Verification of radiosurgery target point alignment with an electronic portal imaging device (EPID)

Abstract

A procedure has been developed using an electronic portal imaging device (EPID) to verify that the center of a patient's lesion is aligned with the center of a treatment cone prior to treatment in a linac-based stereotactic radiosurgery procedure. The coordinates of the lesion center are set on the Brown-Roberts-Wells phantom base using a target simulator. A 3 mm tungsten ball, mounted on the target simulator, is used as the reference point for the planned isocenter. The target simulator is then attached to an adapter mounted on the linac couch, and an EPID image of the simulated target is acquired. The center of the circular-shaped radiation field is calculated from the centroid of the segmented EPID image, and the center of the tungsten ball is identified by an automated computer search algorithm. A summation filter is used to find the position of the lowest radiation intensity coincident with the center of the ball. The alignment error is defined as the difference between the center of the radiation field and the center of the ball. The accuracy of this method was tested and found to be within 0.2 mm. The advantage of the EPID-based procedure is that it can give quantitative offset values quickly for immediate readjustment. We have found that the method is also a convenient tool for testing room laser alignment and the accuracy of the treatment cones.

Isocenter verification for linac‐based stereotactic radiation therapy: review of principles and techniques

“…( 23 ) Dust or marks on the film can lead to spikes in images. ( 40 ) The film results for Winston‐Lutz test are not quantitative, ( 26 , 52 , 59 ) and are based on manual evaluations ( 24 , 34 ) and visual inspection — which makes them highly dependent on the skills of the observer. ( 11 , 36 , 52 , 60 ) The uncertainty introduced by operator judgment has been reported to be 0.3 to 0.4 mm, ( 36 , 61 , 62 ) although this could be reduced by scanning the films and using software analysis.…”

“…( 40 ) The film results for Winston‐Lutz test are not quantitative, ( 26 , 52 , 59 ) and are based on manual evaluations ( 24 , 34 ) and visual inspection — which makes them highly dependent on the skills of the observer. ( 11 , 36 , 52 , 60 ) The uncertainty introduced by operator judgment has been reported to be 0.3 to 0.4 mm, ( 36 , 61 , 62 ) although this could be reduced by scanning the films and using software analysis. ( 14 , 36 , 58 ) In addition, film adjustment and processing are time‐consuming, ( 11 , 36 , 52 ) which makes it difficult to perform the test before each treatment.…”

“…The isocenter verification process must be performed before each SRS/SRT treatment. ( 36 , 52 – 55 ) …”

A quality assurance method with submillimeter accuracy for stereotactic linear accelerators

“…For convenience, the algorithms could be generalized to process an image from an electronic portal imaging device (EPID) instead of data from the film scanner. ( 12 , 13 ) We began a study with GAFCHROMIC ( 14 ) EBT film (International Specialty Products, Wayne, NJ), but it requires at least 200 to 300 MU to obtain a reasonable image, and the scanned image was not as good as for XV film. For the Winston‐Lutz tests in this study, we exclusively used XV film and 50 MU per shot to first discover the best attainable accuracy with the system.…”

Guide to clinical use of electronic portal imaging

“…The EPID has also been put to use for quality assurance of treatment machines 86 and of treatment techniques, such as radiosurgery 87 and dynamic wedge and MLC therapy 88 , 89 . In each case, the EPID has allowed more precise, quantitative results to be obtained with much less effort than would have been achievable using conventional QA tools.…”