Patient position verification with oblique radiation beams.

Purpose: In this study we investigated whether the position of head and neck cancer patients during radiotherapy could be determined from portal images of oblique radiation beams. Currently applied additional anterior posterior (AP) and lateral verification beams could then be abandoned.

Method: The patient position was determined from portal images of the oblique radiation beams and compared with that determined from AP and lateral verification beams. Seven hundred and fifty-one portal images of 18 different patients were analyzed.

Results: The set-up errors of patients that were treated with oblique gantry angles could be determined with the same accuracy from the oblique beams as from the AP and lateral verification beams in the ventrodorsal and craniocaudal direction. An additional AP beam was necessary to obtain the same accuracy in the lateral direction, because the used beam directions were relatively close to lateral. The position verification of patients treated with both oblique gantry angles and isocentric table rotations was more accurate if AP and lateral verification beams were used.

Conclusions: For patients treated with an irradiation technique with oblique gantry angles (and no isocentric table rotations) position verification can be performed by using these oblique radiation beams.