SU-E-T-196: Commissioning for Volumetric Modulated Radiation Therapy on Varian Clinac 21EX

Recently the volumetric arc therapy (VMAT) technology such as RapidArc is widely distributed in Japan. These technologies are normally provided by the high spec linear accelerator such as Trilogy, Novalis Tx, Synergy, et al. The specific DICOM-file is generally used for commissioning of these technologies. On the other hand, we had to apply RapidArc into historic linear accelerator. This title expresses an experience how we performed the commissioning of RapidArc with the old linear accelerator.

Two Varian's linear accelerators "Clinac 21EX" equipped with Millenium multi-leaf collimator and a Varian's treatment planning system "Eclipse ver.8.9" were used for this study. The commissioning for RapidArc was performed in energy 4,6,10,15 MV (Max-DR: 250, 600, 400, 600 MU/min). Commissioning procedure composed two categories: the general machine QA for DMLC-IMRT procedure and the specific RapidArc QA procedure. In RapidArc QA procedure, we modified DICOM-file to apply into the potential spec of Clinac 21EX optimally. The specific MLC-motion sequence and the gantry rotation speed were created by the dedicated programs (Shaper and DicomEdit, Varian) for RapidArc QA procedure. Each tolerance value was defied by the data from daily/monthly QA and the paper by Ling et al.

As the results of the general machine QA procedure, the variance of radiation output during static/dynamic gantry rotation was less than 1%. The deference of fence tests during static/dynamic gantry rotation and RapidArc were less than 1 mm in each. However, the results of the RapidArc QA were worse than the latest machine (especially variable gantry speed) and it was careful to define tolerance level.

The procedure of commissioning for RapidArc on historic linear accelerator was proposed. Several minor revisions for DICOM-file should be required for suitable commissioning and it may ensure the tolerance limit for gantry/MLC-leaf motion speeds.