Dependence of the tray transmission factor on collimator setting and source-surface distance.

When blocks are placed on a tray in megavoltage x-ray beams, generally a single correction factor for the attenuation by the tray is applied for each photon beam quality. In this approach, the tray transmission factor is assumed to be independent of field size and source-surface distance (SSD). Analysis of a set of measurements performed in beams of 13 different linear accelerators demonstrates that there is, however, a slight variation of the tray transmission factor with field size and SSD. The tray factor changes about 1.5% for collimator settings varying between 4x4 cm and 40 x 40 cm for a 1 cm thick PMMA tray and approximately 3% for a 2 cm thick PMMA tray. The variation with field size is smaller if the source-surface distance is increased. The dependence on the collimator setting is not different, within the experimental uncertainty of about 0.5% (1 s.d.), for the nominal accelerating potentials and accelerator types applied in this study. It is shown that the variation of the tray transmission factor with field size and source-surface distance can easily be taken into account in the dose calculation by considering the volume of the irradiated tray material and the position of the tray in the beam. A relation is presented which can be used to calculate the numerical value of the tray transmission factor directly. These calculated values can be checked with only a few measurements using a cylindrical beam coaxial miniphantom.

Effect of electron contamination on scatter correction factors for photon beam dosimetry.

Physical quantities for use in megavoltage photon beam dose calculations which are defined at the depth of maximum absorbed dose are sensitive to electron contamination and are difficult to measure and to calculate. Recently, formalisms have therefore been presented to assess the dose using collimator and phantom scatter correction factors, Sc and Sp, defined at a reference depth of 10 cm. The data can be obtained from measurements at that depth in a miniphantom and in a full scatter phantom. Equations are presented that show the relation between these quantities and corresponding quantities obtained from measurements at the depth of the dose maximum. It is shown that conversion of Sc and Sp determined at a 10 cm depth to quantities defined at the dose maximum such as (normalized) peak scatter factor, (normalized) tissue-air ratio, and vice versa is not possible without quantitative knowledge of the electron contamination. The difference in Sc at dmax resulting from this electron contamination compared with Sc values obtained at a depth of 10 cm in a miniphantom has been determined as a multiplication factor, Scel, for a number of photon beams of different accelerator types. It is shown that Scel may vary up to 5%. Because in the new formalisms output factors are defined at a reference depth of 10 cm, they do not require Scel data. The use of Sc and Sp values, defined at a 10 cm depth, combined with relative depth-dose data or tissue-phantom ratios is therefore recommended. For a transition period the use of the equations provided in this article and Scel data might be required, for instance, if treatment planning systems apply Sc data normalized at d(max).

Comparison of parametrization methods of the collimator scatter correction factor for open rectangular fields of 6-25 MV photon beams.

PURPOSE: To facilitate the use of the collimator scatter correction factor, Sc, parametrization methods that relate Sc to the field size by fitting were investigated. MATERIALS AND METHODS: Sc was measured with a mini-phantom for five types of dual photon energy accelerators with energies varying between 6 and 25 MV. Using these Sc-data six methods of parametrizing Sc for square fields were compared, including a third-order polynomial of the natural logarithm of the field size normalized to the field size of 10 cm2. Also five methods of determining Sc for rectangular fields were considered, including one which determines the equivalent field size by extending Sterling's method. RESULTS: The deviations between measured and calculated Sc-values were determined for all photon beams and methods investigated in this study. The resulting deviations of the most accurate method varied between 0.07 and 0.42% for square fields and between 0.26 and 0.79% for rectangular fields. A recommendation is given as to how to limit the number of fields for which Sc should be measured in order to be able to accurately predict it for an arbitrary field size.

Anti-GBM nephritis in the mouse: role of granulocytes in the heterologous phase.

The role of polymorphonuclear granulocytes (PMNs) was studied in a model of anti-GBM nephritis in mice, in which PMN depletion was induced by total body irradiation of 7.5 Gy. Both in complement-normal B10.D2 new and in C5-deficient B10.D2 old mice, PMN depletion completely prevented the albuminuria occurring after injection of low doses of anti-GBM serum, and severely depressed the albuminuria after injection of high doses. In immunofluorescence, glomerular deposition of antibody and C3 was similar to that in control mice. The glomerular influx of PMNs in both the complement-normal and C5-deficient controls was inhibited to 10% or less of control values. Fibrin deposition or necrosis did not occur. Injection of F(ab')2 fragments of the anti-GBM antibody in non-irradiated mice caused only limited PMN influx and reduced the albuminuria to physiological levels, although the binding of 125I labeled F(ab')2 fragments to the glomeruli was as high as 82% of that of the complete antibody. We conclude that the albuminuria in this model is Fc-dependent and largely, if not completely, dependent on the influx of PMNs in the glomeruli. Among the many experimental models of anti-GBM nephritis, this is the first one in which the heterologous phase is complement-independent but PMN-dependent.

Volumetrical and morphological responses of human laryngeal squamous cell carcinoma xenografts treated with fractionated irradiation.

Xenografts of both primary human laryngeal squamous cell carcinoma and its metastases were irradiated with five daily fractions of 5.0 Gy. Tumor volume changes, morphology, mitotic index and mitotic figures were studied. Primary xenografts disappeared within 17 +/- 3 days. Grafts of metastases showed complete regression within 26 +/- 5 days, or regrowth after a delay period. Mitotic activity was higher in the grafts of metastases. The number of mitotic figures decreased and ceased within 14 days in the primary tumor, while some were still occasionally noted in the grafts of metastases. Telophase stages were practically absent after the first fraction. This study suggests that the response of metastases to radiation therapy is lower than the response of the primary tumor.

A computer model of spermatogenesis in the rat; correlation with flow cytometric data based on autoradiographic cell-cycle properties.

A computer model of rat spermatogenesis was created, based on autoradiographic studies of durations of the phases of the cell cycle (G1, S, G2 and mitotic phases) of each germ-cell type. With this model it is possible to predict and to gain insight into the changes of the DNA content occurring during the normal process of spermatogenesis. The relative proportions of haploid, diploid, S phase and tetraploid germ cells with increasing age of the rats were calculated. Calculated and actual experimental flow cytometry data were compared to test the accuracy of the model, and these show good agreement. The present work demonstrates that single-parameter DNA analysis of testicular cells is primarily a reflection of germ cells in the spermatocyte and spermatid stages of development, and of non-germ cells. The FCM single-parameter DNA analysis of testicular cells is relatively insensitive to changes in the stem cell and spermatogonial stages of germ-cell development.