This paper describes measurements done to investigate the accuracy of dose calculation algorithms when used to calculate the dose under shielding blocks. Because the prescribed dose is sometimes limited by the dose to organs at risk within the irradiated volume, it is important to know the dose under a block accurately. Five different algorithms from four treatment planning systems were used to calculate the dose under two narrow lead alloy blocks each placed centrally in a 15 x 15-cm beam of 6- and 18-MV X-rays, respectively. Measurements were done in a water phantom with the same geometrical set-up. All measured data have a common feature; a high surface dose (up to about 20%) decreasing linearly within the first 1-2 cm to a minimum ranging from 13 to 6.5%, and then increasing to a maximum depending on the X-ray energy and block width. Beyond the maximum, the dose decreases approximately linearly due to absorption of the primary beam. The first part of the curve is due essentially to secondary electrons. Beyond the minimum, the X-ray scatter dose component increases, due to increasing phantom scatter, to a maximum which is greater for 6 MV than for 18 MV X-rays. Most algorithms could not reproduce measured data accurately.(ABSTRACT TRUNCATED AT 250 WORDS)