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Purpose: Respiratory motion is one of the major challenges in radiotherapy. In this work, a comprehensive and clinically plausible set of 4D numerical phantoms, together with their corresponding "ground truths," have been developed and validated for 4D radiotherapy applications., Methods: The phantoms are based on CTs providing density information and motion from multi-breathing-cycle 4D Magnetic Resonance imagings (MRIs). Deformable image registration (DIR) has been utilized to extract motion fields from 4DMRIs and to establish inter-subject correspondence by registering binary lung masks between Computer Tomography (CT) and MRI. The established correspondence is then used to warp the CT according to the 4DMRI motion. The resulting synthetic 4DCTs are called 4DCT(MRI)s. Validation of the 4DCT(MRI) workflow was conducted by directly comparing conventional 4DCTs to derived synthetic 4D images using the motion of the 4DCTs themselves (referred to as 4DCT(CT)s). Digitally reconstructed radiographs (DRRs) as well as 4D pencil beam scanned (PBS) proton dose calculations were used for validation., Results: Based on the CT image appearance of 13 lung cancer patients and deformable motion of five volunteer 4DMRIs, synthetic 4DCT(MRI)s with a total of 871 different breathing cycles have been generated. The 4DCT(MRI)s exhibit an average superior-inferior tumor motion amplitude of 7 ± 5 mm (min: 0.5 mm, max: 22.7 mm). The relative change of the DRR image intensities of the conventional 4DCTs and the corresponding synthetic 4DCT(CT)s inside the body is smaller than 5% for at least 81% of the pixels for all studied cases. Comparison of 4D dose distributions calculated on 4DCTs and the synthetic 4DCT(CT)s using the same motion achieved similar dose distributions with an average 2%/2 mm gamma pass rate of 90.8% (min: 77.8%, max: 97.2%)., Conclusion: We developed a series of numerical 4D lung phantoms based on real imaging and motion data, which give realistic representations of both anatomy and motion scenarios and the accessible "ground truth" deformation vector fields of each 4DCT(MRI). The open-source code and motion data allow foreseen users to generate further 4D data by themselves. These numeric 4D phantoms can be used for the development of new 4D treatment strategies, 4D dose calculations, DIR algorithm validations, as well as simulations of motion mitigation and different online image guidance techniques for both proton and photon radiation therapy., (© 2022 The Authors. Medical Physics published by Wiley Periodicals LLC on behalf of American Association of Physicists in Medicine.)

Background: With 4DCT the risk of introducing positional systematic errors in lung cancer radiotherapy can be minimised. A common approach is to plan on the phase bin of the 4DCT best representing the tumour's time-weighted mean position also called the midventilation scan. However breathing irregularities can introduce uncertainties and potentially misrepresent both the tumour trajectory and the determination of the midventilation phase. In this study we evaluated the robustness of the midventilation approach in the presence of irregular breathing patterns., Methods: A LEGO Mindstorms(®) phantom with compact balls simulating lung tumours was constructed. The breathing curves loaded in the phantom were either acquired from a human volunteer or constructed with various magnitudes (ranging from 12 to 29 mm) as well as various irregularities of motion pattern. Repeated 4DCT scans were performed while tumour trajectories were recorded with two motion tracking systems., Results: The time-weighted mean tumour position is accurately represented in 4DCT scans, even for irregular breathing patterns: the position presentation in the midventilation scan was always within in one standard deviation of the global position presentation (3 mm and 2 mm for regular and irregular breathing patterns, respectively). The displacement representation tended to be underestimated in 4DCT scans., Conclusion: The midventilation approach is robust even in the presence of breathing irregularity. The representation of the tumour trajectory in 4DCT scans is affected by breathing irregularity and the extent of tumour motion can be underestimated, which will affect the calculation of patient-individualised margins based on the 4DCT scan., (Copyright © 2013 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.)

Background: In lung cancer radiotherapy, planning on the midventilation (MidV) bin of a four-dimensional (4D) CT scan can reduce the systematic errors introduced by respiratory tumour motion compared to conventional CT. In this study four different methods for MidV bin selection are evaluated., Material and Methods: The study is based on 4DCT scans of 19 patients with a total of 23 peripheral lung tumours having peak-to-peak displacement ≥ 5 mm in at least one of the left-right (LR), anterior-posterior (AP) or cranio-caudal (CC) directions. For each tumour, the MidV bin was selected based on: 1) visual evaluation of tumour displacement; 2) rigid registration of tumour position; 3) diaphragm displacement in the CC direction; and 4) carina displacement in the CC direction. Determination of the MidV bin based on the displacement of the manually delineated gross tumour volume (GTV) was used as a reference method. The accuracy of each method was evaluated by the distance between GTV position in the selected MidV bin and the time-weighted mean position of GTV throughout the bins (i.e. the geometric MidV error)., Results: Median (range) geometric MidV error was 1.4 (0.4-5.4) mm, 1.4 (0.4-5.4) mm, 1.9 (0.5-6.9) mm, 2.0 (0.5-12.3) mm and 1.1 (0.4-5.4) mm for the visual, rigid registration, diaphragm, carina, and reference method. Median (range) absolute difference between geometric MidV error for the evaluated methods and the reference method was 0.0 (0.0-1.2) mm, 0.0 (0.0-1.7) mm, 0.7 (0.0-3.9) mm and 1.0 (0.0-6.9) mm for the visual, rigid registration, diaphragm and carina method., Conclusion: The visual and semi-automatic rigid registration methods were equivalent in accuracy for selecting the MidV bin of a 4DCT scan. The methods based on diaphragm and carina displacement cannot be recommended without modifications.

Background: Proton therapy of lung cancer holds the potential for a reduction of the volume of irradiated normal lung tissue. In this work we investigate the robustness of intensity modulated proton therapy (IMPT) plans to motion, and evaluate a geometrical tumour tracking method to compensate for tumour motion., Material and Methods: Seven patients with a nine targets with 4DCT scans were selected. IMPT plans were made on the midventilation phase using a 3-field technique. The plans were transferred and calculated on the remaining nine phases of the 4DCT, and the combined dose distribution was summed using deformable image registration (DIR). An additional set of plans were made in which the proton beam was simply geometrically shifted to the centre of the gross tumour volume (GTV), i.e. simulating tracking of the tumour motion but without on-line adjustment of the proton energies. A possible interplay effect between the dynamics of the spot scanning delivery and the tumour motion has not been considered in this work., Results: Around 97-100% of the GTV was covered by 95% of the prescribed dose (V95) for a tumour displacement of less than about 1 cm with a static beam. For the remaining three of nine targets with a larger motion the tracking method studied provided a marked improvement over static beam; raising the GTV V95 from 95 to 100%, 82 to 98% and 51 to 97%, respectively., Conclusion: The possibility of performing DIR and summing the dose on the 4DCT data set was shown to be feasible. The fairly simplistic tracking method suggested here resulted in a marked improvement in GTV coverage for tumours with large intra-fractional motion (>1 cm displacement), indicating that on-line adjustment of the proton energies may be redundant.

Background and Purpose: Four-dimensional computed tomography (4DCT) is used for breathing-adapted radiotherapy planning. Irregular breathing, large tumour motion or interpolation of images can cause artefacts in the 4DCT. This study evaluates the impact of artefacts on gross tumour volume (GTV) size., Material and Methods: In 19 4DCT scans of patients with peripheral lung tumours, GTV was delineated in all bins. Variations in GTV size between bins in each 4DCT scan were analysed and correlated to tumour motion and variations in breathing signal amplitude and breathing signal period. End-expiration GTV size (GTVexp) was considered as reference for GTV size. Intra-session delineation error was estimated by re-delineation of GTV in eight of the 4DCT scans., Results: In 16 of the 4DCT scans the maximum deviations from GTVexp were larger than could be explained by delineation error. The deviations were largest in the bins adjacent to the end-inspiration bin. The coefficient of variation of GTV size was significantly correlated to tumour motion in the cranio-caudal direction, but no significant correlation was found to breathing signal variations., Conclusion: We found considerable variations in GTV size throughout the 4DCT scans. Awareness of the error introduced by artefacts is important especially if radiotherapy planning is based on a single 4DCT bin., (Copyright 2010 Elsevier Ireland Ltd. All rights reserved.)