Determining the Effect of Source Surface Distance Variations on Percentage Depth Dose in Isocentric Radiation Therapy with 6MV Photons

The percent depth dose (PDD) is used as a dose calculation method in radiation therapy. PDD varies with source surface distance (SSD) according to inverse square law. In most clinical situations it is necessary to convert standard SSD to that must be used in practice. Therefore; this research proposes a new factor to calculate PDD corresponding to SSD.
Electa compact accelerator for 6MV photon beam, Scanditronix blue phantom the size of 50 ×50 ×50cm3 and two ionization chamber with sensitive volume of 0.13 cc were used. Dosimetry was done for field sizes: 8×8, 10×10¡ 6.4×6.4 cm2 at SSD=80 and 100cm. Ks0 and Ks taking into account the collimator scatter, for fields in the standard SSD and the practical SSD, respectively. The ratio of Ks0 and Ks was applied to correct the Maynord F factor.
This corrected Maynord F factor revealed better results for 8×8 and 10×10 cm2 fields further more the corrected Maynord F factor, reached the more precision in buildup region for the 6.4×6.4 cm2 field for PDD calculations.
For small fields with less collimator scatter, Maynord F factor method is more accurate to calculate PDD changes with varying SSD. Although, for large depths or SSDs, using the corrected Maynord F factor will show better results for calculating PDD variations with SSD changes.