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

Abstract

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 ...

Inverse 4D conformal planning for lung SBRT using particle swarm optimization

“…Our present scope is limited to four dimensional conformal radiotherapy (4D CRT) planning through optimizing aperture fluence-weights. Despite noticeable improvements in plan quality which have been shown to be achievable through inverse planned 3D CRT (Fraass et al 2012), typically, 3D CRT plans are generated using forward planning (Wu et al 2004). We generate our 4D CRT plans using inverse planning due to the need to optimize over a significantly larger solution space.…”

Penalization of aperture complexity in inversely planned volumetric modulated arc therapy

“…The details of the optimization system can be found in Ref. 20 which discusses a unique inverse planning method referred to as inverse-optimized 3D conformal planning. For initial testing, we created a new plan for a paraspinal SBRT case that was previously treated in our clinic.…”

Radiotherapy Planning Using an Improved Search Strategy in Particle Swarm Optimization

“…Depending on the RT technique, the number of adjustable parameters varies between tens to thousands. For instance, although typically ≤ 12 gantry angles (beams) are considered in both conventional CRT and IMRT, the number of apertures through which a beam is radiated increases from one in CRT to hundreds in IMRT [5, 6]. Arc-based therapies, as another example, employ large number of beams and deliver radiation on continuous rotations of the radiation source [7].…”