Central Axis Depth Dose Data for Use in Radiotherapy

Abstract

• We found an excellent fit to central axis electron PDD data from a depth of <0.4 cm to a depth corresponding to 10-20% of maximum dose using a sixth-order polynomial. R 2 for the curve fits ranged from 0.998 to 1.000. Figure 6 shows a set of clinical data for a Varian Clinac 2100C plotted with the curves generated by the polynomial fit. • Figure 7 shows the set of standards generated for the Varian Clinac 2100C and the standard error bars. Abstract A sample of 136 machines with a total of 760 electron beam PDD tables was selected from the Radiation Dosimetry Services (RDS) database. For each beam, a set of data points was extracted from the tables for a 10x10 cm 2 cone if available and a 15x15 cm 2 cone otherwise. The depths selected for the fitting ranged from 0.4 cm from the surface to the depth of 10% to 20% of the maximum dose. A sixth order polynomial was fitted to the data for each set of beam data using MicroSoft Excel TM software. For all data sets evaluated the regression coefficient was larger than 0.998. Data were stratified by machine manufacturer, model and beam energy for all analyses. Outliers were discarded and not included in the standard calculations. Most outliers were found to be ionization data instead of PDD data. For this study we investigated the feasibility of developing a set of standard curves (average curves). These standard curves were developed to facilitate the analysis of TLD irradiated with an electron beam energy when the institution monitored did not provide complete PDD data for that beam, also for reviewing new beam data submitted by institutions to RDS, and to provide PDD shape information when reported data are sparse. The standard curves proposed can be used as a reference when commissioning a new linear accelerator.