Comparison of hybrid volumetric modulated arc therapy (VMAT) technique to double arc VMAT technique in the treatment of prostate cancer

Abstract

Iroduction

The purpose of this study is to compare a hybrid delivery technique to a standard VMAT technique for treatment of prostate cancer.  This Hybrid technique combines the use of Static IMRT fields to supplement a single arc pass which could theoretically be administered in one gantry rotation.   A variety of treatment plans were chosen to include a combination of larger patients, smaller patients, larger targets, as well as compromised critical OARs that require special consideration

Methods

11 patients having been previously treated for prostate cancer with VMAT techniques were replanned with a hybrid technique on Varian Treatment Planning System (TPS).  Multiple Static IMRT fields (2 to 3) were planned initially based on critical OAR to reduce dose but provide some PTV coverage. This was used as a base dose plan to provide 30-35% coverage for a single arc VMAT plan.

Results

The clinical VMAT plan was used as a control for the purposes of comparison. Average of all OAR sparing between the hybrid technique and VMAT showed the hybrid plan delivering less dose in almost all cases except for V80 of the bladder and maximum dose to right femoral head.

• PTV coverage was superior with the VMAT technique. See discussion.
• Monitor unit differences varied, with the hybrid plan able to deliver fewer units 37% of the time, similar results 18% of the time, and higher units 45% of the time. On average, the hybrid plan delivered 10% more monitor units.

Discussion

• Hybrid technique enables the planner to more easily control dose to critical OARs.   The use of static IMRT allows for additional input into the TPS and serves as a portal for the planner to more directly control dose distributions.
• Due to the nature of utilizing multiple optimizers to create the hybrid plan, coverage to the PTV suffered. Since the base dose plan is unaware of the subsequent VMAT arc, it cannot effectively yield homogeneous coverage to the PTV.  It is believed that the inhomogeneous base dose actually presents an additional restriction on the VMAT optimizer to achieve constraints.
• The total MUs for both techniques are similar.
• Delivery of such a hybrid plan could potentially reduce the treatment time compared to multiple arc VMAT treatments and decrease the associated risk of prostate motion during treatment.
• Software allowing a single VMAT arc to be optimized simultaneously with sections of static IMRT delivery should correct issues with the inability of the current optimizer to match the base dose plan to yield better PTV coverage.

Conclusion

• Dosimetrically the plans are similar, with the hybrid plan achieving better constraints on the OARs.
• The hybrid treatment could be delivered in a single gantry rotation, potentially reducing treatment time and increasing target localization compared to multiple arc VMAT.