A volumetric modulated arc therapy-based dynamic conformal arc technique with limited monitor units (VMATliMU) to reduce multileaf collimator interplay effects: A computational phantom study for stage I non-small-cell lung cancer.

Volumetric modulated arc therapy (VMAT) for lung cancer involves complex multileaf collimator (MLC) motion, which increases sensitivity to interplay effects with tumour motion. Current dynamic conformal arc methods address this issue but may limit the achievable dose distribution optimisation compared with standard VMAT. This study examined the clinical utility of a VMAT technique with monitor unit limits (VMATliMU) to mimic conformal arc delivery and reduce interplay effects while maintaining plan quality. VMATliMU was implemented by applying monitor unit limitations during VMAT reoptimisation to minimise MLC encroachment into target volumes. Using mesh-type reference computational phantom CT images, treatment plans were generated for a simulated stage I lung cancer case prescribed to 45 Gy in three fractions. VMATliMU, conventional VMAT, VMAT with leaf speed limitations, dynamic conformal arc therapy, and constant dynamic conformal arc therapy were compared. Plans were optimised for multiple isodose line prescriptions (50%, 60%, 70%, 80%, and 90%) to investigate the impact of dose distribution. Evaluation parameters included MLC positional accuracy using area difference ratios, dosimetric indices, gradient metrics, and organ-at-risk doses. VMATliMU prevented MLC encroachment into the internal target volume across 60%-90% isodose lines, showing superior MLC accuracy compared with other methods. At the challenging 50% isodose line, VMATliMU had 4.5 times less intrusion than VMAT with leaf speed limits. VMAT plans had better dosimetric indices than dynamic conformal arc plans. VMATliMU reduced monitor units by 5.1%-19.2% across prescriptions. All plans met the clinical dose constraints, with the aortic arch below tolerance and acceptable lung doses. VMATliMU combines VMAT's dosimetric benefits with the dynamic conformal arcs's simplicity, minimising MLC encroachment while maintaining plan quality. Reduced monitor units lower low-dose exposure, treatment time, and interplay effects. VMATliMU is usable in existing planners with monitor unit limits, offering a practical solution for lung stereotactic body radiation therapy.