PTV-based VMAT vs. robust IMPT for head-and-neck cancer: A probabilistic uncertainty analysis of clinical plan evaluation with the Dutch model-based selection

• Published in: Radiotherapy and oncology 2023-09, Vol.186, p.109729-109729, Article 109729
• Main Authors: Rojo-Santiago, Jesús, Korevaar, Erik, Perkó, Zoltán, Both, Stefan, Habraken, Steven J.M., Hoogeman, Mischa S.
• Format: Article
• Language: English
• Subjects: Head-and-neck Cancer; Intensity Modulated Proton Therapy; Polynomial Chaos Expansion; Probabilistic planning; Volumetric Modulated Arc Therapy
• ISSN: 0167-8140; 1879-0887
• DOI: 10.1016/j.radonc.2023.109729

•Probabilistic dose to near-minimum volume v = 99.8% correlated best with clinical PTV-D98% for VMAT and VWmin-D98%,CTV doses for IMPT.•Despite differences between photons and protons, the planning comparison between VMAT and IMPT leads to consistent CTV dose, NTCP and TCP results.•The nominal NTCP is a good estimator to qualify patients for IMPT. In the Netherlands, head-and-neck cancer (HNC) patients are referred for proton therapy (PT) through model-based selection (MBS). However, treatment errors may compromise adequate CTV dose. Our aims are: (i) to derive probabilistic plan evaluation metrics on the CTV consistent with clinical metrics; (ii) to evaluate plan consistency between photon (VMAT) and proton (IMPT) planning in terms of CTV dose iso-effectiveness and (iii) to assess the robustness of the OAR doses and of the risk toxicities involved in the MBS. Sixty HNC plans (30 IMPT/30 VMAT) were included. A robustness evaluation with 100,000 treatment scenarios per plan was performed using Polynomial Chaos Expansion (PCE). PCE was applied to determine scenario distributions of clinically relevant dosimetric parameters, which were compared between the 2 modalities. Finally, PCE-based probabilistic dose parameters were derived and compared to clinical PTV-based photon and voxel-wise proton evaluation metrics. Probabilistic dose to near-minimum volume v = 99.8% for the CTV correlated best with clinical PTV-D98% and VWmin-D98%,CTV doses for VMAT and IMPT respectively. IMPT showed slightly higher nominal CTV doses, with an average increase of 0.8 GyRBE in the median of the D99.8%,CTV distribution. Most patients qualified for IMPT through the dysphagia grade II model, for which an average NTCP gain of 10.5 percentages points (%-point) was found. For all complications, uncertainties resulted in moderate NTCP spreads lower than 3 p.p. on average for both modalities. Despite the differences between photon and proton planning, the comparison between PTV-based VMAT and robust IMPT is consistent. Treatment errors had a moderate impact on NTCPs, showing that the nominal plans are a good estimator to qualify patients for PT.