative output from a linear accelerator.

The radiation output in air on the central axis of a linac photon beam has been modeled as the sum of two components. These are a point source representing radiation direct from the target and a distributed source representing scatter in the flattening filter and primary collimator. By fitting only two parameters, the ratio of the two components for a 20 x 20 field and a width parameter for the distributed source this semi-empirical model describes the relative outputs of square, symmetric rectangular, and asymmetric rectangular fields with an average error of 0.25% for the field sizes studied.

Matching photon and electron fields in the treatment of head and neck tumors.

Radiotherapy of head and neck tumors frequently involves joining photon and electron fields. In such situations narrow penumbras combined with relatively small positioning errors can lead to significant "hot" and "cold" spots in the vicinity of the join-up. The objective of this work was to devise penumbra spreading techniques which lead to a relatively uniform dose distribution in the join-up region of these fields and which reduce the effect of positioning errors on dose uniformity. A stepped edge attenuator was used to obtain a wider penumbra for the 4-MV x-ray beam and a Lucite scatterer was used for the 10-MeV electron beam. The resulting composite beam profiles from these "modified" abutting photon and electron fields are provided and the effects of positioning errors on dose uniformity across the junction are illustrated. These profiles are compared with those resulting from "unmodified" adjacent electron and photon beams.

A proposal for the classification of field placement errors in radiotherapy.

There is an increasing awareness of the high frequency of field placement errors occurring in radiotherapy. If such errors are to be rectified systematically to provide a sustainable improvement in field placement accuracy over a course of treatment, the origins of the errors require unambiguous identification. From an examination of the procedures taking place between the exposure of the prescription and treatment films, we propose resolving field placement errors into four groups: patient motion, entrance field location, field shape or size, and beam direction. We also suggest means by which the components of clinically observed field placement errors may be resolved in practice.

The sensitivity of the thoracolumbar spinal cord of the mouse to hyperthermia.

Twelve millimeters of the thoracolumbar spinal cord of mice has been treated with a radiofrequency heating system which has been shown previously to produce localized and controllable elevation of temperature. The severity of neurological damage was assessed by measuring the reduction in the reflex leg extension of the hind legs of the mice from video-recorded images and by scoring the performance of the mice by a negative geotaxis test. The response to treatment was rapid with maximum paralysis occurring within a few days after treatment. Only minor symptoms were observed in those animals which had not developed paralysis within 2 weeks. A 40% reduction in the reflex leg extension was chosen as an end point, and the percentage of mice having reached the end point for different thermal doses was determined in groups of nine mice. The ED50 for heating for 1 h was 43.1 degrees C and for heating at 45 degrees C was 10.8 min. An increase in temperature by 1 degree C required a decrease in time by a factor of 2.25 to produce the same effect. Thermotolerance was observed 24 h after preheating at 45 degrees C for 1.9 min with a thermotolerance ratio of 1.7. The rapid response and high sensitivity of the spinal cord will have to be taken into consideration in the clinical application of hyperthermia.

Quantitative computed tomography of the brain with xenon enhancement: a phantom study with the GE9800 scanner.

A recently proposed application of quantitative computed tomography is in the study of cerebral blood flow and partition coefficient using stable xenon as a freely diffusible, radio-opaque tracer. Central to the method is the calibration factor describing the relationship between CT number and xenon concentration in the brain. In this paper we examine the influence of temporal fluctuations, kVp, radial position and beam hardening on this calibration factor through the analysis of a series of phantom measurements. We conclude that under clinically realistic conditions and with correlations for temporal fluctuations, the error associated with the calibration factor is less than 2%. Furthermore, errors of this magnitude translate into errors of less than 3% in derived blood flow and partition coefficient values obtained using xenon-enhanced computed tomography.

The role of noise in the measurement of cerebral blood flow and partition coefficient using xenon-enhanced computed tomography.

Monte Carlo simulations have been used to study the accuracy which can be expected in the quantification of blood flow and the partition coefficient using xenon-enhanced computed tomography in the presence of noise. We have demonstrated that the markedly asymmetric frequency distribution of estimates increases in size rapidly with an increase in the standard error of the input CT data. On the basis of our results, we recommend that controllable sources of noise (eg. CT number drift) be corrected and that estimates be obtained by averaging CT data and then fitting, rather than averaging blood flow and partition coefficients derived from individual pixels, as the latter procedure results in the introduction of considerable bias.

Physical aspects of external beam radiotherapy for the treatment of malignant pleural mesothelioma.

The optimization of radiotherapy for the treatment of malignant mesothelioma highlights many of the currently outstanding problems in clinical radiation physics. The experimental investigation of an intuitively attractive irradiation technique with combined photon and electron beams using a specially constructed phantom has established that, due to the penetration in low density material of both primary electrons and those secondary to photon irradiation, the normal lung tissue is not spared to any significant degree by such a technique. Furthermore, great care needs to be exercised in the treatment planning calculations for this approach if absolute dosimetry errors as large as 50% are to be avoided.

Thermal dosimetry of spinal cord heating in the mouse.

Three systems for the localized heating of the spinal cord of the mouse have been evaluated by measuring the temperatures in the spinal canal (Tsp); at a reference location dorsal to the spine (Tdo), and by numerically calculating temperature distributions throughout two-dimensional transverse cross-sections through the middle of the heated region. The systems assessed were water bath heating alone, water bath-rf combination and rf heating alone with oblique, dorsally located electrodes. It has been established that (1) for all systems delta T (where delta T = Tdo-Tsp) decreased throughout a 1 h heating period-this was attributed to changes in blood flow; (2) there existed a considerable variation in the experimental value of delta T, particularly for rf heating. The resulting error in the estimation of Tsp from a measured value of Tdo can be reduced by making use of the observed correlation between delta T and the slope of a temperature decay curve measured at the beginning of the heating period; (3) rf alone best spares adjacent visceral and superficial tissues from significant elevation of temperature.

Precision of determining compliance with prescribed fields from conventional portal films.

Three hundred and seventy measurements of field placement errors (FPEs) have been made by a total of 16 observers on 20 prescription-treatment film pairs taken during routine radiotherapy for cancer of the prostate. Analysis of the distributions of the measured FPEs has yielded the precision of the measurement under a variety of conditions. We report here the influence on the precision of determining FPEs of the following factors: the clinical duties of the observers, the quality of the treatment film, the relative magnification of prescription and treatment films, and whether double-exposure techniques were employed.

Minimizing the self-heating artefacts due to the microwave irradiation of thermocouples.

The self-heating of metallic thermocouples in therapeutic microwave fields has long been recognized as a source of temperature artefacts in clinical hyperthermia dosimetry. We examine several techniques by which the probe and tissue heating artefacts resulting from self-heating may be quantitatively assessed, and discuss these in the context of their applicability to clinical hyperthermia.

Heat production in microwave-irradiated thermocouples.

It has been known for some time that the irradiation of metallic thermometers by microwaves during clinical hyperthermia can lead to artifactual readings. We describe here a series of measurements in which this effect has been quantitatively studied. In particular, the data yield values for the conversion coefficient describing the rate of heat production per unit length of a thermocouple array per watt applied power which can be compared with the rate of heat production in the same volume of tissue. The degree of artifact in the temperature recording depends on the thermal resistance of the protective materials surrounding the array, and this thermal resistance has also been determined. It has been shown that measures taken to reduce the temperature artifact do not compromise the response time of the probe.

A search for nonthermal effects of 434 MHz microwave radiation on whole human blood.

Whole human blood was subjected to a microwave environment at 434 MHz for 6 hr with external electric fields corresponding to free space power densities up to 598 mW cm-2 and the levels of hemoglobin, sodium, and potassium in the plasma were monitored. Under geometrical conditions in which the field strength within the samples was unknown, measurements indicated increased red cell membrane fragility following irradiation. It was not possible to exclude localized heating as an explanation of this effect. However, with a known and reasonably uniform electric field distribution within spherical specimens, increased membrane fragility was not observed. We are therefore unable to confirm previously reported results which indicate a nonthermal deleterious effect of microwave radiation on erythrocytes.