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A slit method to determine the focal spot size and shape of TomoTherapy system
Author(s) -
Chen Quan,
Chen Yu,
Chen Mingli,
Chao Edward,
Sterpin Edmond,
Lu Weiguo
Publication year - 2011
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3589133
Subject(s) - collimator , full width at half maximum , optics , slit , amplitude , gaussian , tomotherapy , physics , beam (structure) , collimated light , laser , medicine , quantum mechanics , radiation therapy
Purpose : To obtain accurate x‐ray source profile measurements using a slit‐collimator, the slit‐collimator should have a narrow width, large height, and be positioned near the source. However, these conditions may not always be met. In this paper, the authors provide a detailed analysis of the slit measurement geometry and the relationship between the slit parameters and the measured x‐ray source profile. The slit model allows the use of a shorter and more easily available slit‐collimator, while accurate source profile measurements can still be obtained. Methods : Measurements were performed with a variety of slit widths and/or slit to source distances. The relationship derived between the slit parameters and the measured profile was used to determine the true focal spot profile through a least square fit of the profile data. The model was verified by comparing the predicted profiles at a variety of slit‐collimator parameters with the measured results on the TomoTherapy Hi‐Art system. Results : Both the treatment beam and the imaging beam were measured. For treatment mode, it was found that a source consisting of one Gaussian with a 0.75 mm full‐width‐half‐maximum (FWHM) and 72% peak amplitude and a second Gaussian with a 2.27 mm FWHM and 18% peak amplitude matched measurement profiles. The overall source profile has a FWHM of 0.93 mm, but with a higher amplitude in the tail region than a single Gaussian. For imaging mode, the source consists of one Gaussian with a 0.68 mm FWHM and 82% peak amplitude and a second Gaussian with a 1.83 mm FWHM and 18% peak amplitude. The overall source profile has a FWHM of 0.77 mm. Conclusions : Our study of the focal spot measurement using slit‐collimators showed that accurate source profile measurements can be achieved through fitting of measurement results at different slit widths and source‐to‐slit distances (SSD). Quantitative measurements of the TomoTherapy linac focal spot showed that the source distribution could be better described with a model consisting of two Gaussian components rather than a single Gaussian model as assumed in previous studies.