Po‐Poster ‐ 06: Beam spot motion of medical linear accelerators

Abstract

Megavoltage cone‐beam computed tomography (MV‐CBCT) is a volumetric imaging method that can improve patient setup verification techniques. MV‐CBCT utilizes the treatment beam to obtain projections at every 1–2° around the patient. For this to be clinically acceptable, total dose received by the patient from all imaging sessions must be kept to a minimum and typically should be ⩽5% of the prescribed dose. This necessitates the use of extremely low doses (<<1MU) in the acquisition of each projection. At such low dose levels beam spot instability is known to exist and can compromise image quality. The purpose of this work is to quantify the beam spot motion of Siemen's accelerator for a conventional 6 MV beam and 5.4 MV “imaging beam” generated with a beryllium target. This was accomplished by using an a‐Si flat panel detector to image a cone‐beam geometric calibration phantom and using a calibration algorithm to derive the spot motion in reference frames fixed in space and/or attached to the gantry. Motion of the beam spot was observed immediately after beam startup primarily in the gun‐target direction. The maximum fixed motion of the 6MV beam spot was 1.1±0.3 mm and is similar to that observed for the low Z beam (1.2±0.1 mm). However, the beam spot position of the latter stabilized at about 0.5 MU compared to 6 MU for the 6 MV beam and had much less fluctuations once stabilized. The beam spot position of the conventional beam was much less reproducible than the low Z beam.