Methods of improving the accuracy of high-energy X-ray absorbed dose calculated using a convolution approach

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Using a linear accelerator producing triplet-energy beams of 4-, 10-, and 15-MV X-rays. the following experimental and theoretical studies were performed.1.On the basis of X-ray intensity transmission data, we developed a method to estimate X-ray spectra and then constructed its calculation program. Under this situation, the primary X-ray spectrum variation caused by using wedges and compensators can be easily evaluated with accuracy and the primary X-ray spectrum variation within an irradiated body can also be easily evaluated with accuracy : Therefore, more accurate dose calculations can now be performed.2.On the basis of the X-ray spectrum, the in-water attenuation coefficient was expressed as a function of depth, and then the zero-area tissue-maximum ratio (TIR) was formulated. Some of the constants in the zero-area TMR formulation were utilized for constructing primary and scatter dose spread functions.3.Sets of tissue-phantom ratios (TPRs) were measured using water and 3-layed co … More rk phantoms for several irradiation fields. Sets of dose OCRs (off-center ratios) were also measured using water and 3-layed cork phantoms for several irradiation fields. These sets of data were utilized to reconfirm such fundamental functions, for dose calculation using a convolution method, as primary-dose spread functions, scatter-dose spread functions, contaminant electron intensity functions, and collimator OCR functions.4.The concept of "collimator OCR" was introduced for evaluation of the shielding effect due to a pair of lower and upper jaws. The concept of "source OCR" was also introduced for evaluation of the incident X-ray intensity at an infinite field. It was proposed that the incident X-ray intensity at a point for a finite field was expressed as the product of the collimator OCR and source OCR functions.5.A dose calculation approach was proposed to regard the multi-leaf collimator (MLC), the wedge, the shielding blocks, and the couch as parts of an irradiated body.6.Speeding Up of the 3D dose calculation soft was attempted by utilizing the fact that both primary and scatter dose spread functional expressions could be integrated analytically. Less