SU-E-T-167: Comprehensive Evaluation of EPID Image Acquisition for Integrating and Temporal Dosimetry of Fixed-Gantry IMRT and ArcIMRT

To evaluate EPID for dosimetry applications of arc and static-gantry IMRT with sliding window (SW) and/or step-and-shoot (SS) deliveries Methods: IMRT beams (SW & SS) were designed that generate beam hold-offs and dose rate modulation due to MLC motion under 10 × 10 cm jaw. An arcIMRT beam was designed by adding gantry movement to the SW field. A 10 cm × 10 cm open beam was also used. Despite differences in delivered dose rates/pulse characteristics, the four beams should deliver the same total dose. For each beam, various MUs with 6 MV beam at 300MU/min were irradiated on EPID which operated in image acquisition of integration mode (IM), continuous scanning mode with synchronization (CMs) and without (CMn) to beam pulses. Acquired images were evaluated in repeatability, dose linearity, and reproducibility (reproduce open beam output in IM).

In IM, repeatability, dose linearity, and reproducibility were within 1% for all dose levels and beams. In CMs, they were within 1-2% if dose rate was maintained steady (1) for SW beam (needed a minimum 1.3 MU/cm MLC motion) and (2) arcIMRT beam (needed a minimum 1 MU/degree and 2.8 MU/cm MLC motion) and (3) if a minimum of 38-40 MU per shoot was used for SS beam. Nonlinearity was observed for fewer MUs. This is due to the response of EPID to pulse-length reduction for fixed-gantry therapy and pulse dropping for arc therapy. The latter produces in-planar non-uniformity making EPID unsuitable for temporal dosimetry of arcIMRT. Sacrifice in temporal resolution then became necessary such as multi-frames per image (eg. ∼1 sec/image). In CMn the results were similar to those of CMs. However, they showed artifacts, thus this mode was not preferred.

We found conditions under which integrating and temporal EPID dosimetry can be used for IMRT and arcIMRT dose deliveries.