The overshoot phenomenon in step-and-shoot IMRT delivery

The control loop in the Varian DMLC system (V4.8) requires approximately 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 (DeltaMU) 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 DeltaMU exceeds the segments' planned MU. Furthermore, QA filming often requires reducing the total MU by a factor of 4-6, increasing the potential for dosimetric error. This study measured DeltaMU over a range of MU/min and MU/segment. At >5 MU/segment, the DeltaMU was stable, corresponding to a delay of 62 msec. DeltaMU became larger and more variable at <1 MU/segment. The behavior was modeled in a computer program that predicted the change in delivered MU/segment and total change in delivered MU to each beamlet. Beams were analyzed for patients receiving 5 field prostate or 9 field head and neck treatments. At 400 MU/min, 28% and 16%, respectively, of the planned segments were skipped. For QA filming, up to 75% of the segments were skipped. The cumulative error averaged <0.1 MU/beamlet, but individual beamlets had errors exceeding 200%. The effect is most significant for low dose regions. Recommendations are given for deciding when to treat or do QA studies with lower MU/min. In general, treatments are not significantly affected, but QA films taken at reduced MU may be improved if irradiated at lowered MU/min.