Abstract
Tumour tracking is an advanced radiotherapy technique for precise treatment of tumours subject to organ motion. In this work, we addressed crucial aspects of dose delivery for its realisation in pencil beam scanning proton therapy, exploring the momentum acceptance and global achromaticity of a Gantry beamline to perform continuous energy regulation with a standard upstream degrader. This novel approach is validated on simulation data from three geometric phantoms of increasing complexity and one liver cancer patient using 4D dose calculations. Results from a standard high-to-low beamline ramping scheme were compared to alternative energy meandering schemes including combinations with rescanning. Target coverage and dose conformity were generally well recovered with tumour tracking even though for particularly small targets, large variations are reported for the different approaches. Meandering in energy while rescanning has a positive impact on target homogeneity and similarly, hot spots outside the targets are mitigated with a relatively fast convergence rate for most tracking scenarios, halving the volume of hot spots after as little as 3 rescans. This work investigates the yet unexplored potential of having a large momentum acceptance in medical beam line, and provides an alternative to take tumour tracking with particle therapy closer to clinical translation.
Highlights
Tumour tracking is an advanced radiotherapy technique for precise treatment of tumours subject to organ motion
The 4D CT images used in this study were obtained by warping one phase of the patient 4DCT with the deformation fields derived from 4D MR imaging to generate 20 motion phases with temporal resolution of 2.7 H z32
Patient breathing has been modelled using a multiresolution B-spline deformable image registration algorithm implemented in the Plastimatch software[33]
Summary
Tumour tracking is an advanced radiotherapy technique for precise treatment of tumours subject to organ motion. Magnetic resonance imaging has been integrated in radiotherapy linear accelerators from commercial vendors, opening up new possibilities for on-line soft tissue imaging in the context of advanced radiotherapy workflows and motion m itigation[17] The translation of these techniques to particle therapy is challenging, and presents a number of very specific issues dictated by the finite range of the treatment beam and its dependence on the density of tissues crossed. Successful tumour tracking with particles is strictly interrelated with accurate modelling of the whole patient anatomy in the beam p ath[18] and the rapid, on the fly, adaptation of beam settings to avoid overshooting into healthy tissues or severe distortions of the dose in the target[19] This latter aspect, related to on-line control of dose delivery to track intra-fractional organ motion is the subject of this contribution, with specific reference to proton treatments delivery with pencil beam scanning (PBS). Treatments are delivered as a sequence of discrete iso-energy layers while scanning transversally through the target with limited capacity for fast energy m odulation[21]
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