AbstractID: 6886 Title: Comparison of Leaf Sequencing Algorithms Against the Efficient Frontier of
Monitor Unit and Segment Usage
Different algorithms have been devised to create the sequence of leaf positions (or
segments) that will generate a desired intensity map with static field IMRT. The goal has
been to use fewer monitor units and segments in order to reduce machine leakage and
treatment time. None of the published methods minimizes the number of segments for a
given number of monitor units. This study investigates the excess number of monitor
units and segments produced by competing leaf sequencing algorithms. Tests were
performed using random maps to remove selection bias and permit any available
algorithm to be evaluated. Tests of three algorithms are reported here. One, credited to
Bortfeld, minimizes monitor units. The other two, called areal and reducing, aim to
decrease segment number at the price of an increased number of monitor units.
An integer program was used to find the minimum number of segments needed to
complete a sequence with the minimum number of monitor units. By allowing
bidirectional leaf movement, the number of segments can be reduced >140% from the
unidirectional sequence provided by the Bortfeld algorithm without requiring more
monitor units. The bidirectional sequences of the areal and reducing algorithms exceed
the minimum number of monitor units by >24%, yet still use >35% more segments than
needed by sequences expending the minimum number of monitor units found by the
integer program. The shortfall of any future algorithm can be measured by comparing
solutions using against the optima found by the integer program.