Evaluation of a new hybrid VMAT-IMRT multi-criteria optimization plan generation algorithm.

To evaluate the Varian 'Fast hybrid multi-criteria optimization (MCO) volumetric modulated arc therapy (VMAT)' (H-VMAT) tool for both its dosimetric accuracy and calculation time. This is a new function within V15.6 of the Varian Eclipse treatment planning system that allows VMAT optimization and dose calculation using the graphical processing unit (GPU). In versions prior to V15.6 VMAT MCO calculations were only possible using central processing unit (CPU) not GPU. We termed this approach as native VMAT (N-VMAT). The study consisted of a cohort of 53 patients representing a range of anatomical treatment sites; bladder (5), brain (6), gynaecological (5), head & neck (5), lung (7), mediastinum (7) prostate (6), oesophagus (7), and rectum (5). Each case was planned to that of a clinical standard (Base) which was compared to a H-VMAT and N-VMAT approach. The study analyzed plan calculation time data, dose to organ at risk (OAR) and target coverage for each approach. Negligible dosimetric differences were found between the H-VMAT and N-VMAT approach for the cohort of patients evaluated. The largest dosimetric changes where observed in the optic chiasm and lacrimal gland where the H-VMAT achieved a max dose of 50.9 ± 7.7 Gy and 8.0 ± 0.5 Gy in comparison to the N-VMAT 53.1 ± 6.3 Gy and 10.2 ± 2.9 Gy, respectively. Several OAR's provided indistinguishable dose outcomes, namely; brainstem, heart, kidney's, lens, parotid, and spinal cord. Large time savings were found using the H-VMAT technique when compared to N-VMAT, being 5 to 40 times faster or up to 75 minutes time saving (average of 25 minutes). Negligible dosimetric change between the 2 techniques and large time savings were observed with the GPU enabled H-VMAT approach. We have shown that the H-VMAT technique has been safely implemented and is ready for clinical use.