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Beam focal spot position: The forgotten linac QA parameter. An EPID-based phantomless method for routine Stereotactic
Jacek M Chojnowski1,2, Michael P Barnes3,4, Jonathan R Sykes2,5
1Department of Radiation Oncology, Coffs Harbour Base Hospital, Coffs Harbour, NSW, Australia.
Journal of Applied Clinical Medical Physics
|August 9, 2017
Summary
A new phantomless method using an Electronic Portal Imaging Device (EPID) accurately measures linear accelerator focal spot alignment. This technique ensures high geometric accuracy for stereotactic treatments, improving patient safety and treatment quality.
Area of Science:
- Medical Physics
- Radiation Oncology
- Radiotherapy Technology
Background:
- High geometric accuracy is crucial for modern stereotactic radiotherapy.
- Isocenter coincidence between imaging and treatment beams is essential for precision.
- Focal spot alignment with the collimator rotation axis impacts beam positioning and accuracy.
Purpose of the Study:
- To develop and validate a novel, phantomless method for measuring linear accelerator (linac) focal spot alignment.
- To assess the feasibility of integrating this method into routine linac Quality Assurance (QA) programs.
- To provide a more accurate and efficient alternative to traditional focal spot measurement techniques.
Main Methods:
- A phantomless method was developed utilizing the Electronic Portal Imaging Device (EPID).
- The technique leverages the differential heights of the Multi-Leaf Collimator (MLC) and jaws.
- The method was validated on four linacs and benchmarked against an ion chamber-based approach.
Main Results:
- The novel EPID-based method demonstrated high reproducibility, within ±0.012 mm (1 SD).
- Results showed excellent agreement with the ion chamber-based method (0.001 ± 0.015 mm, 1 SD).
- The method is practical for routine linac QA.
Conclusions:
- The developed phantomless EPID method offers a reproducible and accurate solution for linac focal spot alignment verification.
- This technique can enhance the geometric accuracy of stereotactic treatments.
- The method is suitable for routine integration into departmental linac QA protocols.
