2320 Volumetric Modulated Arc Therapy (VMAT) for Delivery of Prostate Radiotherapy: Reduction in
Treatment Time and Monitor Unit Requirements Compared to Intensity Modulated Radiotherapy
D. Palma, E. Vollans, K. James, S. Nakano, V. Moiseenko, R. Shaffer, M. McKenzie, J. Morris, K. Otto, BC Cancer
Agency, Vancouver, BC, Canada
Purpose/Objective(s): Volumetric Modulated Arc Therapy (VMAT) is a novel form of intensity modulated
radiotherapy (IMRT) in which an optimized 3D dose distribution may be delivered in a single gantry rotation. VMAT
can be delivered using either a constant (cdr‐VMAT) or variable (vdr‐VMAT) dose rate during rotation. The vdr‐
VMAT algorithm is the predecessor to Varian's RapidArc. The goal of this study was to compare monitor unit (MU)
requirements and treatment time for VMAT, 3D‐conformal radiotherapy (3D‐CRT) and 5‐field IMRT techniques.
Materials/Methods: Ten CT data sets from patients who underwent radiotherapy for prostate cancer were used
to create 3D‐CRT, 5‐field IMRT, cdr‐VMAT and vdr‐VMAT plans. Outcomes evaluated included equivalent uniform
doses (EUD) to target volumes and organs at risk, MU requirements, and treatment time for delivery of a 2 Gy
Results: Both vdr‐VMAT and cdr‐VMAT plans required fewer MU than IMRT plans (relative reductions of 42% and
38% respectively; p=0.005), but more than 3D‐CRT (p=0.005). Treatment times for both cdr‐VMAT and vdr‐VMAT
were significantly less than with IMRT plans. The most favorable EUD were achieved with vdr‐VMAT, although
differences in dose distributions compared to IMRT are unlikely to be clinically significant.
Conclusions: VMAT plans achieved a substantial reduction in MU requirements compared to IMRT plans, which
should result in decreased whole body scatter dose. VMAT plans may allow for more patient throughput than
Author Disclosure: D. Palma, None; E. Vollans, None; K. James, None; S. Nakano, None; V. Moiseenko, None; R.
Shaffer, None; M. McKenzie, None; J. Morris, None; K. Otto, Dr. Otto is the recipient of a medical research grant
from Varian Medical Systems Inc., B. Research Grant.