Volumetric modulated arc radiotherapy (VMAT): When is one revolution enough?

  • Prof Peter Metcalfe, Centre for Medical Radiation Physics, University of Wollongong, Australia
  • Dr N Hardcasle, 2. Dept. Human Oncology and Medical Physics, University of Wisconsin-Madison, Australia
  • Ms J McNamara, Illawarra Cancer Care Centre, Australia
  • Ms J Wong, Centre for Medical Radiation Physics, University of Wollongong, Australia
  • Dr M Carolan, Illawarra Cancer Care Centre, Australia
  • Prof A Rosenfeld, Centre for Medical Radiation Physics, University of Wollongong, Australia

Ten VMAT prostate radiotherapy plans have been optimised using SmartArc (Philips pinnacle version 9.0). The plans were optimised using EUD biological objectives for the rectum and physical dose objectives for the target. The prescription dose was 78Gy with a homogeneous 7mm margin the coverage of the PTV was compared using the D95% (dose to 95% of the PTV), V95% (volume of PTV with dose > 95% of the prescription was always better than 74.1Gy. The plans showed superior dose to fixed gantry seven field IMRT at V25. In all other respects the plans were equivalent. The dose distributions were delivered on a Varian 21eX. The fixed gantry deliveries took on average 4.5 minutes to deliver and the single arc VMATs took on average 1.5 minutes to deliver the dose (1).
It is possible to deliver an equivalent dose to simple plans such as these with a partial arc of about 2/3rds gantry rotation hence avoiding dose delivery through the couch. When the seminal vesicles are included however a full arc or sometimes two arcs may be required.
The average 5 field 3DCRT delivery time for these plans was 3.5 minutes, hence VMAT is faster. Arc rotation with variable dose rate presents new dosimetry verification challenges. Two systems Delta4 and magic plate are examined as VMAT dose distribution verification devices.
(1)Hardcastle et al Medical Dosimetry In Print 2010.