Technical Note: Real‐time image‐guided adaptive radiotherapy of a rigid target for a prototype fixed beam radiotherapy system

Purpose Fixed beam radiotherapy systems utilize couch movement and rotation instead of gantry rotation in order to simplify linear accelerator design. We investigate the ability to deliver fixed beam treatments with the same level of clinical accuracy as conventional (rotating beam) treatments using real‐time image guidance to maintain this accuracy in the presence of rigid target motion. Methods A prototype fixed beam radiotherapy system was built using a standard linac with the beam fixed in the vertical position and a computer controlled rotation stage that rotated a rigid phantom about the superior–inferior axis. Kilovoltage Intrafraction Monitoring (KIM) and real‐time beam adaptation with MLC tracking was applied to a five‐field IMRT treatment plan with motion introduced to the phantom. The same IMRT treatment was also delivered with real‐time adaptation using the conventional rotating beam geometry. Film dosimetry was used to measure the dose delivered with a fixed beam compared to a rotating beam, as well as to compare treatments delivered with and without real‐time adaptation. Results The dose distributions were found to be equivalent between the fixed beam and rotating beam geometry for real‐time adaptive radiotherapy using KIM and MLC tracking beam adaptation. Gamma analysis on the films showed agreement >98% using a 2%/2 mm criteria with adaptation for static shifts and periodic motion. Conclusions Fixed beam treatments with real‐time beam adaptation are dosimetrically equivalent to conventional treatments with a rotating beam, even in the presence of rigid target motion. This suggests that, for a rigid target, the high clinical accuracy of real‐time adaptive radiotherapy can be achieved with simpler beam geometry.