AbstractID: 8545 Title: Mechanical evaluation of a CT/linac therapy system
The purpose of this study was to characterize the mechanical accuracy and
precision of a new CT/linac therapy system. The layout of this system is such that after
the patient is positioned for treatment under the linac, the couch can be rotated through
180 degrees for CT scanning. Translations in the vertical or transverse directions may be
needed for alignment with the CT scanner aperture. The entire CT gantry then moves on
rails across the patient to acquire an image set. Ultimately, this CT image set will be used
to determine, and correct for, the patient’s position and anatomy relative to that of the
treatment planning image set.
Five mechanical sources of variation in the relationship between the radiotherapy
isocenter and CT isocenter were identified and investigated: couch position at the linac
side after a rotation, couch position at the CT side after a rotation, couch position as given
by the digital readouts, effect of couch flexibility, and reported CT coordinates. With the
exception of couch flexibility, all were found to have standard deviations of 0.5mm or
less. The couch was found to flex vertically by more than 1mm, but the difference in flex
between the linac side and CT side was found to be less than 1mm. Alignment protocols
can be compared by quadrature addition of the appropriate variations. The results will be
used in the development of reliable and effective localization and treatment protocols.