An adequate dosimetry protocol for synchrotron radiation and the specific features of the ID17 Biomedical Beamline at the European Synchrotron Radiation Facility are essential for the preparation of the forthcoming clinical trials in the synchrotron stereotactic radiation therapy (SSRT). The main aim of this work is the definition of a suitable protocol based on standards of dose absorbed to water. It must allow measuring the absolute dose with an uncertainty within the recommended limits for patient treatment of 2%–5%.Methods:
Absolute dosimetry is performed with a thimble ionization chamber (PTW semiflex 31002) whose center is positioned at equivalent depth in water. Since the available synchrotron beam at the ESRF Biomedical Beamline has a maximum height of 3 mm, a scanning method was employed to mimic a uniform exposition of the ionization chamber. The scanning method has been shown to be equivalent to a broad beam irradiation. Different correction factors have been assessed by using Monte Carlo simulations.Results:
The absolute dose absorbed to water at 80 keV was measured in reference conditions with a 2% global uncertainty, within the recommended limits. The dose rate was determined to be in the range between 14 and 18 Gy/min, that is to say, a factor two to three times higher than the 6 Gy/min achievable in RapidArc or VMAT machines. The dose absorbed to water was also measured in a RW3 solid water phantom. This phantom is suitable for quality assurance purposes since less than 2% average difference with respect to the water phantom measurements was found. In addition, output factors were assessed for different field sizes.Conclusions:
A dosimetry protocol adequate for the specific features of the SSRT technique has been developed. This protocol allows measuring the absolute dose absorbed to water with an accuracy of 2%. It is therefore satisfactory for patient treatment.
The authors warmly thank Pierre Deman for the figures in the text and Christian Nemoz, Herwig Requardt, Michel Renier, and Thierry Brochard for their valuable support. The ESRF is gratefully acknowledged for the availability of beam time and the support given to the development of the SSRT project.
II. MATERIALS AND METHODS
II.A. Monte Carlo source code
II.B. Irradiation conditions for SSRT
II.C. Absolute dosimetry
II.C.1. Determination of absorbed dose under reference conditions
II.C.2. Scanning method
II.C.3. Absolute dosimetry of monochromatic synchrotron radiation at ID17 Biomedical Beamline
III. RESULTS AND DISCUSSION
III.A. Absolute dosimetry in the clinical trial settings
III.B. Scatter and output factors
III.C. Dosimetry in a solid water phantom
III.D. Patient dosimetry
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