[Vade-mecum of helical CT scanning: estimation of doses, choice of parameters]. / Vade-mecum du scanner hélicoïdal: estimation des doses, choix des paramètres.

Implementation of EURATOM directive 97/43 requires increased scrutiny over the use CT imaging. All CT imaging centers will be required to perform routine QA procedures including measurements of exposure related to frequently performed types of CT examination. CT technologists and radiologists will be required to try and identify ways of decreasing patient exposure for CT examinations while insuring acquisition of diagnostic studies. In turn, this assumes understanding of the exposure parameters displayed on the CT images. Based on this, radiologists will be in a position to compare exposure parameters in their practice to national standards and perform necessary adjustments. Also, it should become possible to more adequately estimate patient exposure related to specific CT examinations.

[Dosimetry of small-size photon beams]. / Dosimétrie des faisceaux de photons de faibles dimensions.

A high performance standard radiotherapy treatment unit could be used to perform stereotactic radiosurgery. The dosimetric aspects of stereotactic irradiation with small size photon beams (energies from 5 to 25 MV produced by electron linear accelerator or gamma-rays produced by cobalt-60 treatment unit) are analyzed. The diameter of circular beams used are 5 to 40 millimeters wide at the isocenter of the treatment unit. The dosimetry of small x-ray fields is complicated by two factors: the relationship between detector size and field size dimensions, and the lack of lateral electron equilibrium. The large dose gradients outside the beam's central axis require dosimetry techniques with higher spatial resolution. To determine the best dosimetry system for measurements at the beam's small focal point, particularly for measurement of the field size dependent on output factors, several different detectors were investigated: ionization chamber, silicon diode, diamond detector, thermoluminescent dosimeter, and film. Ionization chamber, which presents a sensitive volume smaller than 0.02 cm3, is the most commonly recommended detector for field diameter above 8 mm. Current representative examples of dosimetric measurements for different x-ray energies, including percent depth dose, tissue maximum ratios, beam profiles (off axis ratios), and output factors, are presented and discussed. As well, the dosimetric characteristics of small photon beams are detailed.

Convolution calculations of dose in the buildup regions for high energy photon beams obliquely incident.

The central dose received from high energy photon beams that are obliquely incident on an absorber is markedly different from that absorbed when such beams are normally incident at the surface. In this work, we calculate the dose differences using a convolution of energy deposition kernels in a phantom presenting an oblique entrance surface. The dose distributions were calculated with oblique incident 1.25, 3, and 6 MeV monoenergetic and polyenergetic photon beams angled from 0 degrees to 80 degrees. In order to study the dose variations with surface obliquity, we introduced an obliquity factor. For each energy, the obliquity factor was calculated as a function of depth and field size. We found that, ignoring the electron contamination from the air and from the treatment machine head, the influence of obliquity can be described in terms of upstream and downstream contribution of the electrons set in motion by the primary photons. In actual beams, especially for large field sizes, the electron contamination becomes significant and tends to reduce the influence of surface obliquity. Results indicate that the obliquity factor is highly dependent on the beam energy and depth, and are in good agreement with our experimental results measured for 10 and 25 MV x-ray beams. In this paper a theoretical explanation of these dose variations due to oblique incidence of the beam is presented.

[Evaluation of individual dosimetric data on patients in an international radiotherapy trial]. / Evaluation des données dosimétriques individuelles des patients d'un essai thérapeutique international en radiothérapie.

A quality control programme has been developed in the radiotherapy department of the Institut Gustave-Roussy for an international clinical trial on the use of Etanidazole as a radiosensitizer in association with radiotherapy for head and neck tumors. Twenty-nine european centers belonging to Germany, United Kingdom, Austria, France and Italy contributed to this trial during 374 patients has been included from 1987 to 1988. This programme concerned the control of the external radiotherapy equipment available in the participating centers and the examination of the patient treatment data. The authors present, in this paper, the methods applied and the main results obtained during the analysis of the individual patient treatment data. This analysis shows that 83% of the evaluable treatments were done in compliance with the protocol recommendations and 17% contained a major deviation and among them only 3% were judged unacceptable.

[Changes of the dose at the surface in oblique incidence of high energy photon beams]. / Variation de la dose à l'entrée en incidence oblique pour les faisceaux de photons de haute énergie.

For oblique incident photon beams, the absorbed dose is markedly different from normal incident beams. Our aim was to study the changes of the dose at the surface and in the build-up region for oblique incident 4, 6, 10, 15, 25 MV photon beams angled 0 degree to 80 degrees for square fields ranging from 5 x 5 to 30 x 30 cm2. All doses were measured as a function of the depth along the beam axis and used to define an obliquity factor. For all energies investigated, we have studied the obliquity factor as a function of depth, field sizes, energy and angle of incidence. Our results show that the obliquity factor is highly dependent on these different parameters. In addition, by considering the equivalent squares of the entrance fields distorted by obliquity, an analytical method has been developed to predict the dose at the surface of high energy photon beams for various angles and field sizes.

Characteristics of contamination electrons in high energy photon beams.

A simple method to estimate the contribution of contaminating electrons to the dose, and to evaluate their dosimetric characteristics is proposed. The method is based on a normalisation of the tissue--maximum ratio curves to a constant primary photon fluence. The contribution of the contaminating electrons to the dose is calculated by subtracting the dose relative to a small field from the dose relative to the field under consideration. The method includes the determination of the mean energy, the linear apparent attenuation coefficient, the 50% range and the maximum range of the contaminating electrons. The extrapolated surface dose normalised to a constant primary photon fluence has been found to be constant for a constant collimator opening whatever may be the source distance.

Effects of hypothyroidism on bronchial reactivity in non-asthmatic subjects.

The effect of hypothyroidism on non-specific bronchial reactivity was studied in 11 patients without pulmonary disease (mean age 40 (SD 13) years) who had had a total thyroidectomy and radioiodine treatment for thyroid cancer 41 (36) months before the study. All patients when mildly hyperthyroid while having long term thyroxine replacement treatment and once when hypothyroid two weeks after stopping triiodothyronine for the purpose of screening for metastases. Bronchial reactivity was assessed by measuring specific airways conductance (sGaw) after increasing doses of inhaled carbachol (45-1260 micrograms). The dose producing a 35% decrease in sGaw (PD35) was determined from the cumulative log dose-response curve by linear regression analysis. Mean baseline sGaw values were similar when the patients were hypothyroid and when they were hyperthyroid (1.35 (0.36) and 1.41 (0.56) s-1 kPa-1). The interstudy coefficients of variation of baseline sGaw were higher in the thyroid patients than in a euthyroid control group (14% versus 8%). Geometric mean PD35 was lower when the patients were hypothyroid (97 micrograms) than when they were mildly hyperthyroid (192 micrograms). It is concluded that acute hypothyroidism increases non-specific bronchial reactivity in nonasthmatic subjects.

[Advanced testicular seminoma: treatment of residual masses after chemotherapy. A review of 3 cases]. / Séminomes testiculaires évolués: traitement des masses résiduelles après chimiothérapie. Réflexions à partir de 3 observations.

Based on 3 cases of advanced testicular seminoma classified as stage IIC, the treatment of residual masses after chemotherapy is discussed. The excellent therapeutic response confirmed by histological study of the residual masses, which show a fibrotic or necrotic appearance, allows the adoption of a strategy using imaging techniques: either there is persistence of a gland mass syndrome and resection needs to be carried our or retroperitoneal fibrosis is visualised and careful follow up would appear to be sufficient.

A comparison of build-up and depth dose characteristics of different photon beams for the treatment of Hodgkin's disease.

Measurements have been performed of build-up and depth-dose characteristics of photon beams under Hodgkin's disease treatment conditions as applied in two hospitals (AVL, Amsterdam and IGR, Villejuif). Although different types of accelerators, photon energies, field sizes and SSD are employed, similar dose distributions along the beam axis have been obtained in both centers. In order to explain this unexpected good agreement, the influence of the geometrical conditions of irradiation on the build-up and depth-dose distribution has been studied in detail for five photon beams (8 MV-25 MV) of three types of accelerators.

On the use of a quality index to specify high energy photon beams.

It is important to specify the beam quality in a simple and nonambiguous way in order on one hand to make comparisons easier between treatments performed in various hospitals, or at different times in the same hospital and on the other hand to facilitate the choice of numerical values for factors like restricted mass-collision stopping-power ratios and perturbation correction factors used in the conversion of ionization measurements into absorbed dose. We have adopted for high-energy photon beam specification a quality index (I) defined by the ratio (I20/I10) of ionizations measured with a constant source-detector distance for a reference field size 10 X 10 cm2. We have found that this quality index is independent of the source detector distance. On the other hand, the apparent linear attenuation coefficient measured on the exponential part of the tissue-maximum ratio curve can be calculated for any field size from the value of I for most high energy photon beams. In order to check the validity of the quality index for other linacs from other manufacturers, we have compared our results to published data related to various photon beams in a wide energy range: 2.5 to 45 MV.