Session Item

By optimizing beam weights according to a pre-treatment motion model, together with the delivery timeline, 4D optimized (4DO) Pencil Beam Scanned (PBS) proton plans can inherently mitigate the detrimental effects of organ motion. However, such plans are sensitive to changes between the nominal motion for plan optimization (PM) and the actual motion during delivery (DM). We propose and validate in silico, motion guided 4D optimized plan delivery approach (MG4dOPD) to improve robustness, by controlling motion variability within an uncertainty band around the PM.

The two-field 4DO plan were calculated (PTV) for 10 lung cancer cases, using 4DCT(MRI) datasets [1] from 5 patient geometries, each modulated by 2 deformable motions (fig2A). Based on PM using the tumour isocenter as surrogate, uncertainty bands for limiting DM were generated using an adaptive temporal motion model (TMM) (fig1A). To simulate DMs with controlled variabilities (fig1B), 1D surrogate motions with irregular patterns were generated within uncertainty bands of three widths (±2/4/6mm), each for 10 scenarios. A subject-specific spatial motion model (SMM) was established by correlating surrogate motions with deforming anatomy using Principle Component Analysis [2], for the purpose of PBS proton 4D dose calculations. SMM is then used to estimate the 3D deforming motion and associated 4DCTs from each simulated DMs. Moreover, 4DO plans with/without rescanning in the optimization stage were also calculated. The impact of different uncertainty band widths was investigated and compared to the uncontrolled DM without limiting variabilities.  Results were then compared to conventional 3D optimized plans (3DO), calculated on a geometric ITV (gITV: encompassing PTVs of all PM phases) based on averaged or inhalation CT’s and with/without volumetric rescanning (VS). All plans were quantified by DVH and V95 in CTV.

By applying MG4dOPD, the robustness of 4DO plans for motion variability was significantly improved on average by 3.7±4.9% (V95) when DMs were limited within the pre-defined uncertainty bands of ±4mm (Fig.1C and Fig.2B). Further improvements of V95 by 7.0±5.6% can be obtained when additionally incorporating a small number of rescans (VS2) into the 4D optimization. When motion variation was limited to ±4mm, 4DO plans were comparable to the ideal optimized plan quality, with averaged differences of V95 being 3.0±1.2% over all cases. Comparing to the best rescanned 3DO plans (avCT-VS8) (Fig.2B and Fig2C), MG4dOPD improved V95 by 3.3% (1.1-14.4%) in median (range), with best improvements observed for cases with large target and motion. 

MG4dOPD is an effective approach for preserving the advantage of 4D optimized plans for PBS proton therapy delivery to lung cancer, if motion variability could be restricted by e.g. on-line visual feedback.   

[1] A. Duetschler et al. 2021 Radiother Oncol. 161:S260-S261

[2] Ye Zhang et al 2013 Phys. Med. Biol. 58 8621