Measurements and calculations of the absorbed dose distribution around a 60Co source.

The data from Meisberger et al. [Radiology 90, 953-957 (1968)] are often used as a basis for dose calculations in brachytherapy. In order to describe the absorbed dose in water around a brachytherapy point source, Meisberger provided a polynomial fit for different isotopes taking into account the effect of attenuation and scattering. The validity of the Meisberger coefficients is restricted to distances up to 10 cm from the source, which is regarded to be satisfactory for most brachytherapy applications. However, for more distant organs it may lead to errors in calculated absorbed dose. For this reason dose measurements have been performed in air and in water around a high activity 60Co source used in high dose rate brachytherapy. Measurements were carried out to distances of 20 cm, using ionization chambers. These data show that at a distance of about 15 cm the amount of scattered radiation virtually equals the amount of primary radiation. This emphasizes the contribution of scattered radiation to the dose in healthy tissue far from the target volume, even with relatively high energy photon radiation of 60Co. It is also shown that the Meisberger data as well as the approach of Van Kleffens and Star [Int. J. Radiat. Oncol. Phys. 5, 557-563 (1979)] lead to significant errors in absorbed dose between distances of 10 and 20 cm from the source. In addition to these measurements, the Monte Carlo code has been used to calculate separately primary dose and scattered dose from a cobalt point source. The calculated results agree with the experimental data within 1% for a most distant dose scoring region.

Evidence for diploidy in metacyclic forms of African trypanosomes.

The DNA contents of bloodstream form trypanosomes (life cycle stages circulating in the blood of the vertebrate host) of four African Trypanosoma species and of metacyclic forms (the life cycle stage that is injected into the vertebrate by the tsetse fly during its bite) of the same four species were measured by cytofluorometry of individual cells or nuclei. The results showed unambiguously that the metacyclic forms cannot be considered to be products of meiosis containing only half of the DNA of bloodstream forms, in contrast to what was previously reported for Trypanosoma brucei [Zampetti-Bosseler, F., Schweizer, J., Pays, E., Jenni, L. & Steinert, M. (1986) Proc. Natl. Acad. Sci. USA 83, 6063-6064] during an attempt to localize the gametes in the life cycle after experimental evidence of sexual gene exchange in this parasite was reported.

Determination of three parameters describing the uncollimated electron beam in air.

The parameters that describe the electron dose distribution phi (r, theta, z) produced in air by an uncollimated clinical electron beam are accurately determined. For the determination of these parameters the multiple scattering theory of Fermi is assumed. A new method which determines the angular variance at the phantom surface is introduced and the results appear to be in good agreement with the multiple scattering theory. Knowledge of the values of these parameters is essential for a numerical determination of the dose distribution in air and in the patient.

Description of the uncollimated electron beam in air by means of a directional pencil beam model.

The dose distribution of an uncollimated electron beam in air, described by phi(theta x, x, z), is shown to be mathematically equivalent to a directional pencil beam model. In this model the multiple scattering theory of Fermi is assumed. After implementation of the model by using a computer program, dose distributions of electron fields of various shapes were calculated and compared with dose distributions obtained by experiment. In general, the distributions appear to agree well, but in some situations noticeable differences can be observed, which is probably due to the fact that large-angle scattered electrons are not considered in this model.

Cell kinetics of hypoxic cells in a murine tumour in vivo: flow cytometric determination of the radiation-induced blockage of cell cycle progression.

Cells from the small cell population of viable cells in the large necrotic centre of murine M8013 tumours were investigated with respect to their cell kinetics. Flow cytometry (FCM) of this part of subcutaneously transplanted tumours revealed the presence of tumour cells with G1, S and G2 + M phase DNA-contents. These severely hypoxic cells could have stopped cell cycle progression due to the nutritional deprivation, irrespective of their position within the cell cycle. Labelling methods, used to disclose the cell kinetics of this cell population, are hampered by the absence of a transport system in these large necrotic areas. Therefore, FCM was used to monitor radiation-induced changes in the cell cycle distribution. From this investigation it was concluded that hypoxic cells in the necrotic centre of the M8013 tumour progress through the cell cycle. As well as a cell population with a cell cycle time (Tc) of approximately 84 hr, a subpopulation with a Tc of approximately 21 hr occurred.

DNA-flow cytometry of squamous cell carcinomas from the human uterine cervix: the identification of prognostically different subgroups.

The relevance of DNA-index, alone and in combination with other parameters, was assessed for patients with squamous cell carcinomas of the uterine cervix by using the method of flow cytometry (FCM). Tumours with a diploid or a tetraploid DNA-content appeared to be associated with a less favourable prognosis than was observed in the group with non-diploid/non-tetraploid tumours. DNA-index in combination with either the age of patients or the histological grade of tumours, enabled the separation of prognostically different subgroups. Particularly, the combination of an age under 51 years with a DNA-index of 1 or 2 appeared to define a group with a low survival rate. Furthermore, within the group of well and moderately differentiated tumours, it was possible to identify prognostically different subgroups by combining this parameter with DNA-index, i.e. a non-diploid/non-tetraploid DNA-content of these tumours was associated with a more favourable prognosis. From this study it is concluded that FCM clearly contributes to the recognition of subgroups of patients with different prognosis.

A three-dimensional description of heating patterns in vascularised tissues during hyperthermic treatment.

A three-dimensional finite difference computer model has been developed to calculate temperature distributions in vascularised tissues in hyperthermia. Besides offering the possibility of calculating temperature distributions according to the conventional ' bioheat transfer' method, this model allows the introduction of several discrete blood vessels and can describe their influence on the temperature distribution. The model can be used to evaluate all types of heating techniques. First calculations on discrete large vessels show inhomogeneities caused by these vessels in the temperature distribution in tissue in hyperthermia. The theory and model presented can form the basis of a new bioheat transfer theory, with a vessel-temperature related bioheat transfer heat-sink term able to describe the small-scale temperature variation problems in local hyperthermia.

Calculation of the dose distribution around 137Cs tubes used in clinical brachytherapy.

The dose distribution in water around a CDC.J 137Cs tube is calculated with a computer. In the transverse direction the calculated dose rates showed to be almost unsusceptible for the value of the platinum absorption coefficient. In the longitudinal direction, however, the value of the linear absorption coefficient plays a more prominent role. An effective absorption coefficient for the filter material was determined by measurement, using a photographic film. This was done by relating the filmdensity along the source axis with the filmdensity along the transverse axis, where the dose rate is well known. In this way an effective linear absorption coefficient for platinum was obtained which gives a good agreement between the experimental and theoretical results. For use in a computer program for clinical use, the dose distribution around a cesium tube has been calculated in a specific spherical coordinate system using 101 radii.