Abstract
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.
Highlights
Quality Assurance of medical linear accelerators is necessary for safe treatment of radiotherapy patients
This study describes a first phantomless method of evaluating focal spot alignment to collimator rotation axis with no additional tools or assumptions necessary
When measurements are performed for both jaws and MLC, the difference in position of the isocenter centroids can be used as a measure of the alignment of the beam focal spot position with respect to collimator rotation axis
Summary
Quality Assurance of medical linear accelerators is necessary for safe treatment of radiotherapy patients. Recommendations on tests required with frequencies and tolerances are described in specialized publications such as AAPM TG-1421 and IPEMB81.2 Modern linear accelerators have improved accuracy and precision in treatment delivery and allowed clinical implementation of advanced treatment modalities such as frameless Cranial Stereotactic Radiosurgery (CSRS) and Stereotactic Body Radiotherapy (SBRT). These treatment techniques demand more stringent linac quality assurance programs with additional complex checks which lead to increased time and resource demands.[3]. The variables that influence the treatment isocenter component of WL include mechanical isocenter size and shape, alignment of collimators and beam focal spot position relative to collimator rotation axis,[6] and their stability with gantry rotation
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