Session Item

Basic beam data validation was carried out for the new Varian Halcyon platform using a 3D water phantom (w.p.). The data acquired was compared with the preconfigured AAA reference data set provided by the vendor to validate the algorithm clinically.

The beam data was acquired on a 6FFF x-ray beam from a Varian Halcyon using the 3D-Scanner w.p. from Sun Nuclear (SN). Depth dose curves, crossline and diagonal dose profiles at different depths and fixed SSD=90cm were measured using a SNC125c ion chamber from 4x4 cm² to the maximum allowed field size of 28x28 cm².Output factors were measured at SSD=95cm and 5 cm depth. The nominal linac output and the beam quality index were determined according to TRS-398.

The measured depth dose curves, dose profiles and diagonal profiles showed very good agreement with the preconfigured beam data, differences were within 1%-1mm.

Field output factors differences were less than 0.5% for all the field sizes measured with a mean dose difference of -0.1%.

Measured linac output for the 10x10 cm² square field size at SSD=90cm and 10cm depth was 1cGy per MU, in concordance with machine calibration specifications from the vendor.The output was also verified calculating the dose on a virtual water phantom on the TPS with a 0.4% difference against reference value.

The difference between the in-water measured quality index TPR_{20, 10} of 0.627 and the one calculated on a virtual water phantom from isocentric doses at 10 cm and 20 cm depth was 0.77%.

The ArcCheck measured data from modulated plans were compared to the planned dose using the SNCPatient software leading to a mean value of 99.8% points passing 3%-3mm gamma evaluation with a 10% threshold.

The portal dosimetry for the same tests gave a mean value of 99.8% points passing 3%-3mm gamma evaluation in absolute dose mode.

Measurements in water regarding basic beam dose distribution are agree with the data provided by Varian with enough accuracy to use the photon beam model for clinical purposes.