Reduced Order Constrained Optimization (ROCO):

Purpose: We use reduced-order constrained optimization (ROCO) to create clinically acceptable IMRT plans quickly and automatically for advanced lung cancer patients. Our new ROCO implementation works with the treatment planning system and full dose calculation used at Memorial Sloan-Kettering Cancer Center, and we have implemented mean dose hard-constraints, along with the point-dose and dose-volume constraints that we used for our previous work on the prostate. Methods: ROCO consists of three major steps. First, we sample the space of treatment plans by solving a series of optimization problems using penalty-based quadratic objective functions. Next, we find an efficient basis for this space via principal component analysis (PCA); this reduces the dimensionality of the problem. Finally, we solve a constrained optimization problem over this basis to find a clinically acceptable IMRT plan. Dimensionality reduction makes constrained optimization computationally efficient. Results: We apply ROCO to 12 stage III non-small-cell lung cancer (NSCLC) cases, generating IMRT plans that meet all clinical constraints and are clinically acceptable, and demonstrate that they are competitive with the clinical treatment plans. We also test how many samples and PCA modes are necessary to achieve an adequate lung plan, demonstrate the importance of long range dose calculation for ROCO, and evaluate the performance of non-specific normal tissue (“rind”) constraints in ROCO treatment planning for the lung. Finally, we show that ROCO can save time for planners; we estimate that, in our clinic, planning using our approach would save a median of 105 minutes for the patients in our study. Conclusions: New challenges arise when applying ROCO to the lung site, which include the lack of a class solution, a larger treatment site, an increased number of parameters and beamlets, a variable number of beams and beam arrangement, and the customary use of rinds in clinical plans to avoid high-dose areas outside the PTV. In our previous work, use of an approximate dose calculation in the hard constraint optimization sometimes meant that clinical constraints were not met when evaluated with the full dose calculation. This difficulty has been removed in the current work by using the full dose calculation in the hard constraint optimization. We have demonstrated that ROCO offers a fast and automatic way to create IMRT plans for advanced NSCLC, which extends our previous application of ROCO to prostate cancer IMRT planning.