Three-dimensional dose verification for intensity modulated radiation therapy (IMRT) using an amorphous silicon electronic portal imaging device (EPID)

The aim of this research was to provide a 3D dose verification of intensity modulated radiation therapy using an a-Si EPID for patient pre treatment verification. Current verification methods for IMRT include point doses and 2D dose verification of field by field or composite plans by using a combination of ionisation chambers, film and 2D detector arrays. These dosimetric methods do not provide a complete 3D data set for comparison with the radiation therapy treatment planning system. Another alternative method is using dose projections measured with the EPID compared to the calculated TPS dose projections The method reported here is an extension of the original 3D method, EPIdose1. EPIdose recreates the dose in the midplane of a virtual cylindrical phantom for each IMRT from EPID images acquired without any phantom present. This method uses a kernel optimisation at one depth of 10 cm. This has been extended in this work to optimise the kernel for other depths. These methods were compared using Chi analysis. A further step was to include a 2D Gamma analysis to compare EPIdose to the Eclipse TPS dose calculation, EPIdose to film, and film to the Eclipse TPS dose calculation. Results show that the implementation of the depth dependent kernel gave an overall improvement of the original kernel in EPIdose. In some cases the depth dependent kernel and the original kernel gave comparable results and in only one case there was a small advantage with the original kernel.