Monte Carlo evaluation of in infinitely long cylinders of water, polyethylene and PMMA with diameters from to

Abstract

Monte Carlo simulations were used to evaluate the radiation dose to infinitely long cylinders of water, polyethylene, and poly(methylmethacrylate) (PMMA) from 10 to in diameter. Radiation doses were computed by simulating a divergent primary beam striking the cylinder at , and the scattered radiation in the and directions was integrated out to infinity. Doses were assessed using the total energy deposited divided by the mass of the volume of material in the primary beam. This approach is consistent with the notion of the computed tomography dose index (CTDI) integrated over infinite , which is equivalent to the dose near the center of an infinitely long CT scan. Monoenergetic x‐ray beams were studied from 5 to , allowing polyenergetic x‐ray spectra to be evaluated using a weighted average. The radiation dose for a CT slice was assessed at the center, edge, and over the entire diameter of the phantom. The geometry of a commercial CT scanner was simulated, and the computed results were in good agreement with measured doses. The absorbed dose in water for x‐ray spectrum with no bow tie filter for a cylinder diameter was about per mGy air kerma at isocenter for both the peripheral and center regions, and dropped to for a water phantom at the periphery, where the corresponding value for the center location was . The influence of phantom composition was studied. For a diameter of , the dose coefficients were 1.23 for water, 1.02 for PMMA, and 0.94 for polyethylene (at ). For larger diameter phantoms, the order changed—for a phantom, the dose coefficient of polyethylene (0.25) was greater than water (0.21) and PMMA (0.16). The influence of the head and body bow tie filters was also studied. For the peripheral location, the dose coefficients when no bow tie filter was used were high (e.g., for a water phantom at at a diameter of , the dose coefficient was 0.97). The body bow tie filter reduces this value to 0.62, and the head bow tie filter (which is not actually designed to be used for a object) reduces the dose coefficient to 0.42. The dose in CT is delivered both by the absorption of primary and scattered x‐ray photons, and at the center of a water cylinder the ratio of scatter to primary (SPR) doses increased steadily with cylinder diameter. For water, a spectrum and a cylinder diameter of , the SPR was 4, and this value grew to 9 for a diameter of and to over 16 for a cylinder. A freely available spreadsheet was developed to allow the computation of radiation dose as a function of object diameter , composition (water, polyethylene, PMMA), and beam energy .