Towards FLASH proton therapy: the impact of treatment planning and machine characteristics on achievable dose rates

Authors

  • Steven van de Water Center for Proton Therapy, Paul Scherrer Institute, Villigen, Switzerland
  • Sairos Safai Center for Proton Therapy, Paul Scherrer Institute, Villigen, Switzerland
  • Jacobus M. Schippers Center for Proton Therapy, Paul Scherrer Institute, Villigen, Switzerland;  Department of Radiation Therapy, Rijksuniversiteit Groningen, Groningen, the Netherlands
  • Damien C. Weber Center for Proton Therapy, Paul Scherrer Institute, Villigen, Switzerland;  Department of Radiation Oncology, University Hospital of Bern, Bern, Switzerland;  Department of Radiation Oncology, University Hospital of Zürich, Zürich, Switzerland
  • Antony J. Lomax Center for Proton Therapy, Paul Scherrer Institute, Villigen, Switzerland;  Department of Physics, ETH Zürich, Zürich, Switzerland

DOI:

https://doi.org/10.1080/0284186X.2019.1627416

Abstract

Background: This study aimed at evaluating spatially varying instantaneous dose rates for different intensity-modulated proton therapy (IMPT) planning strategies and delivery scenarios, and comparing these with FLASH dose rates (>40 Gy/s).

Material and methods: In order to quantify dose rates in three-dimensions, we proposed the ‘dose-averaged dose rate’ (DADR) metric, defined for each voxel as the dose-weighted mean of the instantaneous dose rates of all spots (i.e., pencil beams). This concept was applied to four head-and-neck cases, each planned with clinical (4 fields) and various spot-reduced IMPT techniques: ‘standard’ (4 fields), ‘arc’ (120 fields) and ‘arc-shoot-through’ (120 fields; 229 MeV only). For all plans, different delivery scenarios were simulated: constant beam intensity, variable beam intensity for a clinical Varian ProBeam system, varied per energy layer or per spot, and theoretical spot-wise variable beam intensity (i.e., no monitor/safety limitations). DADR distributions were calculated assuming 2-Gy or 6-Gy fractions.

Results: Spot-reduced plans contained 17–52 times fewer spots than clinical plans, with no deterioration of plan quality. For the clinical plans, the mean DADR in normal tissue for 2-Gy fractionation was 1.7 Gy/s (median over all patients) at maximum, whereas in standard spot-reduced plans it was 0.7, 4.4, 7.1, and 12.1 Gy/s, for the constant, energy-layer-wise, spot-wise, and theoretical spot-wise delivery scenarios, respectively. Similar values were observed for arc plans. Arc-shoot-through planning resulted in DADR values of 3.0, 6.0, 14.1, and 24.4 Gy/s, for the abovementioned scenarios. Hypofractionation (3×) generally resulted in higher dose rates, up to 73.2 Gy/s for arc-shoot-through plans. The DADR was inhomogeneously distributed with highest values at beam entrance and at the Bragg peak.

Conclusion: FLASH dose rates were not achieved for conventional planning and clinical spot-scanning machines. As such, increased spot-wise beam intensities, spot-reduced planning, hypofractionation and arc-shoot-through plans were required to achieve FLASH compatible dose rates.

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Published

2019-10-03

How to Cite

van de Water, S., Safai, S., Schippers, J. M., Weber, D. C., & Lomax, A. J. (2019). Towards FLASH proton therapy: the impact of treatment planning and machine characteristics on achievable dose rates. Acta Oncologica, 58(10), 1463–1469. https://doi.org/10.1080/0284186X.2019.1627416

Issue

Vol. 58 No. 10 (2019): Acta Oncologica

Section

Articles