Possibilities and challenges when using synthetic computed tomography in an adaptive carbon-ion treatment workflow

Barbara Knäusl; Peter Kuess; Markus Stock; Dietmar Georg; Piero Fossati; Petra Georg; Lukas Zimmermann

doi:10.1016/j.zemedi.2022.05.003

Possibilities and challenges when using synthetic computed tomography in an adaptive carbon-ion treatment workflow

Z Med Phys. 2023 May;33(2):146-154. doi: 10.1016/j.zemedi.2022.05.003. Epub 2022 Jun 25.

Authors

Barbara Knäusl¹, Peter Kuess², Markus Stock³, Dietmar Georg⁴, Piero Fossati³, Petra Georg³, Lukas Zimmermann⁵

Affiliations

¹ Medical University of Vienna, Department of Radiation Oncology, Währinger Gürtel 18-20, 1090 Vienna, Austria; MedAustron Ion Therapy Center, Wiener Neustadt, Marie-Curie-Straße 5, 2700 Wiener Neustadt, Austria. Electronic address: barbara.knaeusl@meduniwien.ac.at.
² Medical University of Vienna, Department of Radiation Oncology, Währinger Gürtel 18-20, 1090 Vienna, Austria; MedAustron Ion Therapy Center, Wiener Neustadt, Marie-Curie-Straße 5, 2700 Wiener Neustadt, Austria.
³ MedAustron Ion Therapy Center, Wiener Neustadt, Marie-Curie-Straße 5, 2700 Wiener Neustadt, Austria.
⁴ Medical University of Vienna, Department of Radiation Oncology, Währinger Gürtel 18-20, 1090 Vienna, Austria.
⁵ Medical University of Vienna, Department of Radiation Oncology, Währinger Gürtel 18-20, 1090 Vienna, Austria; Faculty of Health, University of Applied Sciences Wiener Neustadt, Johannes-Gutenberg-Straße 3, 2700 Wiener Neustadt, Austria; Competence Center for Preclinical Imaging and Biomedical Engineering, University of Applied Sciences Wiener Neustadt, Johannes-Gutenberg-Straße 3, 2700 Wiener Neustadt, Austria.

Abstract

Background and purpose: Anatomical surveillance during ion-beam therapy is the basis for an effective tumor treatment and optimal organ at risk (OAR) sparing. Synthetic computed tomography (sCT) based on magnetic resonance imaging (MRI) can replace the X-ray based planning CT (X-rayCT) in photon radiotherapy and improve the workflow efficiency without additional imaging dose. The extension to carbon-ion radiotherapy is highly challenging; complex patient positioning, unique anatomical situations, distinct horizontal and vertical beam incidence directions, and limited training data are only few problems. This study gives insight into the possibilities and challenges of using sCTs in carbon-ion therapy.

Materials and methods: For head and neck patients immobilised with thermoplastic masks 30 clinically applied actively scanned carbon-ion treatment plans on 15 CTs comprising 60 beams were analyzed. Those treatment plans were re-calculated on MRI based sCTs which were created employing a 3D U-Net. Dose differences and carbon-ion spot displacements between sCT and X-rayCT were evaluated on a patient specific basis.

Results: Spot displacement analysis showed a peak displacement by 0.2 cm caused by the immobilisation mask not measurable with the MRI. 95.7% of all spot displacements were located within 1 cm. For the clinical target volume (CTV) the median D_50% agreed within -0.2% (-1.3 to 1.4%), while the median D_0.01cc differed up to 4.2% (-1.3 to 25.3%) comparing the dose distribution on the X-rayCT and the sCT. OAR deviations depended strongly on the position and the dose gradient. For three patients no deterioration of the OAR parameters was observed. Other patients showed large deteriorations, e.g. for one patient D_2% of the chiasm differed by 28.1%.

Conclusion: The usage of sCTs opens several new questions, concluding that we are not ready yet for an MR-only workflow in carbon-ion therapy, as envisaged in photon therapy. Although omitting the X-rayCT seems unfavourable in the case of carbon-ion therapy, an sCT could be advantageous for monitoring, re-planning, and adaptation.

Keywords: Adaptive carbon-ion therapy; Deep learning; MR based workflow; Synthetic CT.

MeSH terms

Head
Humans
Magnetic Resonance Imaging / methods
Radiotherapy Planning, Computer-Assisted* / methods
Tomography, X-Ray Computed* / methods
Workflow