SU‐E‐T‐654: Direct Aperture Optimization Using An Inverse Form of Back Projection

Purpose: Intensity modulated radiotherapy plans using direct aperture optimization (DAO) promise high dosimetric quality and fast treatment delivery. We present a new and fast DAO approach where IMRT and arc therapy optimization are treated as inverse forms of CT reconstruction using multiplicative algebraic reconstruction technique (MART). Methods: We developed a modified MART for DAO, and the multiplicand is termed as ‘index’, indicating how much the current dose should be changed. Index is assigned to each voxel, each beamlet and each segment to optimize fluence map, MLC segment shape, and segment weight, respectively. In addition, a column generation method is developed to add new segments to improve dose conformity. Results: DAO using index‐dose can be completed within a few minutes. Without sacrificing plan quality, the implementation of the DAO in our in‐house treatment planning system reduced average IMRT treatment time from 13 min to 8 min for the prostate, and from 15 min to 9 min for the head and neck. The DAO approach is also applied for arc therapy in a head and neck case using rotational IMRT with burst mode. Conclusion: We have demonstrated a new direct aperture optimization technique using an inverse form of back projection approach. The method effectively reduces the number of segments needed for IMRT plans, and thus reduces treatment time. Our approach also provides a feasible solution for the rotation IMRT with burst mode.