[en] External beam dose calculation programs for the PC-12 have been divided into two main classes. One is the ''Beam Generator'' program which computes the single beam dose distribution with normal incidence using compiled TAR/SAR values. The other is the ''Superimpose Beams'' program which makes corrections for patient curvature, any change in SSD and heterogeneity, using the effective SSD method. The present work has been conducted for the purpose to investigate how well the computed dose distributions agree with measured ones for 6 MV X-ray at each class of the program. In the ''Beam Generator'' program, open and partially blocked beams are in error less than 2 percent of the maximum dose in useful region, but for wedged fields error increases considerably depending on wedge angle and the depth in phantom. In the ''Superimpose Beams'' program, isodose curves for 0 - 60⁰ oblique incidence and for +-20 cm deviation from the standard SSD are in error within 5 mm discrepancy in 50 percent line on the central axis, but for lung correction the error increases considerably with depth. These errors have to be taken into consideration when the system is used for high energy X-ray sources. (author)