a b s t r a c tBackground and purpose: A planning target volume (PTV) in photon treatments aims to ensure that the clinical target volume (CTV) receives adequate dose despite treatment uncertainties. The underlying static dose cloud approximation (the assumption that the dose distribution is invariant to errors) is problematic in intensity modulated proton treatments where range errors should be taken into account as well. The purpose of this work is to introduce a robustness evaluation method that is applicable to photon and proton treatments and is consistent with (historic) PTV-based treatment plan evaluations. Materials and methods: The limitation of the static dose cloud approximation was solved in a multiscenario simulation by explicitly calculating doses for various treatment scenarios that describe possible errors in the treatment course. Setup errors were the same as the CTV-PTV margin and the underlying theory of 3D probability density distributions was extended to 4D to include range errors, maintaining a 90% confidence level. Scenario dose distributions were reduced to voxel-wise minimum and maximum dose distributions; the first to evaluate CTV coverage and the second for hot spots. Acceptance criteria for CTV D98 and D2 were calibrated against PTV-based criteria from historic photon treatment plans. Results: CTV D98 in worst case scenario dose and voxel-wise minimum dose showed a very strong correlation with scenario average D98 (R 2 > 0.99). The voxel-wise minimum dose visualised CTV dose conformity and coverage in 3D in agreement with PTV-based evaluation in photon therapy. Criteria for CTV D98 and D2 of the voxel-wise minimum and maximum dose showed very strong correlations to PTV D98 and D2 (R 2 > 0.99) and on average needed corrections of À0.9% and +2.3%, respectively. Conclusions: A practical approach to robustness evaluation was provided and clinically implemented for PTV-less photon and proton treatment planning, consistent with PTV evaluations but without its static dose cloud approximation. Ó 2019 The Authors. Published by Elsevier B.V. Radiotherapy and Oncology 141 (2019) 267-274 This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).The use of margins in photon radiotherapy is a long established and universally adopted method to provide adequate target coverage under the presence of uncertainties. The CTV-PTV margin provides a geometrical buffer zone around the target within which the desired dose is achieved for the majority of treatments; criteria of 95% of the prescription dose in 90% of the patient population has found general appeal [1,2]. The suitability of a geometricallyexpanded buffer zone arises from the (relative) insensitivity of megavoltage photon dose distributions to density changes in the beam path. By and large, the biggest risk to a photon dose distribution is a geometrical miss -a translation of the CTV relative to the beam. Therefore, the static dose cloud approximation (dose distribution is invariant to errors)...

Objectives: To describe the measurements and to present the results of the beam commissioning and the beam model validation of a compact, gantry-mounted, spot scanning proton accelerator system with dynamic layer-by-layer field collimation. Methods: We performed measurements of depth dose distributions in water, spot and scanned field size in air at different positions from the isocenter plane, spot position over the 20 × 20 cm2 scanned area, beam monitor calibration in terms of absorbed dose to water and specific field collimation measurements at different gantry angles to commission the system. To validate the beam model in the treatment planning system (TPS), we measured spot profiles in water at different depths, absolute dose in water of single energy layers of different field sizes and inversely optimised spread-out Bragg peaks (SOBP) under normal and oblique beam incidence, field size and penumbra in water of SOBPs, and patient treatment specific quality assurance in homogeneous and heterogeneous phantoms. Results: Energy range, spot size, spot position and dose output were consistent at all gantry angles with 0.3 mm, 0.4 mm, 0.6 mm and 0.5% maximum deviations, respectively. Uncollimated spot size (one sigma) in air with an air-gap of 10 cm ranged from 4.1 to 16.4 mm covering a range from 32.2 to 1.9 cm in water, respectively. Absolute dose measurements were within 3% when comparing TPS and experimental data. Gamma pass rates >98% and >96% at 3%/3 mm were obtained when performing 2D dose measurements in homogeneous and in heterogeneous media, respectively. Leaf position was within ±1 mm at all gantry angles and nozzle positions. Conclusions: Beam characterisation and machine commissioning results, and the exhaustive end-to-end tests performed to assess the proper functionality of the system, confirm that it is safe and accurate to treat patients. Advances in knowledge: This is the first paper addressing the beam commissioning and the beam validation of a compact, gantry-mounted, pencil beam scanning proton accelerator system with dynamic layer-by-layer multileaf collimation.

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