Evaluation of the calculated PDD of a 3D electron beam algorithm in a radiotherapy planning system

The authors evaluated calculations of Percent Depth Dose (PDD) when the grid size and position were changed in the Cadplan radiotherapy planning system. A Scanditronix diode and a plane parallel ionization chamber were used to obtain measured PDD. The measurements were performed on Clinac 2100 C with 6, 9, 12, 16 and 20 MeV electrons, for all electron applicators and representative clinical fields. Cadplan calculations for electrons are based on a generalized Gaussian pencil beam model. The PDD were extracted from the calculated dose distributions with a variable step (1-1.5 mm) between the points (100 points is the maximum) regardless of the grid size, using the Cadplan dose vertical line utility. The curves showed a good agreement, within ±2 mm or ±3% for all electrons beams. For small grid size (1.25-2.5 mm, used in general for low energy electrons), the PDD were discontinuous and not as smooth as the measured PDD. These discrepancies or discontinuities were in general within ±3% for the fields and energies measured. In conclusion, Cadplan calculations for PDD are in good agreement with measurements. When a small grid size is used, the calculated PDD are not as smooth as the measured PDD