Inverse‐optimized 3D conformal planning: Minimizing complexity while achieving equivalence with beamlet IMRT in multiple clinical sites

Purpose: Inverse planned intensity modulated radiation therapy (IMRT) has helped many centers implement highly conformal treatment planning with beamlet‐based techniques. The many comparisons between IMRT and 3D conformal (3DCRT) plans, however, have been limited because most 3DCRT plans are forward‐planned while IMRT plans utilize inverse planning, meaning both optimization and delivery techniques are different. This work avoids that problem by comparing 3D plans generated with a unique inverse planning method for 3DCRT called inverse‐optimized 3D (IO‐3D) conformal planning. Since IO‐3D and the beamlet IMRT to which it is compared use the same optimization techniques, cost functions, and plan evaluation tools, direct comparisons between IMRT and simple, optimized IO‐3D plans are possible. Though IO‐3D has some similarity to direct aperture optimization (DAO), since it directly optimizes the apertures used, IO‐3D is specifically designed for 3DCRT fields (i.e., 1–2 apertures per beam) rather than starting with IMRT‐like modulation and then optimizing aperture shapes. The two algorithms are very different in design, implementation, and use. The goals of this work include using IO‐3D to evaluate how close simple but optimized IO‐3D plans come to nonconstrained beamlet IMRT, showing that optimization, rather than modulation, may be the most important aspect of IMRT (for some sites). Methods: The IO‐3D dose calculation and optimization functionality is integrated in the in‐house 3D planning/optimization system. New features include random point dose calculation distributions, costlet and cost function capabilities, fast dose volume histogram (DVH) and plan evaluation tools, optimization search strategies designed for IO‐3D, and an improved, reimplemented edge/octree calculation algorithm. The IO‐3D optimization, in distinction to DAO, is designed to optimize 3D conformal plans (one to two segments per beam) and optimizes MLC segment shapes and weights with various user‐controllable search strategies which optimize plans without beamlet or pencil beam approximations. IO‐3D allows comparisons of beamlet, multisegment, and conformal plans optimized using the same cost functions, dose points, and plan evaluation metrics, so quantitative comparisons are straightforward. Here, comparisons of IO‐3D and beamlet IMRT techniques are presented for breast, brain, liver, and lung plans. Results: IO‐3D achieves high quality results comparable to beamlet IMRT, for many situations. Though the IO‐3D plans have many fewer degrees of freedom for the optimization, this work finds that IO‐3D plans with only one to two segments per beam are dosimetrically equivalent (or nearly so) to the beamlet IMRT plans, for several sites. IO‐3D also reduces plan complexity significantly. Here, monitor units per fraction (MU/Fx) for IO‐3D plans were 22%–68% less than that for the 1 cm × 1 cm beamlet IMRT plans and 72%–84% than the 0.5 cm × 0.5 cm beamlet IMRT plans. Conclusions: The unique IO‐3D algorithm illustrates that inverse planning can achieve high quality 3D conformal plans equivalent (or nearly so) to unconstrained beamlet IMRT plans, for many sites. IO‐3D thus provides the potential to optimize flat or few‐segment 3DCRT plans, creating less complex optimized plans which are efficient and simple to deliver. The less complex IO‐3D plans have operational advantages for scenarios including adaptive replanning, cases with interfraction and intrafraction motion, and pediatric patients.