Beam focal spot intrafraction motion and gantry angle dependence: A study of Varian linac focal spot alignment

Luke Slama, Hans Lynggaard Riis, Mahsheed Sabet, Michael P Barnes, Martin Ebert, Sun Chan, Pejman Rowshan Farzad

Research output: Contribution to journalArticle

Abstract

The characteristics of the focal spot of the linear accelerator (linac) play a role in determining the resulting dose distribution within the patient, and hence probability of treatment success. A direct measurement of focal spot position is not recommended by AAPM Task Group 142, but factors influenced by focal spot position, such as beam symmetry and isocentre position, are. Traditional methods of measuring focal spot position are time consuming and can only be performed at gantry 0°. The presented method has been proposed using a phantom of novel design to accurately measure the position of the focal spot relative to the collimator’s axis of rotation (CAX) at any gantry angle, and to measure the intra-fraction movement of the focal spot relative to the mean position during treatment. The method was reproducible to within 0.012 mm/0.029 mm (mean/max) for the three Varian linacs tested. The focal spot position was shown to deviate from the CAX by up to 0.386 mm during gantry rotation. The focal spot position was more unstable at the start of treatment, with the worst performing linac having an initial displacement of up to 0.15 mm from its mean position before stabilizing to within 0.01 mm after 3 s. The method proposed is a beneficial addition to the quality assurance (QA) schedule of any clinic, allowing quick determination of source position and movement at any gantry angle. Measurement of focal spot allows the possibility of fine-tuning the electron beam steering system to improve the standard of the photon beam and of stereotactic treatments.
Original languageEnglish
Pages (from-to)41-47
Number of pages7
JournalPhysica Medica
Volume63
DOIs
Publication statusPublished - Jul 2019

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gantry cranes
Particle Accelerators
linear accelerators
alignment
Therapeutics
Photons
axes of rotation
Appointments and Schedules
collimators
Electrons
beam steering
photon beams
assurance
schedules
tuning
electron beams

Cite this

Slama, Luke ; Riis, Hans Lynggaard ; Sabet, Mahsheed ; Barnes, Michael P ; Ebert, Martin ; Chan, Sun ; Rowshan Farzad, Pejman. / Beam focal spot intrafraction motion and gantry angle dependence : A study of Varian linac focal spot alignment. In: Physica Medica. 2019 ; Vol. 63. pp. 41-47.
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Beam focal spot intrafraction motion and gantry angle dependence : A study of Varian linac focal spot alignment. / Slama, Luke; Riis, Hans Lynggaard; Sabet, Mahsheed; Barnes, Michael P; Ebert, Martin; Chan, Sun; Rowshan Farzad, Pejman.

In: Physica Medica, Vol. 63, 07.2019, p. 41-47.

Research output: Contribution to journalArticle

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