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arxiv: 2606.03562 · v1 · pith:72NEUK2Vnew · submitted 2026-06-02 · ⚛️ physics.med-ph

Magnet-Free Proton Therapy with 4D Pencil Beam Delivery Optimisation

Nair N von Muehlenen , Florentin Bieder , Ye Zhang , Philippe C Cattin This is my paper

Pith reviewed 2026-06-28 07:29 UTC · model grok-4.3

classification ⚛️ physics.med-ph
keywords proton therapy4D deliverymotion managementpencil beam scanninggantry-freemagnet-freerespiratory motiontreatment planning
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The pith

A 4D pencil beam delivery strategy makes magnet-free and gantry-free proton therapy feasible for tumors that move with breathing.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper develops and tests a four-dimensional pencil beam delivery method that folds respiratory motion directly into the treatment plan for proton therapy. It evaluates the method on a mobile phantom across scanner configurations that omit gantries or scanner magnets. The central goal is to show that clinically acceptable dose distributions remain possible even with these simplifications. If the approach holds, proton therapy systems could drop substantial hardware while still handling motion-affected targets within realistic treatment times.

Core claim

The 4D planning tool generated treatment plans that achieved clinically acceptable dose distributions across all configurations. In magnet-free configurations, static beam operation substantially increased treatment time and reduced dose conformity. In contrast, configurations using a single scanner magnet, without a gantry, maintained acceptable conformity within practical treatment times. The proposed 4D delivery strategy demonstrates feasibility for treating mobile targets with simplified, gantry-free and magnet-free scanner designs.

What carries the argument

The 4D pencil beam delivery strategy, which incorporates respiratory motion into a dynamic treatment plan to optimize beam delivery timing and position.

If this is right

  • Single-magnet gantry-free setups can deliver acceptable dose conformity in practical treatment times.
  • Magnet-free configurations need dynamic beam operation; static operation increases time and lowers conformity.
  • Synchronizing patient breathing with the 4D delivery can further improve accuracy during irregular or interrupted breathing.
  • The approach reduces system complexity while preserving dosimetric performance for motion-affected tumors.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The method could lower the space and cost barriers that currently limit proton therapy to specialized centers.
  • Validation against actual patient motion data would be the next required step before clinical translation.
  • Similar 4D optimization might extend to other beam modalities that face respiratory motion challenges.
  • Real-time motion monitoring integrated with the 4D plan could reduce reliance on the phantom model.

Load-bearing premise

The mobile phantom used in the study sufficiently represents real patient respiratory motion and irregular breathing patterns to support claims of clinical feasibility.

What would settle it

A direct comparison on real patients showing that irregular breathing produces dose distributions outside clinical tolerance or treatment times exceeding practical limits would falsify the feasibility claim for simplified scanner designs.

Figures

Figures reproduced from arXiv: 2606.03562 by Florentin Bieder, Nair N von Muehlenen, Philippe C Cattin, Ye Zhang.

Figure 1
Figure 1. Figure 1: Schematic of the phantom and the corresponding patient coordinate system. (a) The phantom consists of a target volume representing a liver tumour (red sphere), the liver (blue rectangle) and a rib bone (green wedge). The surrounding tissue can represent water or fatty tissue. (b) Patient coordinate system for sitting and lying patients with respect to the beam delivery system. The beam always lies along th… view at source ↗
Figure 2
Figure 2. Figure 2: Schematic showing proposed path within the phantom sphere in a Scanner Y setup. (a) sectioned spot positions of the 40 mm phantom sphere along the x-axis according to breathing depth. (b) The proposed pathway within a section. 5 [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Schematic showing an example of the correction of beam positions within the phantom in an SB and SVB setup. The blue points represent the planned positions, while the orange squares show the visited positions. The green arrows represent a table movement to the next section. (a) Shows an example for SB scanning mode. (b) Shows an example for SVB scanning mode. 2.4 Weight Optimisation For a complete 4D treat… view at source ↗
Figure 4
Figure 4. Figure 4: Schematic showing the calculation of the radiation delivered during the treatment. The 3D CT image of the phantom is converted into an RSP map to calculate the beam deformation resulting from density differences. Additionally, the phantom is deformed according to the breathing motion based on the current time step. To optimise the weights to generate a homogeneous radiation field, we first define constrain… view at source ↗
Figure 5
Figure 5. Figure 5: Calculated delivery time for the phantom spheres with diameter 30 mm Figure (a), 40 mm Figure (b) and 50 mm Figure (c). Each delivery mode and set of different dwell times, assuming regular breathing without interruption. 10 [PITH_FULL_IMAGE:figures/full_fig_p010_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Radiation map for SB delivery system, all three phantom diameters. The top row of maps shows radiation distribution for regular and predicted breathing. The bottom row illustrates the outcome of the delivery plan under non-predicted breathing conditions. 11 [PITH_FULL_IMAGE:figures/full_fig_p011_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: DVH of the SB scanner mode, for regular breathing and irregular breathing. (Red) DVH for the 30 mm diameter phantom. (Green) DVH for the 40 mm diameter phantom. (Blue) DVH for phantom of 50 mm diameter. The respective V95%, D95% and DmaxTV values for regular and irregular breathing are provided in Section C in the Supplementary Material. 12 [PITH_FULL_IMAGE:figures/full_fig_p012_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Radiation map for SVB delivery system, all three phantom diameters. The top row of maps shows radiation distribution for regular and predicted breathing. The bottom row illustrates the outcome of the delivery plan under non-predicted breathing conditions. 13 [PITH_FULL_IMAGE:figures/full_fig_p013_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: DVH of the SVB scanner mode, for regular breathing and irregular breathing. (Red) DVH for the 30 mm diameter phantom. (Green) DVH for the 40 mm diameter phantom. (Blue) DVH for phantom of 50 mm diameter. The respective V95%, D95% and DmaxTV values for regular and irregular breathing are provided in Section C in the Supplementary Material. 14 [PITH_FULL_IMAGE:figures/full_fig_p014_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Radiation map for Scanners XY delivery system, all three phantom diameters. The top row of maps shows radiation distribution for regular and predicted breathing. The bottom row illustrates the outcome of the delivery plan under non-predicted breathing conditions. 15 [PITH_FULL_IMAGE:figures/full_fig_p015_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: DVH of the SB scanner mode, for regular breathing and irregular breathing. (Red) DVH for the 30 mm diameter phantom. (Green) DVH for the 40 mm diameter phantom. (Blue) DVH for phantom of 50 mm diameter. The respective V95%, D95% and DmaxTV values for regular and irregular breathing are provided in Section C in the Supplementary Material. 4 Disscusion This paper presents a 4D pencil beam delivery optimisat… view at source ↗
Figure 1
Figure 1. Figure 1: Illustration of the optimal path of dose deposition in gantry-free setups for the fastest treatment according to scanner properties and configurations in an exemplary grid of size 5 × 2 × 4. The red sphere represents the centre of a spot, the blue-green line represents the order in which the positions are attended, and the blue part represents the path’s beginning. (a) The example path for the Stationary B… view at source ↗
Figure 2
Figure 2. Figure 2: Lookup Table for transforming Hounsfield Units into relative stopping power. B.2 Beam Calculation [PITH_FULL_IMAGE:figures/full_fig_p024_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Integral dose, interpolated from the depth-dose-look-up table, given by the PSI. 0 50 100 150 200 250 300 350 Water equivalent depth [mm] 1 2 3 4 5 Standard deviations [PITH_FULL_IMAGE:figures/full_fig_p025_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Standard deviation, given by the PSI. 4 [PITH_FULL_IMAGE:figures/full_fig_p025_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: shows an example of an irregular breathing pattern. Both amplitude and frequency are modulated randomly [PITH_FULL_IMAGE:figures/full_fig_p026_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: shows the delivery time for a dwell time of 10 ms plotted against the target volume size. SB SVB ScannerX ScannerY ScannerXY 30 35 40 45 50 55 0 200 400 600 800 1000 1200 1400 Delivery Time Dwell Time 10 ms Delivery Time [s] Phantom Size [mm] [PITH_FULL_IMAGE:figures/full_fig_p026_6.png] view at source ↗
read the original abstract

Objective. Motion management is a critical challenge in proton therapy for mobile tumours. This study aims to develop and evaluate a novel four-dimensional (4D) pencil beam delivery strategy that incorporates respiratory motion into a dynamic treatment plan to improve dose conformity and treatment efficiency. Approach. To assess this 4D pencil beam delivery strategy, a mobile phantom was used. The generated 4D treatment plans were assessed with various scanner configurations, including gantry-free and magnet-free scanner heads. For each setup, the treatment time, dose conformity, and robustness against irregular breathing patterns were quantified. The influence of scanner head design and patient-specific motion irregularities on overall plan quality was evaluated. Main Results. The 4D planning tool generated treatment plans that achieved clinically acceptable dose distributions across all configurations. In magnet-free configurations, static beam operation substantially increased treatment time and reduced dose conformity. In contrast, configurations using a single scanner magnet, without a gantry, maintained acceptable conformity within practical treatment times. Significance. The proposed 4D delivery strategy demonstrates feasibility for treating mobile targets with simplified, gantry-free and magnet-free scanner designs. Further improvements could be achieved by synchronising the patient's breathing with 4D delivery, which may enhance dose accuracy during irregular or interrupted breathing. By reducing system complexity while preserving dosimetric performance, this approach offers a pathway toward more accessible and cost-effective proton beam therapy for motion-affected tumours.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 1 minor

Summary. The paper develops and evaluates a 4D pencil beam delivery strategy for proton therapy that incorporates respiratory motion into dynamic treatment planning. Using a mobile phantom, it assesses treatment plans across scanner configurations including gantry-free and magnet-free designs, quantifying treatment time, dose conformity, and robustness to irregular breathing. The central claim is that single-magnet gantry-free setups achieve clinically acceptable dose distributions within practical times, demonstrating feasibility for simplified, magnet-free systems.

Significance. If validated, the work could reduce system complexity and cost for motion-managed proton therapy while preserving dosimetric performance. The phantom-based evaluation of multiple scanner heads provides a concrete test of the 4D optimization approach, but the absence of quantitative metrics and limited motion realism constrain the strength of the feasibility conclusion.

major comments (3)
  1. [Abstract] Abstract: The main results claim 'clinically acceptable dose distributions' and 'acceptable conformity' for single-magnet setups, yet supply no numerical values for conformity indices, target coverage metrics, OAR doses, or statistical comparisons; without these, the load-bearing feasibility claim cannot be verified from the reported data.
  2. [Abstract] Abstract: Robustness against irregular breathing is stated to have been quantified, but the mobile phantom motion model is not described or benchmarked against clinical 4DCT distributions (amplitude variation, baseline drift, hysteresis); this directly affects the central claim that the strategy is robust for real patient motion.
  3. [Abstract] Abstract: Treatment-time and conformity results for magnet-free static-beam operation are contrasted with single-magnet results, but no error bars, repeated measurements, or sensitivity analysis to phantom parameters are mentioned, leaving the quantitative advantage of the 4D strategy unsupported.
minor comments (1)
  1. [Abstract] The abstract uses 'gantry-free and magnet-free scanner heads' without defining the exact hardware simplifications or beam-line constraints assumed in the 4D planning tool.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the constructive comments on our manuscript. We address each point below and will revise the abstract accordingly to improve clarity and support for the claims.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The main results claim 'clinically acceptable dose distributions' and 'acceptable conformity' for single-magnet setups, yet supply no numerical values for conformity indices, target coverage metrics, OAR doses, or statistical comparisons; without these, the load-bearing feasibility claim cannot be verified from the reported data.

    Authors: We agree that the abstract would benefit from explicit numerical support. In the revised version we will insert the key quantitative results obtained from the phantom measurements, including conformity index values, target coverage metrics (e.g., D95, V95), and relevant OAR doses for the single-magnet configuration. Because the study is a controlled phantom experiment, formal statistical comparisons across patient cohorts are not applicable; we will therefore report the measured values with their observed ranges. revision: yes

  2. Referee: [Abstract] Abstract: Robustness against irregular breathing is stated to have been quantified, but the mobile phantom motion model is not described or benchmarked against clinical 4DCT distributions (amplitude variation, baseline drift, hysteresis); this directly affects the central claim that the strategy is robust for real patient motion.

    Authors: The phantom motion parameters (amplitude, period, and irregular patterns) are fully specified in the Methods section. We will add a concise description of these parameters to the abstract. Our work is a phantom-based feasibility study and does not contain a direct benchmark against clinical 4DCT distributions; we will therefore qualify the robustness claim in both the abstract and discussion to reflect this scope. revision: partial

  3. Referee: [Abstract] Abstract: Treatment-time and conformity results for magnet-free static-beam operation are contrasted with single-magnet results, but no error bars, repeated measurements, or sensitivity analysis to phantom parameters are mentioned, leaving the quantitative advantage of the 4D strategy unsupported.

    Authors: We will revise the abstract to report the observed ranges or standard deviations from the phantom deliveries and to clarify that each configuration was evaluated once under controlled conditions. A full sensitivity analysis to all phantom parameters lies beyond the present scope; we will note this limitation explicitly while retaining the comparative demonstration that was performed. revision: yes

Circularity Check

0 steps flagged

No circularity; results from independent phantom evaluation

full rationale

The paper develops and evaluates a 4D pencil beam delivery strategy through direct assessment on a mobile phantom, generating treatment plans and quantifying treatment time, dose conformity, and robustness for multiple scanner configurations including gantry-free and magnet-free setups. All load-bearing claims derive from these external experimental and simulation measurements rather than from any self-referential definitions, fitted parameters renamed as predictions, or self-citation chains. The derivation chain remains self-contained against the phantom-based benchmarks with no reductions by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Abstract-only review limits visibility into parameters; the central feasibility claim rests on the unstated premise that phantom motion adequately proxies patient data.

axioms (1)
  • domain assumption The mobile phantom accurately simulates patient respiratory motion and irregular breathing for evaluating 4D plans and scanner configurations.
    The study uses phantom results to claim feasibility and robustness for clinical mobile targets.

pith-pipeline@v0.9.1-grok · 5791 in / 1227 out tokens · 36282 ms · 2026-06-28T07:29:36.564376+00:00 · methodology

discussion (0)

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Reference graph

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