AbstractID: 7476 Title: Independent monitor unit calculations for linac-based stereotactic treatments
utilizing a number of discrete arcs.
We have used three to five arcs for stereotactic radiosurgery and
radiotherapy (SRS, SRT) treatments. A combination of external circular
collimators and the four linac X-ray jaws define the apertures. The dose
contribution to the isocenter per arc divided by the dose rate at the average
depth of the arc is used to independently check the monitor unit calculations
of the planning computer. The dose rate is a product of the TMR and the
output factor. The latter includes the effects of phantom scatter, jaw scatter
and jaw penumbra. A spreadsheet-based program has been developed to
perform the independent check of the treatment times. The collimator
setting, radiosurgery cone size and average depth per arc are input. The
equivalent jaw setting and effective field size are then calculated. Based on
these, the contributions of the jaw scatter, phantom scatter and jaw
penumbra to the output factor are computed using exponential fit functions.
The TMR is interpolated using a two- dimensional lookup table of the TMR
as a function of depth and field size. In the event where only the external
circular collimators are needed, one-dimensional lookup tables for the output
factor and exponential fitting parameters of the TMR versus cone size are
used instead. We have found that the spreadsheet-based independent physics
calculation of the monitor units is easy to implement and its use results in
substantial time savings for the physicist reviewing the radiosurgery
calculations.