A comparison study of out-of-field photon dosimetry between two Varian linear accelerators

Abstract

Author(s): Aswathi Ra, D. Khanna, Hridya V T, Hridya V T, Sathish Padmanabhan and P. Mohandass

Purpose: The present study compares the components of out of field dose of two linear accelerators and quantifies (i) the variation of out-of-field dose with the detector, (ii) the phantom scatter, collimator scatter and head leakage contribution towards out-of-field dose for two linear accelerators (LINAC), and (iii) the variation of out-of-field dose with Field Size (FS)

Materials and Methods: The out-of-field measurements were obtained from Varian Unique Power Linear Accelerator (VUP) and compared with the Varian Truebeam® STx Linear accelerator (VTB) using PTW chambers of different volumes and slab phantoms. The measurements with different chambers were performed for 10 cm2 ×10 cm2 FS with VUP with collimators 00 and 900 . The out-of-field dose components were measured for a small and intermediate FS for VUP and VTB. The measured results were compared with the TPS calculated.

Conclusion: Higher out-of-field was observed with Semi flex chamber with a collimator 90? and with a larger FS. Among the machine, VUP showed a lesser scatter factor. From TPS measurements, it was clear that TPS was not modelled for collimator scatter and head leakage.

Results: Comparing the out-of-field dose contribution semi flex showed a higher dose than other chambers. Compared to the individual scatter component, the phantom scatters component shows high with Semi flex chamber, the collimator scatter with the farmer, and head leakage with Semi flex. With collimator rotation of 900 , Semi flex and pinpoint showed an increase in out-offield dose concerning collimator zero. All the components of out-of-field dose increase with FS. When comparing the scatter components of two Linacs, VUP showed a lesser scatter than VTB.

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