Hardware-sensitive optimization for intensity modulated radiotherapy

The multileaf collimator (MLC) hardware constraints are usually neglected i n the process of intensity-modulated beam optimization. Consequently, it is not always possible to deliver planned beam modulation using dynamic MLC. Beam optimization is significantly diminished if the results must be approx imated due to limitations imposed by the delivery device. To overcome this problem, an inverse beam optimization method which incorporates the hardwar e constraints has been developed. The hardware constraints, including the l eaf velocity, the dose rate and the minimum required gap between opposing a nd adjacent leaves, were considered. An iterative search for feasible modul ation was conducted alternately in the dosimetric space and the MLC positio n-time space. The optimization algorithm was designed for a unidirectional leaf trajectory and a constant dose rate. A scheme to reduce tongue-and-gro ove underdosage during optimization was also implemented. Comparisons were made between the solutions produced by this method and conventional optimiz ation disregarding the hardware restrictions. The beam profiles generated b y the conventional method were modified to satisfy the hardware specificati ons. The results indicate that inclusion of MLC constraints during optimiza tion can improve the degree of conformity that is deliverable.