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The Monte Carlo calculation is the most accurate means of predicting radiation dose, but its accuracy is accompanied by an increase in the amount of time required to produce a statistically meaningful dose distribution. In this study, the effects on calculation time by introducing variance reduction techniques and increasing computing power, respectively, in the Monte Carlo dose calculation for a 6 MV photon beam from the Varian 600 C/D were estimated when maintaining accuracy of the Monte Carlo calculation results. The EGSnrcbased BEAMnrc code was used to simulate the beam and the EGSnrcbased DOSXYZnrc code to calculate dose distributions. Variance reduction techniques in the codes were used to describe reducedphysics, and a computer cluster consisting of ten PCs was built to execute parallel computing. As a result, time was more reduced by the use of variance reduction techniques than that by the increase of computing power. Because the use of the Monte Carlo dose calculation in clinical practice is yet limited by reducing the computational time only through improvements in computing power, introduction of reducedphysics into the Monte Carlo calculation is inevitable at this point. Therefore, a more active investigation of existing or new reducedphysics approaches is required.
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