Direct measurement is made for X-ray beams with first-half-value thicknesses in the range 0.5 mm to 8 mm Al and for field sizes ranging from 15 mm diameter to 70 mm square at 100 mm SSD. The measurements were made in a water phantom using a 0.2 cm3 thimble ionization chamber. The surface dose relative to that for full backscatter conditions decreases as the depth of water with lead beneath decreases. Further, for a given thickness of water with lead beneath, the surface dose relative to that for full backscatter conditions decreases with increasing beam energy and field size. The measured data is well described by a simple exponential function, the parameter values of which for the range of beam qualities and field sizes considered in this study are given. The measurements are compared with Klevenhagen's measurements of the build-up of backscatter with thickness of polystyrene.

We are investigating three-dimensional converging stereotactic radiotherapy (3DCSRT) with suitable medium-energy x rays as treatment for small lung tumors with better dose homogeneity at the target. A computed tomography (CT) system dedicated for non-coplanar converging radiotherapy was simulated with BEAMnrc (EGS4) Monte-Carlo code for x-ray energy of 147.5, 200, 300, and 500 kilovoltage (kVp). The system was validated by comparing calculated and measured percentage of depth dose in a water phantom for the energy of 120 and 147.5 kVp. A thorax phantom and CT data from lung tumors (<20 cm3) were used to compare dose homogeneities of kVp energies with MV energies of 4, 6, and 10 MV. Three non-coplanar arcs (0 degrees and +/-25 degrees ) around the center of the target were employed. The Monte Carlo dose data format was converted to the XiO RTP format to compare dose homogeneity, differential, and integral dose volume histograms of kVp and MV energies. In terms of dose homogeneity and DVHs, dose distributions at the target of all kVp energies with the thorax phantom were better than MV energies, with mean dose absorption at the ribs (human data) of 100%, 85%, 50%, 30% for 147.5, 200, 300, and 500 kVp, respectively. Considering dose distributions and reduction of the enhanced dose absorption at the ribs, a minimum of 500 kVp is suitable for the lung kVp 3DCSRT system.

The risk of breast cancer following radiotherapy for Hodgkin's lymphoma appears to be dose related. In this study we compared breast dose in an anthropomorphic phantom for conventional 'mantle'; upper mediastinal/bilateral neck (minimantle) and unilateral neck fields, and evaluated the accuracy of computer planned dose estimates for out-of-field doses. For each field, computer-planned breast dose (CPD) estimates were compared with thermoluminescence dosimetry measurements in five locations within 'breast tissue'. CPD were also compared with ion chamber measurements in a slab phantom. Measured dose and CPD were within 20% of each other up to approximately 10 cm from the field edge. Beyond 10 cm, the CPD underestimated dose by a factor of 2 or more. The mini-mantle reduced the breast dose by a factor of approximately 10 compared with the mantle treatment. Treating the neck field lowered the breast dose by a further 50% or more. Modern involved-field radiotherapy for lymphoma substantially reduces breast dose compared with mantle fields. Computer dosimetery underestimated dose at larger distances from the field. This needs to be considered if computer dosimetery is used to estimate breast dose and, by extrapolation, breast cancer risk.

A high-speed analog tomographic reconstruction device is described. The device uses convolution filtering of the projection data to improve the image quality. This filtering was performed using a tapped analog delay configured as a transversal filter. Filtered back-projections were displayed on a raster, rotated using a dove prism, and integrated using analog techniques. Images of high-contrast objects are presented and are a better approximation to the object than those obtained without filtering and have a spatial resolution of approximately 1 in 300. The electronics used to perform the Radon inversion were analog and provided real-time computations introducing a delay between data input and data output of only 2 ms. The shortest time attained for total data collection and processing was 8 s; however, it is suggested that with new commercially available technologies reconstruction times could be further shortened. The individual components of the device were tested and the final results compared with digital reconstructions.

No abstract