Characterization of linear accelerator X‐ray source size using a laminated beam‐spot camera

A laminated beam‐spot camera of length 20 cm and effective cross‐sectional area [Formula: see text] was designed and constructed for the measurement of X‐ray beam‐spot sizes on different models of Siemens accelerators. With the accelerator gantry at 180° and camera positioned on an accessory tray holder, an XV film placed in contact with the camera at the distal end of it detected those X‐rays that were transmitted through the camera. The FWHM of the detected X‐ray intensity profile in the gun–target (G–T) direction or the orthogonal A–B direction was used as a measure of the beam‐spot size in that direction. Siemens Mevatron MXEs exhibited a beam‐spot size of 1.7 [Formula: see text] in both the in‐plane and cross‐plane directions for 6 MV photon beams. The beam‐spot size observed for a Mevatron MDX‐2 was larger by up to 1 mm, and also was different for the in‐plane and cross‐plane directions. For Siemens PRIMUS accelerators, the beam‐spot size in the in‐plane direction was found to fall in the range [Formula: see text] , whereas the beam‐spot size in the cross‐plane direction fell within [Formula: see text] for 6, 10, and 18 MV photon beams. Assessment of long‐term stability of the beam‐spot size shows the spot size remains fairly stable over time. PACS number: 87.56.B‐, 87.56.bd