The overshoot phenomenon in step-and-shoot IMRT delivery

Abstract

The control loop in the Varian DMLC system (V4.8) requires 65 msec to monitor and halt the irradiation of a segment, causing an overshoot effect: the segment ends on a fractional monitor unit larger than that planned. As a result, the actual MU delivered may differ from that planned. In general, for step-and-shoot treatments, the first segment receives more, the last receives less, and intermediate segments vary. The overshoot for each segment (MU) is small, approximately 0.6 MU at 600 MU/min. Our IMRT planning system (Corvus) produces plans often having more than 20 of the segments with less than 1 MU/segment. Such segments may be skipped if the MU exceeds the segments planned MU. Furthermore, QA filming often requires reducing the total MU by a factor of 46, increasing the potential for dosimetric error. This study measured MU over a range of MU/min and MU/segment. At >5 MU/segment, the MU was stable, corresponding to a delay of 62 msec. MU became larger and more variable at