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

Full text: 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 objcctives for the target. The prescription dose was 78 Gy with a homogeneous 7 mm margin the coverage of the PTV was com pared using the D95% (dose to 95% of the PTY), V95% (volume of PTV with dose >95%) of the prescription was always better than 74.1 Gy. The plans showed superior dose to fixed gantry seven field [MRT at V25. [n all other respects the plans were equivalent. The dose distributions were delivered on a Varian 21 eX. The fixed gantry deliveries took on average 4.5 min to deliver and the single arc VMATs took on average 1.5 min to deliver the dose (I). It is possiblc 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 averagc five field 3DCRT delivery time for these plans was 3.5 min, hence VMA T is faster. Arc rotation with variable dose rate presents new dosimetry verification challenges. Two systems Delta4 and magic plate are examined as VMA T dose distribution verification devices.