Purpose
Conventional in-room CT guided patient alignment employing bony landmarks is compromised for H&N radiotherapy. On-board MRI provides superior soft tissue contrast, allowing patient alignment using the tumor or nearby critical structures. This study sought to use MRI-guided H&N cases to a) retrospectively compare the daily dose delivery accuracy of using MRI soft-tissue alignment and the bony alignment; and b) assess the projected accumulative doses through entire treatment course using weekly MR images.
Methods
20 H&N patients treated on a MRI-guided radiotherapy system (MRIdian, Viewray Inc., OH) were retrospectively analyzed. 149 weekly pre-treatment MR images were manually aligned to the planning target delineated on planning CT/MRI. A near-real-time in-house deformable image registration and dose accumulation framework was developed to calculate daily and accumulative delivery doses. The delivered doses calculated on daily anatomy according to pre-treatment MRIs and then mapped back to the planning CT/MRI for cumulative dose DVH analysis. The daily and cumulative doses for the target(s) and the selected critical structures were calculated and compared for two patient alignment methods using soft-tissue versus bony landmark (i.e., C1/C2).
Results
Soft-tissue alignment greatly improved GTV coverage and normal tissue sparing. The percentage dose delivered at V95 of the GTV is 98.2% for the direct soft-tissue alignment, while GTV coverage of 87.6% was observed using the bony structure alignment. For normal structures such as the spinal cord, the maximum dose was 17% higher using the bony alignment. The dose shows good agreement between soft-tissue alignment and the planned dose.
Conclusion
Soft-tissue contrast from on-board MRI of MRIdian system offers acceptable tumor conspicuity that allows soft-tissue alignment. The soft-tissue alignment resulted in improved dose delivery accuracy to the GTV and better critical structure sparing, compared to bony landmark based alignment technique.