Ferrous-sulfate-doped gelatin gel dosimeters are useful tools for the measurement of three-dimensional absorbed radiation dose distributions. The diffusion of ferric ions through these gels causes degradation with time of the dose distribution image. It would be useful to reduce ferric ion diffusion without decreasing gel sensitivity. The amount of ferric ion diffusion is a function of the time delay after radiation, the gel temperature, and the gel concentration. These effects can be quantified by measuring the ferric ion diffusion coefficient. Determination of the diffusion coefficient by irradiating the lower section of a cylinder of gel, which was then imaged repeatedly over time with a clinical magnetic resonance imager, is described. Analysis of the edge spread function formed at each of several times after irradiation by drawing a profile over the imaged junction between the irradiated and unirradiated halves of the cylinder, gave estimates of the variance of the edge spread function. These variances were used to obtain an estimate of the ferric ion diffusion coefficient for the gel. A method of reducing ferric ion diffusion by adding a chelator and the cross linkage agent formaldehyde is suggested. The chelators investigated were 1,10 phenanthroline, xylenol orange, and bathophenanthroline disulfonic acid. These reduced diffusion to varying extents, and influenced the gel sensitivity. The diffusion coefficient in gels containing xylenol orange was found to be 0.44 mm2h-1. The gel sensitivity was 0.0093 s-1Gy-1. This compared with a diffusion coefficient of 0.82 mm2h-1 for the base line gel that did not contain formaldehyde or chelators. The sensitivity of this base line gel was 0.0129 s-1Gy-1. The addition of xylenol orange produced the most improved gel dosimeter of the gels studied. This gel had a decreased ferric ion diffusion coefficient and a decreased sensitivity. It was still sensitive enough to be useful.

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A number of Monte Carlo codes are available, which can be used to calculate dose distributions n patients with high accuracy. Patient geometry can readily be derived with adequate spatial resolution from CT scans. To perform the Monte Carlo calculation with the same spatial resolution, it is necessary to enter the atomic composition and density of the tissue in each voxel of the CT image. This means entering 65,536 discrete values for a CT slice with a 256 x 256 matrix size. The need for automated methods of setting up the material data files is obvious. Because there is no direct unique relationship between CT numbers and material composition, the aim of our work was to devise a method whereby the atomic composition and density in each voxel could be assigned automatically by indirect derivation from the CT numbers. The set of all tissues types in the human body was divided into subsets that are dosimetrically equivalent, based on Monte Carlo calculated depth dose curves in homogeneous phantoms of each tissue. CT number ranges corresponding to each tissue subset were determined from the calibration curve linking electron density with CT number for the specific CT scanner. Further subdivision was found to be necessary for the lung and bone type tissues. This was done by keeping the atomic composition constant and varying the physical density. It was found that 57 distinct tissue subsets were needed to represent the 16 main tissue types in the body at a 1% dose level. Corresponding CT number intervals of 30 HU were needed in the lung and soft tissue region, whereas in the bone region the intervals could be increased to 100 HU. A computer algorithm was set up to convert automatically from CT number to corresponding equivalent material number for the Monte Carlo preprocessor code.

This paper shows the contribution that Monte Carlo methods make in regard to dose distribution calculations in CT based patient models and the role it plays as a gold standard to evaluate other dose calculation algorithms. The EGS4 based BEAM code was used to construct a generic 8 MV accelerator to obtain a series of x-ray field sources. These were used in the EGS4 based DOSXYZ code to generate beam data in a mathematical water phantom to set up a beam model in a commercial treatment planning system (TPS), CADPLAN V.2.7.9. Dose distributions were calculated with the Batho and ETAR inhomogeneity correction algorithms in head/sinus, lung, and prostate patient models for 2 x 2, 5 x 5, and 10 X 10 cm2 open x-ray beams. Corresponding dose distributions were calculated with DOSXYZ that were used as a benchmark. The dose comparisons are expressed in terms of 2D isodose distributions, percentage depth dose data, and dose difference volume histograms (DDVH's). Results indicated that the Batho and ETAR methods contained inaccuracies of 20%-70% in the maxillary sinus region in the head model. Large lung inhomogeneities irradiated with small fields gave rise to absorbed dose deviations of 10%-20%. It is shown for a 10 x 10 cm2 field that DOSXYZ models lateral scatter in lung that is not present in the Batho and ETAR methods. The ETAR and Batho methods are accurate within 3% in a prostate model. We showed how the performance of these inhomogeneity correction methods can be understood in realistic patient models using validated Monte Carlo codes such as BEAM and DOSXYZ.

SummaryPlatelets were isolated from blood of baboons and treated with neuraminidase to remove platelet membrane sialic acid, a process which artificially ages the platelets. The platelets were then labelled with 111In and their mean life span, in vivo distribution and sites of Sequestration were measured. The effect of removal of sialic acid on the attachment of immunoglobulin to platelets were investigated and related to the Sequestration of the platelets by the spleen, liver, and bone marrow. Removal of sialic acid by neuraminidase did not affect the aggregation of platelets by agonists in vitro, nor their sites of Sequestration. The removal of 0.51 (median, range 0.01 to 2.10) nmol sialic acid/108 platelets shortened their life span by 75 h (median, range 0 to 132) h (n = 19, p <0.001), and there was an exponential correlation between the shortening of the mean platelet life span and the amount of sialic acid removed. The increase in platelet-associated IgG was 0.112 (median, range 0.007 to 0.309) fg/platelet (n = 25, p <0.001) after 0.79 (median, range 0.00 to 6.70) nmol sialic acid/108 platelets was removed (p <0.001). There was an exponential correlation between the shortening of mean platelet life span after the removal of sialic acid and the increase in platelet-associated IgG. The results suggest that platelet membrane sialic acid influences ageing of circulating platelets, and that the loss of sialic acid may have exposed a senescent cell antigen that binds IgG on the platelet membrane. The antibody-antigen complex may then provide a signal to the macrophages that the platelet is old, and can be phagocytosed and destroyed.