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 an incident polyenergetic (PE) spectrum and a monoenergetic (ME) spectrum 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 distributions with measured dose distributions in water from the National Cancer Institute electron collaborative working group (ECWG) data set. Comparisons are made at 9 and 20 MeV for the 15 x 15 cm2 and 6 x 6 cm2 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 dose for the 15 x 15 cm2 field at 100-cm SSD. C(E) is determined by a least square fit to central-axis depth dose for the PE PBRA. Results show that both the PE and ME PBRA accurately calculate central-axis depth dose at 100-cm SSD for the 6 x 6 cm2 and 15 x 15 cm2 field sizes and also at 110-cm SSD for the 15 x 15 cm2 field size. In the penumbral region, the PE PBRA calculation is significantly more accurate than 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 cm2 and 15 x 15 cm2 fields at 100-cm SSD and inside the penumbra at shallow depths for the 6 x 6 cm2 field size at 110-cm SSD. These errors are attributed to the fact that the PBRA does not model collimator scatter in the incident beam. Calculation times for the PE PBRA are approximately 70%-140% greater than those for the ME PBRA. We conclude that the PE PBRA is significantly more accurate than the ME PBRA, and we believe that the increase in time for the PE PBRA will not significantly impact the clinical utility of the PBRA.