SU‐E‐T‐859: A New Approach to Improve IMRT Delivery Efficiency by Intensity Map Discretization

Purpose: To minimize IMRT treatment monitor units and delivery time without compromising dosimetric quality in an in‐house treatment planning system. Methods: In‐house treatment planning system PLanUNC employs a two‐step process to generate a MLC‐IMRT treatment plan. First, the index‐dose gradient minimization dose optimization process produces a continuous intensity map. Second, MLC segmentation process converts the intensity map into MLC segments. This two‐step process is sensitive to MLC segmentation technique and can lead to excessive number of segments and dosimetric degradation. We developed a new method that discretizes the intensity map during optimization. The new method considered each beam in turn: its intensity map was optimized using the current dose distribution; the optimized intensity map was discretized according to its statistical distribution; the dose distribution was updated after discretization. In this process, the peaks and valleys in the continuous intensity map were converted into discrete plateaus, and the optimization of remaining beams mitigated possible degradation of dose conformity introduced by the discretization of previous beams. MLC segmentation was applied to the discretized fluence maps. This new approach is being validated using prostate and head and neck cases. Results: For the prostate, with no compromise to dose conformity, the number of segments was reduced by 40%, and the MUs reduced by ∼10%. For the head and neck, the number of segments was reduced by 20%, and a small dose escalation was observed in adjacent normal tissues; this may be due to complex constraints used in the optimization process and could be accommodated by re‐optimizing the beam weight using fixed beam apertures in the future. Conclusions: Discretization based on statistical distribution of beamlet intensity could effectively reduce IMRT beam complexity and result in shorter treatment time.