Organ at risk dose verification in nasopharyngeal cancer VMAT planning with collimator comparison between single arc and double arc

Volumetric modulated arc therapy (VMAT) is an irradiation technique that enables the improvement of the flexibility of dose delivery from a full range of angles with continuous modulation of beam aperture and dose rate. We compared the homogeneity index (HI), conformity index (CI), dose volume histogram (DVH) and dose differentiation of single arc and double arc VMAT delivered for nasopharyngeal cancer (NPC). Planning for gantry rotation angles of 360° counter clockwise and clockwise (double arc) with collimator (0° and 10°) and 360° clockwise (single arc) were created for ten nasopharyngeal cancer cases applied in RANDO phantom with the Eclipse treatment planning system and delivered using Trilogy linear accelerator. The verification was performed by using gafchromic film EBT3 and placed on the RANDO phantom according to the location of the organs at risk. In this study, we evaluated right parotid, left parotid, right eye, left eye and trachea. The measurement of mean dose using gafchromic film and TPS shows that the resulting values were close to TPS results with the difference being -1.51%. From the result of dose measurement using gafchromic film based on the data from RANDO CT phantom use, it can be concluded that double arc VMAT technique with collimator can reduce the dose in the organ at risk.