Comparison of the scatter factor with various configurations and its impact on the precision of dose calculation

The purpose of this study is to contrast the output factors using non-identical detectors with different setups and their effect on the plan of therapy.

The measurements were obtained for 6MV beams with various volume chambers using Varian True beam™ STx linear accelerator (LINAC). With chambers set up at source to axis distance (SAD) and source to surface distance (SSD) at two different depths, the output factors were measured (both 5 and 10 cm). The smallest output factors were assessed with the SAD technique at 5 cm of depth and the largest output factor was observed with the SSD approach at 10 cm depth and had been moved to the system for treatment planning and variation in calculated and measured dose was noted.

 A variation in measured dose from Treatment Planning System (TPS) calculated ranges from 0 to -2.7 % for small field plans calculated with SAD technique and -0.22 to -2.31 % for plans calculated with SSD technique. For large fields, it ranges from -0.89 to 0.8 % for SAD and -0.6 to 0.64 % for the SSD technique. The Statistical significance was checked and was found to be greater than 0.05.

The percentage difference in output factors at two depths was more prominent for low energy beams (6 MV) than for beams of greater energy. This might be a result of the loss of lateral equilibrium as the depth is changed. The output factor measurement at 10 cm of depth and 100 cm SSD is suitable for small fields (3 x 3 cm2) as it increases the lateral equilibrium and hence reduces the error.