On average, there was no significant difference in MHD between VMAT-DIBH and t-IMRT-DIBH. However, VMAT-DIBH was found to benefit a select group of patients. For patients in whom the MHD was >6.3 Gy with t-IMRT-DIBH, the use of VMAT-DIBH resulted in a benefit in reducing the MHD.

An efficient procedure has been developed using an electronic portal imaging device (EPID) and in-house written software for verification of a target simulator alignment with the radiation isocentre. A 5 mm tungsten ball is aligned to a linac isocentre based on a lasers intersection point. The BrainLab(®) m3™ add-on multileaf collimator (MLC) forms a rectangular open field of 1.8 × 1.8 cm(2). At five different gantry and couch positions, EPID images are acquired. A computer search algorithm determines the centres of both a radiation field and a tungsten ball for each image. Based on the geometric differences between those centres, the optimum three-dimensional shift of a tungsten ball is calculated in order to minimise the misalignment error between a target simulator and a radiation isocentre. A decision can then be made whether or not the tungsten ball and lasers intersection point should be corrected. The accuracy and precision of the procedure has been tested and found to be 0.04 and 0.24 mm respectively at 95% confidence interval. The procedure is also quicker, easier and more reliable to perform compared to the previous method based on irradiating a radiographic film.

Modern day Stereotactic treatments require high geometric accuracy of the delivered treatment. To achieve the required accuracy the IGRT imaging isocenter needs to closely coincide with the treatment beam isocenter. An influence on this isocenter coincidence and on the spatial positioning of the beam itself is the alignment of the treatment beam focal spot with collimator rotation axis. The positioning of the focal spot is dependent on the linac beam steering and on the stability of the monitor chamber and beam steering servo system. As such, there is the potential for focal spot misalignment and this should be checked on a regular basis. Traditional methods for measuring focal spot position are either indirect, inaccurate, or time consuming and hence impractical for routine use. In this study a novel, phantomless method has been developed using the EPID (Electronic Portal Imaging Device) that utilizes the different heights of the MLC and jaws. The method has been performed on four linear accelerators and benchmarked against an alternate ion chamber‐based method. The method has been found to be reproducible to within ±0.012 mm (1 SD) and in agreement with the ion chamber‐based method to within 0.001 ± 0.015 mm (1 SD). The method could easily be incorporated into a departmental routine linac QA (Quality Assurance) program.

A novel phantomless, EPID‐based method of measuring the beam focal spot offset of a linear accelerator was proposed and validated for Varian machines. In this method, one set of jaws and the MLC were utilized to form a symmetric field and then a 180o collimator rotation was utilized to determine the radiation isocenter defined by the jaws and the MLC, respectively. The difference between these two isocentres is directly correlated with the beam focal spot offset of the linear accelerator. In the current work, the method has been considered for Elekta linacs. An Elekta linac with the Agility® head does not have two set of jaws, therefore, a modified method is presented making use of one set of diaphragms, the MLC and a full 360o collimator rotation. The modified method has been tested on two Elekta Synergy® linacs with Agility® heads and independently validated. A practical guide with instructions and a MATLAB ® code is attached for easy implementation.