SU-E-T-463: Biological-Based Optimization and VMAT is Unnecessary for Stereotactic Body Radiation Therapy

This study shows that there is no clear dosimetric benefit of biological-based optimization for either fixed-beam IMRT or VMAT. Other than shorter delivery times, even VMAT does not offer additional advantage to fixed-beam IMRT.

A small number of patients for lung, pancreas, spine and brain CA were planned with fixed-beam IMRT, optimized with (gEUD) and without (DV) biological objectives and, also planned for VMAT with and without gEUD, for comparison. For the lung and brain cases, a non-coplanar 7-11 beam arrangement was used for fixed- beam IMRT and a coplanar 'hybrid' arc simulated VMAT with beams set every 5° spacing. For the other treatment sites, all beams were coplanar. For each case, the fixed-beam IMRT and VMAT plans were optimized with the same objectives. It is important to note that, only 2 segments/beam were allowed for each plan, in order to create small fluence modulation, appropriate for small target volumes during SBRT.

For all plans we noticed that there were minor or no dosimetric differences between fixed- beam IMRT and VMAT, whether DV or gEUD objectives were used or whether fixed-beam IMRT or VMAT is used. Keeping the level of beam modulation as-low-as possible, for small SBRT targets, one can show that VMAT with or without gEUD optimization does not offer any dosimetric advantage against fixed-beam IMRT with multiple non-coplanar beams. This is against the expectation that gEUD-optimization can Result superior plans than DV-optimization. The difference is that, for small target volumes like those encountered in SBRT, the complexity of the fluence is not as high as in large field intensity modulated cases.

The fact that VMAT with or without gEUD can produce as good plans as fixed-IMRT does not make VMAT a preferred treatment modality, other than the fact that requires reduced treatment time.