Practical robustness evaluation in radiotherapy – A photon and proton-proof alternative to PTV-based plan evaluation

Abstract

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)...

Head and neck IMPT probabilistic dose accumulation: Feasibility of a 2 mm setup uncertainty setting

“…In our clinical practice, a target coverage criterion of V 95 of at least 98% in the voxel-wise minimum dose distribution is applied during treatment [19]. The required coverage of the delivered dose distribution should be more than 98% as the coverage in the voxelwise minimum dose distribution is less favorable than any single scenario, but less than 100% as this would include clinically irrelevant volumes.…”

“…Target coverage was assessed using the voxel-wise minimum robustness (multi-scenario) evaluation approach outlined in a recent publication describing the Dutch consensus for proton plan evaluation [19]. During evaluation, 14 different isocenter shifts (i.e.…”

“…During evaluation, 14 different isocenter shifts (i.e. a positive or negative shift along each of the three axes and along each diagonal) and two density shits were considered resulting in a total of 28 scenario's [19]. The voxel-wise minimum of these 28 scenarios was used to assess robust target coverage, where the coverage criteria was a V 95 of both CTVs of at least 98%.…”

Inter-fraction motion robustness and organ sparing potential of proton therapy for cervical cancer

“…At this location a minimum dose of 10 Gy RBE per beam was requested for both posterior beams, to limit dose modulation and improve plan robustness. For PBS-PT, robust optimisation in RayStation was used with a 3% range and a 0.5 cm set-up uncertainty in all orthogonal directions [22,23]. Clinical dose distributions were calculated using the Monte Carlo dose calculation engine, with 1% uncertainty.…”

“…For PBS-PT, 3% range uncertainty was added. This robustness evaluation produced 14 (28) scenarios and the voxelwise minimum (vox min) dose was derived as the composite of minimum dose values per voxel from all scenarios [23,25]. Similar, voxel-wise maximum (vox max) dose was derived as the composite of maximum dose values.…”