Proton beam dosimetry in the presence of magnetic fields using Farmer-type ionization chambers of different radii.

BACKGROUND: Magnetic resonance-guided proton therapy is promising, as it combines high-contrast imaging of soft tissue with highly conformal dose delivery. However, proton dosimetry in magnetic fields using ionization chambers is challenging since the dose distribution as well as the detector response are perturbed. PURPOSE: This work investigates the effect of the magnetic field on the ionization chamber response, and on the polarity and ion recombination correction factors, which are essential for the implementation of a proton beam dosimetry protocol in the presence of magnetic fields. METHODS: Three Farmer-type cylindrical ionization chambers, the 30013 with 3 mm inner radius (PTW, Freiburg, Germany) and two custom built chambers "R1" and "R6" with 1 and 6 mm inner radii respectively were placed at the center of an experimental electromagnet (Schwarzbeck Mess - Elektronik, Germany) 2 cm depth of an in-house developed 3D printed water phantom. The detector response was measured for a 3 × 10 cm2 field of mono-energetic protons 221.05 MeV/u for the three chambers, and with an additional proton beam of 157.43 MeV/u for the chamber PTW 30013. The magnetic flux density was varied between 0.1 and 1.0 Tesla in steps of 0.1 Tesla. RESULTS: At both energies, the ionization chamber PTW 30013 showed a non-linear response as a function of the magnetic field strength, with a decrease of the ionization chamber response of up to 0.27% ± 0.06% (1 SD) at 0.2 Tesla, followed by a smaller effect at higher magnetic field strength. For the chamber R1, the response decreased slightly with the magnetic field strength up to 0.45% ± 0.12% at 1 Tesla, and for the chamber R6, the response decreased up to 0.54% ± 0.13% at 0.1 Tesla, followed by a plateau up to 0.3 Tesla, and a weaker effect at higher magnetic field strength. The dependence of the polarity and recombination correction factor on the magnetic field was ⩽0.1% for the chamber PTW 30013. CONCLUSIONS: The magnetic field has a small but significant effect on the chamber response in the low magnetic field region for the chamber PTW 30013 and for R6, and in the high magnetic field region for the chamber R1. Corrections may be necessary for ionization chamber measurements, depending on both the chamber volume and the magnetic flux density. No significant effect of the magnetic field on the polarity and recombination correction factor was detected in this work for the ionization chamber PTW 30013.

Deductive development and validation of a questionnaire to assess sensitivity to very low and very high frequency sounds: SISUS-Q (Sensitivity to Infra-Sound and Ultra-Sound Questionnaire).

OBJECTIVE: Auditory research and complaints about environmental noise indicate that there exists a significant, small subgroup within the population which is sensitive towards infra- and low-frequency or ultra- and high-frequency sounds (ILF/UHF). This paper reports on the development, factorization and validation of measures of sensitivity towards frequencies outside the common hearing range. DESIGN: A multinational, cross-sectional survey study was run. Principal component analyses and exploratory factor analyses were conducted in a sample of 267 Europeans (from the UK, Slovenia, and Germany). RESULTS: The factor analyses suggested that ILF versus UHF sensitivity constitute different factors, each characterized by sensory perception, stress-responsivity, and behavioral avoidance. A third factor comprising beliefs of dangerousness of ILF and UHF emerged. The factors explained 72% of the variance. The factor-solution was replicated separately for the English (n = 98) and German (n = 169) versions of the questionnaire (Slovenians and UK residents filled out the English version). Acceptable to excellent reliability was found. ILF and UHF sensitivity were moderately related to noise sensitivity in the normal hearing range, suggesting the new measures are not redundant. Correlations with psychiatric and somatic symptoms were small to moderate. ILF sensitivity correlated with neuroticism (small effect) and daytime sleepiness (moderate effect). ILF and UHF sensitivity were related to agreeableness (small effects). Overall, the novel ILF and UHF sensitivity scales seems to provide a solid tool for conducting further research on the role of sensitivity concerning adverse effects of ILF and UHF sound (e.g. health outcomes, annoyance ratings). The questionnaire consortium recommends using the new scales in combination with established measures of normal hearing range sensitivity.

The practical application of scintillation dosimetry in small-field photon-beam radiotherapy.

Plastic scintillation detectors are a new instrument of stereotactic photon-beam dosimetry. The clinical application of the plastic scintillation detector Exradin W1 at the Siemens Artiste and Elekta Synergy accelerators is a matter of current interest. In order to reduce the measurement uncertainty, precautions have to be taken with regard to the geometrical arrangement of the scintillator, the light-guide fiber and the photodiode in the radiation field. To determine the "Cerenkov light ratio" CLR with a type A uncertainty below 1%, the Cerenkov calibration procedure for small-field measurements based on the two-channel spectral method was used. Output factors were correctly measured with the W1 for field sizes down to 0.5×0.5cm2 with a type A uncertainty of 1.8%. Measurements of small field dose profiles and percentage depth dose curves were carried out with the W1 using automated water phantom profile scans, and a type A uncertainty for dose maxima of 1.4% was achieved. The agreement with a synthetic diamond detector (microDiamond, PTW Freiburg) and a plane parallel ionization chamber (Roos chamber, PTW Freiburg) in relative dose measurements was excellent. In oversight of all results, the suitability of the plastic scintillation detector Exradin W1 for clinical dosimetry under stereotactic conditions, in particular the tried and tested procedures for CLR determination, output factor measurement and automated dose profile scans in water phantoms, have been confirmed.