Radiosurgery--Quality control

MapCheck measurements for 50 retrospective patient’s treatment plans suggested that MapCheck could be effectively employed in routine patient specific quality assurance in M6 Cyberknife with beams delivered at different treatment angles. However, these measurements also suggested that for highly intensity modulated MLC plans, field segments of width < 8 mm should further be analyzed with a modified (-4%) correction factor. Results of MC simulations of the M6 Cyberknife using the EGSnrc program for 2-5 millions of incident particles in BEAMnrc and 10-20 millions in DOSXYZnrc have shown dose uncertainties within 2% for open fields from 7.6 x 7.7 mm2 to 100 x 100 mm2. Energy and corresponding FWHM were optimized by comparing with water phantom measurements at 800 mm SAD resulting to E = 7 MeV and FWHM = 2.2 mm. Good agreement of dose profiles (within 2%) and outputs (within 3%) were found between the MC simulations and water phantom measurements for the open fields.

It is hypothesized that the uncertainty of the Synchrony® model from the rotation of a
geometrically asymmetrical single fiducial shall be non-zero during the motion tracking.
To validate this hypothesis, the uncertainty was measured for a Synchrony® model built
for a respiratory motion phantom oriented at different yaw angles on a Cyberknife®
treatment table.
A Mini-ball Cube with three cylindrical GoldMark™ (1mmx5mm Au) numbered
fiducials was placed inside a respiratory phantom and used for all tests. The fiducial with
the least artifact interference was selected for the motion tracking. A 2cm periodic,
longitudinal, linear motion of the Mini-ball cube was executed and tested for yaw
rotational angles, 0° – 90°. The test was repeated over 3 nonconsecutive days. The
uncertainty increased with the yaw angle with the most noticeable changes seen
between20° and 60° yaw, where uncertainty increased from 23.5% to 57.9%. A similar test was performed using a spherical Gold Anchor™ fiducial. The uncertainties found
when using the Gold Anchor™ were statistically lower than those found when using the
GoldMark™ fiducial for all angles of rotation.
For the first time, it is found that Synchrony® model uncertainty depends on fiducial
geometry. In addition, this research has shown that tracking target rotation using a single
fiducial can be accomplished with the Synchrony® model uncertainty as it is displayed on
the treatment console.
The results of this research could lead to decreased acute toxicity effects related to
multiple fiducials.