Investigation of a novel algorithm for true 4D-VMAT planning with comparison to tracked, gated and static delivery

A novel 4D volumetric modulated are therapy (4D-VMAT) planning system is presented where radiation sparing of organs at risk (OARs) is enhanced by exploiting respiratory motion of tumor and healthy tissues.

In conventional radiation therapy, a motion encompassing margin is normally added to the clinical target volume (CTV) to ensure the tumor receives the planned treatment dose. This results in a substantial increase in dose to the OARs. Our 4D-VMAT algorithm aims to reduce OAR dose by incorporating 4D volumetric target and OAR motions directly into the optimization process. During optimization, phase correlated beam samples are progressively added throughout the full range of gantry rotation. The resulting treatment plans have respiratory phase-optimized apertures whose deliveries are synchronized to the patient's respiratory cycle. 4D-VMAT plans reduce dose to the OAR by: (1) eliminating the motion margin, (2) selectively redistributing OAR dose over the OAR volume, and (3) timing larger dose contributions (MU) to respiratory phases where greater separations between the target and OAR occur. Our 4D-VMAT algorithm was tested by simulating a variety of tumor motion amplitudes (0.5-2 cm) in the superior/inferior and anterior/ posterior directions. 4D-VMAT's performance was compared against 3D-VMAT, gated VMAT and dynamic multileaf collimator (DMLC) ideal-tracking VMAT.

Results show that OAR sparing of 4D-VMAT was greater than 3D-VMAT in all cases due to the smaller PTV margin. Compared to DMLC ideal-tracking VMAT, 4D-VMAT's OAR sparing is superior only when the relative distance between the PTV and OAR is changing. For gated VMAT, results compared to 4D-VMAT are phantom dependent. There was negligible difference in plan qualities for the tested case of motion along the anterior/posterior axis. For motions along the superior/inferior axis, gated VMAT's narrow beam-on window reduces the OAR volume directly irradiated by the linac but also allows higher dose accumulation in the exposed OAR. In contrast, 4D-VMAT can reduce the OAR volume exposed to high doses but at the cost of redistributing the OAR dose over a larger volume. Finally for 4D-VMAT, an increase in tumor motion no longer resulted in greater irradiation of the OAR as seen in conventional 3D radiation therapy. OAR dose levels were preserved for increasing target motion along the anterior/posterior axis. For increasing superior/inferior motion, the volume of OAR exposed to high doses actually decreased due to dose redistribution.

Our investigation demonstrated that the 4D-VMAT system has the potential to improve radiation therapy of periodically moving tumors over 3D-VMAT, gating or tracking methods.