Detection of the focal spot motion relative to the collimator axis of a linear accelerator under gantry rotation.

Detection of the focal spot motion relative to the collimator axis of a linear accelerator under gantry rotation.
Phys Med Biol. 2018 Dec 19;:
Authors: Riis HL, Ebert MA, Rowshanfarzad P
Abstract
The potential for delivering high precision radiotherapy using linear accelerators (linacs) has been improved with the development of digital x-ray electronic portal imaging devices (EPID) for acquiring kilovoltage (kV) cone-beam CT and megavoltage (MV) images for patient positioning. EPIDs have also opened the possibilities of developing novel quality assurance and insight into radiotherapy equipment performance.
 The aim of this work was to measure the offset of the focal spot position (FSP) of a linac under gantry rotation relative to the collimator axis using an EPID. The focal spot was assumed to be a point source of MV x-ray generation. 
 A special phantom was designed for measurement of FSP as a function of gantry angle on clinical linacs. The phantom was designed for attachment to the gantry head and supporting two tungsten-carbide ball-bearings at two different distances from the focal spot. 
 The methodology was demonstrated on a series of images acquired of the phantom on three Elekta linacs in clinical use with 6 MV flattening-filter-free (FFF) beams. The gantry and collimator were rotated 360° in steps of 30°. For each position an image of the phantom was acquired using the EPID. Each series consisted o...

Source: Physics in Medicine and Biology - December 19, 2018 Category: Physics Authors: Riis HL, Ebert MA, Rowshanfarzad P Tags: Phys Med Biol Source Type: research