[Dependence of uniformity on the radionuclide in SPECT: test methods]. / Abhängigkeit der Homogenität vom Radionuklid bei SPECT: Methoden der Uberprüfung.

The aim of this study was to investigate test methods to clarify whether the non-uniformity of a gamma camera depends on individual radionuclides, and whether it is necessary to measure a separate correction matrix for each radionuclide used in single photon emission computed tomography (SPECT). Two methods were devised to verify the nuclide-dependence of the gamma camera. In order to test the energy correction of the detectors, the first approach was based on the evaluation of the intrinsic non-uniformity and on the production of images with asymmetrical energy window. The second method was based on the production of correction matrices for different radionuclides, as well as on the subsequent application to phantom data that were also generated with different radionuclides. The investigation of a dualhead gamma camera produced the same results with both methods. One detector head was found to be weakly dependent on the radionuclide, due to the insufficient quality of energy correction. In this case, the phantom or patient data should be corrected using a uniformity correction matrix measured with the same radionuclide. The second detector remained nuclide-independent; in this case the uniformity correction matrix acquired for only one radionuclide was sufficient.

[BANG-polymer dosimetry in the proton therapy of eye neoplasms]. / BANG-Polymergeldosimetrie in der Protonentherapie von Augentumoren.

The use of polymer gel makes it possible to measure three-dimensional dose distributions of ionizing radiation. Phantoms with BANG-1 and BANG-3 gel were irradiated using a 68 MeV proton beam at the eye tumour therapy beam line of the Hahn Meitner Institute in Berlin. Up to twelve treatment fields could be applied to one gel phantom. The investigations consisted of mono-energetic Bragg curves, spread-out Bragg curves of circular fields (diameter 20 mm), and spread-out Bragg curves of patient fields. Magnetic resonance imaging was used to obtain the gel dose distributions. The results were compared to measurements of a water-phantom ionization chamber. The BANG polymer gels showed a significant quenching of Bragg peaks compared to ionization chamber measurements. The BANG-3 gel was found to be unsuitable for further investigations with 68 MeV protons. The use of BANG-1 allowed the verification of wedge slopes and irregular field forms. On the basis of our experience, polymer gels are well suited for quality assurance in proton therapy in principle.

[Properties of a commercial Hi-pSi detector for dosimetry of stereotactic collimators with very small diameters]. / Eigenschaften eines kommerzillen Hi-pSi-Detektors für die Dosimetrie stereotaktischer Kollimatoren mit sehr kleinen Durchmessern.

BACKGROUND: Conformal stereotactic radiosurgery and radiotherapy with linear accelerators and hole collimators yield a dose concentration in the target volume by rotation of the gantry. For small target volumes collimators with isocentre diameters of 4-45 mm are used. In this paper dosimetric measurements with a commercial high doped p-type silicon detector are demonstrated and compared to measurements with diamond detector and ionisation chamber. MATERIAL AND METHODS: The properties of the silicon detector SFD from Scanditronix were investigated with the radiation of a Gammatron S and a Varian 2100 CD at 6 MV. The results were compared with those of a calibrated ionisation chamber (0.3 cm3) and a diamond detector. MEASUREMENTS AND RESULTS: At the beginning the reproductibility of the registered dose and dose rate and the temperature dependence of the Si-detector were investigated at the Gammatron S. For the comparison the absorbed dose was measured with the ionisation chamber in air. The sensitivity decreases slightly with dose and dose rate. After a period of several days without radiation again higher doses were registered. The temperature dependence causes deviations of 0.25%/K. The signal-to-noise ratio and the spatial resolution were investigated with the linear accelerator. The signal-to-noise ratio is clearly lower compared with that of the diamond detector, whereas the resolution is nearly the same. CONCLUSIONS: The Si-detector is qualified for dosimetry of very small fields because of the insignificant dose and dose rate dependence and in spite of some disadvantages regarding dosimetric properties compared with the diamond detector. The advantage is the availability and the cost. Measurement with ionisation chambers are not useful for collimator diameters below 20 mm.