Session Item

Philips released recently MRCAT pelvis for MR-only radiotherapy simulation purpose. Dose calculation is performed on a virtual CT series derived from a modified mDixon acquisition (MRCAT) while the target and OAR contouring are made based on a 3D T2 series. MRCAT pelvis is an extension of the previous MRCAT prostate product released in 2015 and dedicated to prostate treatments. MRCAT series exhibit now continuous HU values (1500 levels in average) rather than stratification in five densities for an easier online CBCT matching. MRCAT generation is robust and can be now used for all types of cancers in the pelvis area (prostate, rectum, bladder, anal canal), and is compatible with different patient position (HFS, HFP). Purpose of this work is to assess dose calculation accuracy on these continuous MRCAT datasets for different pelvic tumor types and geometries in comparison to standard CT-based planning.

Validation study was conducted on the first 15 pelvic patients (8 prostate, 6 rectum, 1 bladder) who benefited from MR simulation in our institution (Ingenia 3T). Patients were scanned using the default manufacturer provided ExamCard. VMAT planning (Pinnacle³ 16.2) was performed on the MRCAT series using the manufacturer provided HU to mass density curve and the resulting beams were recalculated (Collapsed Cone Convolution) either on standard CT following the MR simulation (Brilliance BigBore) or on the CBCT acquired to the first fraction of the treatment (EDGE iCBCT). We used 3D gamma index (1.5% of local dose, 1mm) criteria (PTW Verisoft) and local dose difference in the 30% to evaluate the agreement between dose matrices. We also recorded geometric distortions QA during the first year of operation.

Agreement between MRCAT-based and CT-based dose calculation shows good agreement with a mean dose ratio of 1.007 (0.991-1.014). 3D gamma score is greater than 95% (95.2-99.0%) for all plans checked. No correlation was found with PTV volume or body site with comparable results obtained. One year geometric QA analysis never showed distortions larger than 2mm at ±13cm from magnet center and we only got one MRCAT generation failure in this first cohort of patients (sanity check).

VMAT dose calculation on continuous HU MRCAT pelvis is reliable for various target size and geometries and these datasets can be safely used for radiotherapy planning.