Patient-specific quality assurance strategies for synthetic computed tomography in magnetic resonance-only radiotherapy of the abdomen

• Published in: Physics and imaging in radiation oncology 2023-07, Vol.27, p.100464-100464, Article 100464
• Main Authors: Dal Bello, Riccardo, Lapaeva, Mariia, La Greca Saint-Esteven, Agustina, Wallimann, Philipp, Günther, Manuel, Konukoglu, Ender, Andratschke, Nicolaus, Guckenberger, Matthias, Tanadini-Lang, Stephanie
• Format: Article
• Language: English
• Subjects: MR-guided radiotherapy; MR-Linac; Neural network; Original; PSQA; Quality assurance; synthetic CT
• ISSN: 2405-6316; 2405-6316
• DOI: 10.1016/j.phro.2023.100464

•The use of a separate neural network provided dose equivalence within 2%•The time burden was limited to less than 10 min.•The method was shown to be applicable in presence of air pockets, lung or implants. The superior tissue contrast of magnetic resonance (MR) compared to computed tomography (CT) led to an increasing interest towards MR-only radiotherapy. For the latter, the dose calculation should be performed on a synthetic CT (sCT). Patient-specific quality assurance (PSQA) methods have not been established yet and this study aimed to assess several software-based solutions. A retrospective study was performed on 20 patients treated at an MR-Linac, which were selected to evenly cover four subcategories: (i) standard, (ii) air pockets, (iii) lung and (iv) implant cases. The neural network (NN) CycleGAN was adopted to generate a reference sCT, which was then compared to four PSQA methods: (A) water override of body, (B) five tissue classes with bulk densities, (C) sCT generated by a separate NN (pix2pix) and (D) deformed CT. The evaluation of the dose endpoints demonstrated that while all methods A-D provided statistically equivalent results (p = 0.05) within the 2% level for the standard cases (i), only the methods C-D guaranteed the same result over the whole cohort. The bulk densities override was shown to be a valuable method in absence of lung tissue within the beam path. The observations of this study suggested that the use of an additional sCT generated by a separate NN was an appropriate tool to perform PSQA of a sCT in an MR-only workflow at an MR-Linac. The time and dose endpoints requirements were respected, namely within 10 min and 2%.