Adapting VMAT plans optimized for an HD120 MLC for delivery with a Millennium MLC

Linac downtime invariably impacts delivery of patients' scheduled treatments. Transferring a patient's treatment to an available linac is a common practice. Transferring a Volumetric Modulated Arc Therapy (VMAT) plan from a linac equipped with a standard-definition MLC to one equipped with a higher definition MLC is practical and routine in clinics with multiple MLC-equipped linacs. However, the reverse transfer presents a challenge because the high-definition MLC aperture shapes must be adapted for delivery with the lower definition device. We have developed an efficient method to adapt VMAT plans originally designed for a high-definition MLC to a standard-definition MLC. We present the dosimetric results of our adaptation method for head-and-neck, brain, lung, and prostate VMAT plans. The delivery of the adapted plans was verified using standard phantom measurements.