On-line re-optimization of prostate IMRT plan for adaptive radiation therapy: A feasibility study and implementation

Prostate cancer is a disease that affected approximately 200,000 men in United States in 2006. Radiation therapy is a non invasive treatment option for this disease and is highly effective. The goal of radiation therapy is to deliver the prescription dose to the tumor (prostate) while sparing the surrounding healthy organs (e.g. bladder, rectum, and femoral heads). One limitation of the radiation therapy is organ position and shape variation from day to day. These variations could be as large as half inch. The conventional solution to this problem is to include some margins surrounding the target when plan the treatment. The development of image guided radiation therapy technique allows in-room correction which potentially eliminates the patient setup error however the uncertainty due to organ deformation still remains. Performing a plan re-optimization will take about half hour which is infeasible to perform an online correction. A technique of performing online re-optimization of intensity modulated radiation therapy is developed for adaptive radiation therapy of prostate cancer. The technique is capable of correction both organ positioning and shape changes within a few minutes. The proposed technique involves (1) 3D on-board imaging of daily anatomy, (2) registering the daily images with original planning CT images and mapping the original dose distribution to the daily anatomy, (3) real time re-optimization of the plan. Finally the leaf sequences are calculated for the treatment delivery. The feasibility of this online adaptive radiation therapy scheme was evaluated by clinical cases. The results demonstrate that it is feasible to perform online re-optimization of the original plan when large position or shape variation occurs.