Impact of Beam Energy and Field Configuration on VMAT TBI Plan Quality
Abstract
Purpose
To assess how different treatment planning techniques affect dose homogeneity and OAR sparing in VMAT total-body irradiation (TBI) plans.
Methods
VMAT TBI treatments for five patients were retrospectively replanned. For all cases, the upper body was planned using three VMAT isocenters (head, chest, and pelvis). The legs were planned with two 3D-CRT AP/PA isocenters (upper and lower leg), serving as a base dose for VMAT optimization. Multiple VMAT plan variants were generated, varying the beam energies and number of arcs per isocenter. For the energy comparison, all VMAT plans used four full arcs per isocenter with beam energies of 6 MV only, 10 MV only, or an equal combination of 6 and 10 MV. To evaluate the effect of changing the number of fields, 6 MV plans were generated using two or four full arcs per isocenter with different collimator angles. All plans were normalized so that 100% of the prescription dose covered 85% of the PTV_Body volume. Plans were evaluated by comparing V110% and OAR dose metrics.
Results
For patients with smaller chest and abdominal separation, beam energy had a negligible effect on V110%. However, for the patient with the largest separation (>55 cm laterally), incorporating 10 MV beams reduced V110% from 17.3% (6 MV) to 15.1% (6 and 10 MV) and 14.3% (10 MV). The effect of energy on OAR doses was variable, with no single energy consistently benefiting all OARs. Increasing the number of arcs per VMAT isocenter considerably improved plan quality. The average V110% decreased from 24.7% (two arcs) to 9.4% (four arcs). Similarly, the lung mean dose decreased from 77.4% to 69.7%.
Conclusion
Increasing the number of arcs in VMAT TBI plans improves dose homogeneity and OAR sparing. In patients with lateral separation exceeding approximately 55 cm, adding 10 MV beams may reduce V110%.