AbstractID: 7315 Title: Robotic Variable SAD for Frameless Functional Radiosurgery
Functional radiosurgery requires the delivery of a very high single dose resulting in long
beam-on times. A smaller lesion size delivered in a shorter time is better. Unlike the
isocentric gantry, the robotic arm mounted linac of the Cyberknife may utilize a variable
source target distance. This study investigates with Monte Carlo simulations the effect of
a robotic variable SAD delivery technique to produce a smaller lesion volume while
simultaneously achieving a higher dose rate for functional radiosurgery. An isocentric
80 cm SAD treatment plan from a trigeminal neuralgia patient was utilized. Each beam
location was moved along the central axis to provide 5, 10, 15, and 20 cm clearance
between the collimator and the patient’s skin resulting in separate plans with 45, 50, 55,
and 60 cm SSD for both 5 mm and 7.5 mm collimators. A 3.25 mm collimator was
simulated using EGS4/BEAM and utilized to generate a comparison dose distribution.
The EGS4/MCDOSE code was used to simulate the treatment plans. The required
monitor units to achieve 80 Gy peak dose was determined from the simulations for each
configuration. The beam-on time for a 300 MU/minute output was calculated. The 5
mm collimator at 45 cm SSD achieves the smallest target isodose volume (3.03 times
reduction) and the 7.5 mm collimator at 45 cm SSD achieves the highest dose rate (2.49
times increase). This technique provides dose distributions equivalent to those from
smaller collimators while simultaneously achieving higher dose rates.
Research supported by Accuray Oncology, Inc.