Session Item

This work aims to evaluate the feasibility of a radiation therapy (RT) workflow based only on magnetic resonance (MR) images for head and neck (H&N) cancer, using the resources available in a general hospital, i.e., a diagnostic MR system and a conventional linac to realize full MR guided Radiotherapy (MRgRT). The RT planning performed on MR images (MRI-only radiotherapy) allows overcoming some limitations of the combined use of CT and MR images, like defects in co-registration. Unlike CT, MRI signals do not depend on electron densities (EDs), required by the treatment planning systems (TPS) to calculate the dose distribution. Therefore, it is necessary to associate MR data with ED maps, thus obtaining a synthetic-CT (sCT). 

Thirteen patients with H&N cancer were scanned using a T1w 3D VIBE Dixon gradient echo sequence on a 1.5 T Magnetom Aera scanner (Siemens Healthcare), with a large field of view to cover the entire shoulder region. The patients were acquired in the same setup of CT simulation, using a customized thermoplastic mask fixed on a rigid table and MR compatible markers. An 18-channel body coil placed above the patient's head and shoulders was used. At the same time, the CT acquisitions were performed with a Brilliance CT scanner (Philips Medical System), in order to validate the sCT generation method, comparing the dose distributions. The sCT images are obtained from MR images, using a combination of a multi atlas-based approach and the bulk ED assignment to the contoured organs-at-risk (OARs). Furthermore, air cavities and bone structures should be properly contoured and assigned average EDs. First, MR images were auto-contoured using ADMIRE® software (research version 3.28, Elekta AB) and verified by a radiation oncologist. VMAT plans, optimized with a Monaco TPS (Elekta AB), on the sCT were recalculated on the CT deformably registered on the MR. Deviations between dose calculations on CT and sCT were evaluated using dose-volume histograms (DVH) and gamma analysis.

The mean absorbed dose deviations between sCT and CT plans were within 1% for all DVH parameters for PTVs and <3% for OARs, as shown in Table 1 and Figure 1. The differences were considered clinically acceptable for all the DVH parameters. The 2D-local gamma analysis showed a good agreement between the sCT and CT plan dose distributions with an average passing rate value of 99.1±1.0%, and 96.5±2.6%, using an acceptance criterion of 3%/3mm and 2%/2 mm, respectively.

This study shows that the sCT, generated with the combination of a multi atlas-based approach and the bulk ED assignment, achieved clinically feasible dosimetric results in an MRgRT workflow for H&N cancer radiotherapy. Thus, it represents a significant potential in the daily practices of RT for future clinical implementation of MR-only planning, eliminating the use of dual imaging modality and avoiding multi-modality image registration, as well as for re-planning and adaptive purposes.