TUMOR DOSE ERROR ANALYSIS FOR MEGAVOLTAGE OPEN AND BLOCKED FIELDS USING MEASURED SCATTER CORRECTION FACTORS

Abstract

Introduction. The most important goal in radiation therapy is treating the tumor while protecting the surrounding healthy tissue. To achieve this goal the difference between prescribed and deliveredtumor dose has to be less than±2 percent. This study evaluated the necessity of using output factors for changing output of a reference field to the output of other field sizes.
Methods. Scp was measured in water and Se in a narrow cylindrical phantom built in this research for cobalt and 12 MV beams. The separation of Scp to Sc and Sp parts for blocked fields was evaluated by comparing the measured and calculated Scp values for blocked fields.
Results. Results of this study indicates that if output factors are not used for a 12 MV beam the difference between delivered and prescribed tumor dose is from 13 percent (in largest field) to – 6 percent (in smallest field) ( difference was 8 percent for cobalt beam). The 'method of separating Scp to Se and Sp was verified (maximum difference from measured values was 1.3 percent).
Discussion. In the method of using phantom field size for selecting Scp, tumor dose was more than prescribed dose from 2.4 percent (for a 27 percent block) to 7.8 percent (for a 90 percent block) for a 12 MV beam (the error increased by block size and proximity of block to the center).