Equivalence and Differences Between Robust Optimization Methods and PTV Based Optimization Method for IMRT and IMPT
S Ge*, M Quan, L Zhang, W Liu, R Mohan, University of Texas, MD Anderson Cancer Ctr., Houston, TXSU-E-T-701 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: The similarity and differences between varies robust optimization methods and PTV based optimization method for photon and proton are still confused for many people. Here is the first clear presentation about the relationship between them.
Methods:The dose distributions of a PTV based IMRT and IMPT plan of a non-small cell lung cancer (NSCLC) patient are calculated with the alignment uncertainty of 5mm and range uncertainty of ±3.5% (proton). The data is analyzed to compare the sensitivity of PTV based IMRT and IMPT plan to setup and range uncertainties. Probabilistic, minimax worst case and voxel based worst case optimization methods are compared with PTV safety margin based optimization method with a simple conceptual example.
Results:Relative to the isocenter, the dose distributions (dose cloud) of IMRT plan is very static, and the dose distribution of IMPT plan is sensitive to range and setup uncertainties. For IMPT plan, the PTV coverage decreases only in the edge area when the volume moves outside the dose cloud and the CTV coverage is pretty well. For proton, the isodose line of the prescription dose may break inside the CTV and underdose may happen in the middle of the CTV. Comparing various robust optimization methods with PTV based method shows that the two worst case optimization methods are equal to the PTV based method for photon and more robust for proton.
Conclusion:PTV based optimization method is only robust for photon because it tries to deliver the dose to the whole space where CTV may occupy (PTV) based on nominal anatomy; while the worst case method and minimax worst case methods are robust for both photon and proton because they not only try to deliver the dose to the whole space where CTV may occupy, but also optimize the dose distribution based on changed anatomy.