Author: Caiden Atienza, Pim T. S. Borman, Martin F. Fast, Daniel E. Hyer, Bas W. Raaymakers, Jeffrey E. Snyder, Prescilla Uijtewaal, Peter Woodhead 👨🔬
Affiliation: Department of Radiotherapy, University Medical Center Utrecht, Yale New Haven Health, University of Iowa 🌍
Purpose:
On conventional C-arm linacs, VMAT is the preferred delivery modality due to its superior efficiency and highly conformal dose distributions. MR-linacs offer superior soft-tissue imaging that can guide daily plan adaptation and real-time target tracking, enabling automated beam gating. For the future clinical introduction of VMAT on the MR-linac, it is desirable to establish its compatibility with beam gating, which we explore in this study.
Methods:
All experiments were performed on a 1.5T Unity MR-linac (Elekta AB, SWE), in research mode enabling VMAT with gating. Clinical-grade treatment plans for three lung SBRT patients (4-7.5 Gy/fx prescription) were created: A) dual-arc VMAT plans, and B) step-and-shoot IMRT plans (7-11 beams). Plans were recalculated on the Delta4 Phantom+ MR (Scandidos AB, SWE), placed on a QUASAR Motion Platform (IBA Quasar, CAN), emulating respiratory motion (cos4, 15bpm) with a 5mm end-exhale gating window and 50% duty-cycle (2s/2s on/off). Ungated and gated scenarios were delivered for the IMRT and VMAT plans. The IMRT plans were gated using an artificial gating signal since the clinical delivery software does not accept third-party gating signals.
Results:
The measured dose matched the planned dose in all IMRT and VMAT scenarios, with local gamma pass-rates (2%/2mm) of 90-99%. The MU efficiencies (actual-to-maximum dose rate ratio) of the ungated/gated deliveries were 27%/21%, 60%/37%, and 51%/34% for IMRT and 78%/35%, 80%/36%, and 67/33% for VMAT. The ungated VMAT plans were delivered 24% faster than the IMRT plans, even though the former had 20% more MU. Gating increased the delivery time by 115% for VMAT, versus 45% for IMRT, due to the move-only segments in the IMRT plans, unaffected by gating.
Conclusion:
This study demonstrated that VMAT plans can be delivered on the Unity MR-linac in combination with respiratory beam gating but are more affected by efficiency loss due to gating.