Abstract
We design a practical procedure for measuring translational and rotational misalignment of graticule with collimator axis of rotation and collimator jaws, respectively. The procedure's quantitative results are accurate to less than 0.2 mm (at isocenter) and do not assume alignment of radiation focal spot with collimator axis of rotation. When provided with these quantitative results, the manufacturer can custom‐adjust graticules to the purchaser's collimator head.PACS number: 87.56.Fc
Highlights
Portal images are compared with simulation images to verify fidelity of beam-patient geometry from simulation to treatment.(1) Often the difficulty in this comparison is in finding a common reference.(2) Graticules, both in simulators and treatment linacs, appear as “built-in rulers” in the images and are regarded as the common frame of reference for the comparison
In addition to beam-patient geometry discrepancy, graticules help reveal block discrepancy between simulation and treatment and, more importantly, allow one to distinguish between the two types of errors.(3)
Regardless of focal spot alignment with collimator axis of rotation (CAR), the CAR is aligned with graticule center if and only if graticule dots of first exposure perfectly align with those of the second exposure
Summary
Portal images are compared with simulation images to verify fidelity of beam-patient geometry from simulation to treatment.(1) Often the difficulty in this comparison is in finding a common reference.(2) Graticules, both in simulators and treatment linacs, appear as “built-in rulers” in the images and are regarded as the common frame of reference for the comparison. Improvements in accuracy of gantry rotation and other mechanical components of our linacs warranted a reduction to 1 mm of our threshold for graticule misalignment. If this threshold is exceeded, we order a new graticule. This reduction in threshold increased requirements on the QA procedure in two respects. It is desirable to custom-adjust newly ordered replacement graticules to individual collimator heads. To this end, the procedure must provide accurate quantitative results. In the case of manually adjustable graticules, our procedure can be used to confirm alignment after manual adjustment
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