The use of an aSi-based EPID for routine absolute dosimetric pre-treatment verification of dynamic IMRT fields

In parallel with the increased use of intensity modulated radiation treatment (IMRT) fields in radiation therapy, flat panel amorphous silicon (aSi) detectors are becoming the standard for online portal imaging at the linear accelerator. In order to minimise the workload related to the quality assurance of the IMRT fields, we have explored the possibility of using a commercially available aSi portal imager for absolute dosimetric verification of the delivery of dynamic IMRT fields.

We investigated the basic dosimetric characteristics of an aSi portal imager (aS500, Varian Medical Systems), using an acquisition mode especially developed for portal dose (PD) integration during delivery of a-static or dynamic-radiation field. Secondly, the dose calculation algorithm of a commercially available treatment planning system (Cadplan, Varian Medical Systems) was modified to allow prediction of the PD image, i.e. to compare the intended fluence distribution with the fluence distribution as actually delivered by the dynamic multileaf collimator. Absolute rather than relative dose prediction was applied. The PD image prediction was compared to the corresponding acquisition for several clinical IMRT fields by means of the gamma evaluation method.

The acquisition mode is accurate in integrating all PD over a wide range of monitor units, provided detector saturation is avoided. Although the dose deposition behaviour in the portal image detector is not equivalent to the dose to water measurements, it is reproducible and self-consistent, lending itself to quality assurance measurements. Gamma evaluations of the predicted versus measured PD distribution were within the pre-defined acceptance criteria for all clinical IMRT fields, i.e. allowing a dose difference of 3% of the local field dose in combination with a distance to agreement of 3 mm.