Initial dosimetric evaluation of SmartArc - a novel VMAT treatment planning module implemented in a multi-vendor delivery chain

We performed an initial dosimetric evaluation of SmartArc - a novel VMAT planning module for the Philips Pinnacle treatment planning system. It was implemented in a multi-vendor environment, with the other two major components of the delivery chain being MOSAIQ record and verify system (IMPAC Medical Systems, Sunnyvale, CA) and a Trilogy linac (Varian Medical Systems, Palo Alto, CA). A test suite of structure sets and dose objectives provided by the AAPM for multi-institutional comparison of IMRT dosimetry was used. A total of fifty plans were successfully delivered. The effect of control point spacing on dosimetric accuracy was investigated. When calculated with the 4 degrees spacing, the overall mean point dose errors measured with an ion chamber were 0.5 +/- 1.4 and -0.3 +/- 1.4% for the PTV and OAR, respectively. The gamma(3%, 3 mm) passing rate, measured for absolute dose with a biplanar diode array, was 98.2 +/- 1.6% (range 94.5 -99.9%). Ninety percent of the passing rate values were above 97.7%. With the 6 degrees control point spacing, the highly modulated plans exhibited large dosimetric errors (e. g. gamma(3%, 3 mm) passing rates below 90% and ion chamber point dose errors of 6 -12%), while the results were still acceptable for the simpler cases. The data show that the practical accuracy of the small-arc approximation, which is at the heart of VMAT dose calculations, depends not only on the control point spacing, but also on the size and relative position of the MLC openings corresponding to the consecutive control points. The effect of the minimum allowed separation between the opposing leaves was found to be minimal. It appears that 4 degrees control point spacing may be a good compromise between calculation speed and accuracy. However each institution is encouraged to establish its own treatment planning guidelines based on the case complexity and acceptable error level.