DEPTH-DOSE UNDER NARROW SHIELDING BLOCKS - A COMPARISON OF MEASURED AND CALCULATED DOSE

This paper describes measurements done to investigate the accuracy of dose calculation algorithms when used to calculate the dose under shie lding blocks. Because the prescribed dose is sometimes limited by the dose to organs at risk within the irradiated volume, it is important t o know the dose under a block accurately. Five different algorithms fr om four treatment planning systems were used to calculate the dose und er 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 hav e a common feature; a high surface dose (up to about 20%) decreasing l inearly 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 blo ck width. Beyond the maximum, the dose decreases approximately linearl y due to absorption of the primary beam. The first part of the curve i s due essentially to secondary electrons. Beyond the minimum, the X-ra y 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 al gorithms could not reproduce measured data accurately. Of the five alg orithms considered here, one simply scaled the open field dose by the nominal transmission factor (5 and 6% for 6- and 18-MV X-rays, respect ively), another additionally corrected for phantom scatter but still u nder-estimated the dose at a depth of 5 cm by 30-50% depending on the X-ray energy and block width. Both these algorithms calculated the 18- MV dose to be higher than the 6-MV dose. A full-scatter algorithm gave excellent results for 6-MV X-rays but only for the narrower block, an d it under-estimated the 18-MV X-ray dose by up to 45%. Another gave g ood results for 6-MV X-rays but under-estimated the dose for 18-MV X-r ays by about 50%. One full-scatter algorithm over-estimated the dose b y 25-50% depending on the block width and X-ray energy.