Investigation of the sag in linac secondary collimator and MLC carriage during arc deliveries

Abstract

In modern radiotherapy, it is vitally important to monitor the performance of all linac components including the collimation system. In this study, a simple measurement method and accurate algorithm are introduced for investigation of the secondary and tertiary collimator sag during radiotherapy arc treatments. The method is based on cine EPID images of a ball bearing marker fixed to the gantry head and determines the jaw and MLC sag in all directions relative to the reference at zero gantry angle. Analysis was performed using different field sizes and collimator angles, different linacs and different gantry rotation directions. The accuracy of the method was tested and was less than 0.02 mm. The repeatability and reproducibility of the method was 0.005 and 0.09 mm, respectively. The setup is easy and quick and the algorithm is fast and fully automatic with sub-pixel accuracy. This method is suitable to be included in the routine quality assurance of linacs to monitor the collimator system performance.

A comprehensive study of the mechanical performance of gantry, EPID and the MLC assembly in Elekta linacs during gantry rotation

“…[1][2][3][4][5] Gravity can also induce sagging of the beam collimation system. 3,6,7 Rotation of the gantry during treatment delivery can lead to additional multileaf collimator (MLC) errors (systematic shifts) owing to the displacement of the leaf bank assembly. [7][8][9] Moreover, linear accelerator (linac) rotation can affect gantry-mounted accessories such as the electronic portal imaging device (EPID), since the EPIDsupporting arm is not mechanically perfect and rigidly attached.…”

“…3,6,7 Rotation of the gantry during treatment delivery can lead to additional multileaf collimator (MLC) errors (systematic shifts) owing to the displacement of the leaf bank assembly. [7][8][9] Moreover, linear accelerator (linac) rotation can affect gantry-mounted accessories such as the electronic portal imaging device (EPID), since the EPIDsupporting arm is not mechanically perfect and rigidly attached. With the growing application of EPIDs in preand post-treatment dosimetry verification, [10][11][12][13][14][15] realtime dosimetry verification 16,17 and real-time tumour tracking for intrafraction motion management in modern radiotherapy, [18][19][20] it is essential to characterize and account for the mechanical system imperfections of linacs.…”

“…4,7,21 Our former studies were focused on using EPID-based methods for evaluation of the performance of Varian linacs (Varian® Medical Systems, Palo Alto, CA). In this work, investigation is extended to the behaviour of components of Elekta linacs (Elekta AB, Stockholm, Sweden) at various gantry angles with some additional details.…”

Gantry angle determination during arc IMRT: evaluation of a simple EPID‐based technique and two commercial inclinometers

“…The centers of the BBs were automatically detected in each image using an algorithm already developed and explained in detail by Rowshanfarzad et al 18 , 19 , 20 in the MATLAB programming language (The MathWorks Inc., Natick, MA) with minor modifications. The algorithm finds the position of BBs by determining the minimum signal value in a region of interest with subpixel accuracy.…”

Impact of backscattered radiation from the bunker structure on EPID dosimetry

“…Evaluation of the imager response was based on the pixel values from the central 50% of the fields in order to provide adequately large areas to capture the backscatter signal and to limit the region of interest to within the field, and thus eliminate the effects of any possible sagging of the EPID (23) (or MatriXX) and collimator jaws (24) during rotation. The field edges were determined by developing a code which used the image pixel values, picking the first and the last points with gray scale levels larger than 50% of the maximum signal in the cross‐plane and in‐plane directions.…”