Experimental evaluation of a robust optimization method for IMRT of moving targets

Abstract

Internal organ motion during radiation therapy, if not considered appropriately in the planning process, has been shown to reduce target coverage and increase the dose to healthy tissues. Standard planning approaches, which use safety margins to handle intrafractional movement of the tumor, are typically designed based on the maximum amplitude of motion, and are often overly conservative. Comparable coverage and reduced dose to healthy organs appear achievable with robust motion-adaptive treatment planning, which considers the expected probability distribution of the average target position and the uncertainty of its realization during treatment delivery. A dosimetric test of a robust optimization method for IMRT was performed, using patient breathing data. External marker motion data acquired from respiratory-gated radiotherapy patients were used to build and test the framework for robust optimization. The motion trajectories recorded during radiation treatment itself are not strictly necessary to generate the initial version of a robust treatment plan, but can be used to adapt the plan during the course of treatment. Single-field IMRT plans were optimized to deliver a uniform dose to a rectangular area. During delivery on a linear accelerator, a computer-driven motion phantom reproduced the patients' breathing patterns and a two-dimensional ionization detector array measured the dose delivered. The dose distributions from robust-optimized plans were compared to those from standard plans, which used a margin expansion. Dosimetric tests confirmed the improved sparing of the non-target area with robust planning, which was achieved without compromising the target coverage. The maximum dose in robust plans did not exceed 110% of the prescription, while the minimum target doses were comparable in standard and robust plans. In test courses, optimized for a simplified target geometry, and delivered to a phantom that moved in one dimension with an average amplitude of 17 mm, the robust treatment design produced a reduction of more than 12% of the integral dose to non-target areas, compared to the standard plan using 10 mm margin expansion.

Motion-compensating intensity maps in intensity-modulated radiation therapy

“…These edge-enhanced intensity maps resulted in high-quality dose distributions to the tumor in the face of motion uncertainty, while minimizing dose to the healthy tissue. Such edge-enhanced intensity maps have been delivered on a linear accelerator, and the resulting dose distribution measured using a detector array on a moving platform (to simulate breathing motion) validated previous computational experiments (Vrancic et al, 2009). It is important to note that our analysis is not targeted towards designing clinically deliverable edge-enhanced intensity maps.…”

“…A simple edge-enhanced intensity map delivered using only a few apertures has the potential benefit of reduced treatment delivery time, and better sampling of the motion distribution (Bortfeld et al, 2002). Note that it was shown in Vrancic et al (2009) that the MLC interplay effect is small with a clinically delivered edge-enhanced intensity map. Since robust treatments tend to reduce the heterogeneity of the intensity map (Chan et al, 2006), deriving simple margin or edge-enhanced intensity maps could help the optimizer do that in advance and potentially speed up the optimization.…”

Lung sparing and dose escalation in a robust‐inspired IMRT planning method for lung radiotherapy that accounts for intrafraction motion

“…One must consider then the potential for each MLC segment to sample a different probability distribution of the position of the moving tumor. Vrancic et al 33 have investigated the effects of motion interplay on an IMRT plan generated via a robust optimization strategy. They evaluated experimentally, the dosimetric accuracy of a robust optimization method for IMRT, with a motion phantom which mimics a patient's breathing pattern and a two-dimensional ionization detector array to measure the dose delivered for comparison with the calculated dose.…”

Bridging the gap between IMRT and VMAT: Dense angularly sampled and sparse intensity modulated radiation therapy

“…For each patient, we planned the treatment using Pinnacle TM system (Philips Radiation Oncology Systems, Milpitas, CA), with different numbers of beams (5,7,11,15,23,31,61, and 90 in this study), uniformly distributed in the range of 0 to 360…”

“…These techniques include fixedgantry IMRT (Ref. 1) (typically, with five to ten beams) planned using beamlet-based optimization [2][3][4][5] or direct aperture optimization or segment-based optimization, [6][7][8] volumetric modulated arc therapy (VMAT) [9][10][11] (typically with one to two arcs), and Tomotherapy. 12,13 Each of these techniques captures certain aspect(s) of desirable features of radiation therapy (RT) but compromises in either dose distribution (in fixed-gantry IMRT and VMAT) or delivery efficiency (in Tomotherapy TM ).…”