Feasibility of gated helical tomotherapy using the Real-Time Position Management (RPM) system

We propose a new method of gating helical tomotherapy (HT) delivery for lung lesions that does not require breath holding. Without interrupting the gantry rotation and couch translation, the complete closure of the bMLC leaves would be triggered by the Real Time Position Management (RPM) system (Varian Oncology Systems, Palo Alto, CA), whenever the associated chest (and implied tumour) position moves outside the gating window. Once the initial radiation delivery is complete, additional iterations of the helical treatment are repeated until >95 % of the planned beam projections are delivered. Each iteration commences at a different phase of the respiratory cycle to “fill in” the previously undelivered beam projections. In this study, a gated HT delivery was simulated on the HT unit by delivering a series of “modified” leaf sinograms — altered according to the RPM curve of a real lung patient. The original leaf sinogram created for a stationary target (non‐gated) was modified such that all beam projections whose delivery times fall outside the gating window were nulled. The resulting dose distribution was measured by film dosimetry, and compared to the dose distribution of the non‐gated delivery. The gated HT delivery showed excellent agreement with the non‐gated HT delivery, with a mean dose difference of less than 1% observed in the central plane. In conclusion, the feasibility of gated HT delivery using the RPM system has been demonstrated. Future work will address different breathing patterns and more complicated target shapes treated with intensity modulation. © 2005, American Association of Physicists in Medicine. All rights reserved.