Dynamic wedges – dosimetry and quality control

Abstract

Background

Modern medical accelerators are usually equipped with a dynamic wedge option. It is a form of dose-rate modulation which makes use of the dynamic movement pairs of collimator jaws. Dynamic wedges may replace physical wedges but their use requires more complex dosimetry and quality control procedures. Film dosimetry has been proposed as a quality control tool in dynamic dose distribution. We present examples of extensive systematic calculation errors which were detected during complex dosimetry quality control procedures in two treatment planning systems.

Aim

The aim, in presenting QA procedures and examples of systematic errors which were detected and corrected, is to focus attention on the QA of dynamic accessories used in TPS before they are used in the clinical practice. This is an important issue which may have been frequently overlooked as the configuration of dynamic wedges in many treatment planning systems requires no dose measurement data. Measurements of verifications are often, overlooked.

Materials/Methods

Dynamic wedge dose distributions were generated by Clinac 2300 C/D accelerators (Varian) for beam energies of 6 and 15 MV. Measurements were performed with LA48 linear array of ionization chambers (PTW), and with dosimetric films X-Omat V and EDR-2 (Kodak). The dosimetric characteristics of the film were examined for a wide range of dose values and beam parameters. The results of the measurements were compared with dose distribution calculations produced by the treatment planning systems Helax and CadPlan/Eclipse.

Results

The initial results showed considerable differences between measurements and calculations. Larger differences were observed for larger wedge angles and lower energies. On the basis of these results, TPS manufacturers were able to tune their calculation algorithms which effectively reduced the observed differences from a level of −5.5% and −8% (for 15 and 6 MV respectively) for 60° wedges to a level of ±2% for the Helax system.

Conclusions

A comprehensive quality control procedure for a broad range of dynamic wedge parameters, on a Clinac 2300C/D, made it possible to achieve an improved agreement between measured and calculated results in radiotherapy. Such measurement procedures should be included in the recommendations for periodic quality control tests of accelerators.