The dosimetry and dose verification for 6-MV X-rays were performed for radiosurgery cones of 5- to 40-mm diameter. The total scatter factors decrease slowly from 0.936 (40-mm cone) to 0.893 (10-mm cone; a variation of 5%), but they fall to 0.83 (7.5-mm cone) and 0.67 (5-mm cone). The dmax increases from about 12.9 (5-mm cone) to 16.3 mm (40-mm cone). The full width half maximum (FWHMs) of the beam profiles, measured at 5 cm depth, agree with the cone diameters within 1 mm. The 10–90% beam penumbra/FWHM ratio is 0.23 ± 0.03 (≧20-mm cones); for the smaller-diameter cones this ratio increases reaching 0.84 (5-mm cone). New tissue maximum ratios (TMRs) are reported for the 5-, 7.5-, 32.5-, and 37.5-mm-diameter cones. TMRs for the other diameter cones are consistent with published data. The measured doses in two verification studies using the 12 cones with diameters ≧12.5 mm with a single 360° arc agreed to 2% with the planned doses, and to about 10% for the three smaller cones. In a simulated treatment neglecting tissue heterogeneities (skull bone), the measured doses for two five arc studies (22.5-mm cone) were within 4% of the calculated dose to isocenter.
A two-dimensional pencil-beam algorithm is presented for the calculation of arc electron dose distributions in any plane that is perpendicular to the axis of rotation. The dose distributions are calculated by modelling the arced beam as a single broad beam defined by the irradiated surface of the patient. The algorithm is two-dimensional in that the anatomical cross section of the patient and the skin collimators are assumed identical in parallel planes outside the plane of calculation. The broad beam is modelled as a collection of strip beams, each strip beam being characterised by its planar fluence, mean projected angular direction and a root-mean-square spread about the mean direction. Using these parameters, the dose distribution is calculated using pencil-beam theory. Examples of strip-beam parameters and resulting dose distributions for patient geometries are presented. Features of the algorithm, which include (1) incorporation of pencil-beam theory for the calculation of dose in heterogeneous tissue, (2) run times of only about twice that of comparable-sized fixed electron fields and (3) the input requirement of only a single depth dose and four off-axis dose profiles of measured data, make the algorithm practical for clinical use.
We describe the use of polystyrene wedges to match adjacent electron beams with improved dose uniformity. These wedges were designed to increase the penumbra width at the field junction from about 1.5 to about 3.5 cm, to achieve dose uniformity. Measurements using thermoluminescent dosimeters (TLD) and therapy localization film showed that the use of polystyrene wedges (penumbra generators) produced only a small increase (less than 3%) in the surface dose and a small increase (less than 1%) in the x-ray contamination. Without wedges at the field junction, lateral mismatching of beam edges by 2 or 3 mm may introduce high dose variations (120% or more or 50% or less). Similar 2-3 mm set-up errors did not cause more than +/- 5% dose variations when plastic wedges were used to match the fields. These wedges are particularly useful when matching fields of different beam energies or matching fields on curved surfaces, such as the chest wall.
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