EFFECT OF USING AN INITIAL POLYENERGETIC SPECTRUM WITH THE PENCIL-BEAM REDEFINITION ALGORITHM FOR ELECTRON-DOSE CALCULATIONS IN WATER

This work compares the accuracy of dose distributions computed using a n incident polyenergetic (PE) spectrum and a monoenergetic (ME) spectr um. in the electron pencil-beam redefinition algorithm (PBRA), It also compares the times required to compute PE and ME dose distributions. This has been accomplished by comparing PBRA calculated dose distribut ions with measured dose distributions in water from the National Cance r Institute electron collaborative working group (ECWG) data set. Comp arisons are made at 9 and 20 MeV for the 15 x 15 cm(2) and 6 x 6 cm(2) fields at 100- and 110-cm SSD. The incident PE spectrum is determined by a process that best matches the weighted sum of monoenergetic PBRA calculated central-axis depth doses, each calculated with the energy correction factor, C(E), equal to unity, to the ECWG measured depth do se for the 15 x 15 cm(2) held at 100-cm SSD. C(E) is determined by a l east square fit to central-axis depth dose for the PE PBRA. Results sh ow that both the PE and ME PBRA accurately calculate central-axis dept h dose at 100-cm SSD for the 6 x 6 cm(2) and 15 x 15 cm(2) field sizes and also at 110-cm SSD for the 15 x 15 cm(2) field size. In the penum bral region, the PE PBRA calculation is significantly more accurate th an the ME PBRA for all measurement conditions. Both the PE and ME PBRA exhibit significant dose errors (>4%) outside the penumbra at shallow depths for the 6 x 6 cm(2) and 15 x 15 cm(2) fields at 100-cm SSD and inside the penumbra at shallow depths for the 6 x 6 cm(2) field size at 110-cm SSD. These errors are attributed to the fact that the PBRA d oes not model collimator scatter in the incident beam. Calculation tim es for the PE PBRA are approximately 70%-140% greater than those for t he ME PBRA. We conclude that the PE PBRA is significantly more accurat e than the ME PBRA, and we believe that the increase in time for the P E PBRA will not significantly impact the clinical utility of the PBRA. (C) 1998 American Association of Physicists in Medicine. [S0094-2405( 98)00711-1].