AbstractID: 7354 Title: Independent Monitor Unit Calculation for Serial Tomotherapy
A self-consistent monitor unit (MU) calculation method has been developed which
provides a quick and independent verification of the MUs calculated by the Corvus
inverse planning system for Nomos serial tomotherapy. The method takes into account
the two unique features of Nomos serial tomotherapy: the dynamic arc delivery of
intensity modulated beams and the slice-by-slice multiple-table-position treatment of
large tumor volumes. At each table position, the method converts the intensity modulated
beam profiles planned at fixed gantry angles into conventional beam apertures at each
elemental arc segment corresponding to the arc delivery. The MU calculation is then
made equivalent to that for a conventional arc with gantry-angle dependent beam
apertures. The dose contribution from each elemental arc segment is calculated by using
the Clarkson scatter summation technique based on measured tissue-maximum-ratio and
output factors. For treatments requiring multiple table positions, the MU for each table
position is dependent on the MUs given at other table positions, as the additional
scattered dose from adjacent table positions is proportional to the MU given at those table
positions. The current method takes into account the off-slice scatter contribution selfconsistently by solving a set of coupled linear equations for the MUs of all involved table
positions. The off-slice scatter contribution is also calculated by using the Clarkson
scatter summation technique. Comparison of the MUs calculated by the current method
and the Corvus planning system as well as that deduced from direct measurements will be
discussed for different treatment conditions.