The Swiss IMRT dosimetry intercomparison using a thorax phantom.

PURPOSE: In 2008, a national intensity modulated radiation therapy (IMRT) dosimetry intercomparison was carried out for all 23 radiation oncology institutions in Switzerland. It was the aim to check the treatment chain focused on the planning, dose calculation, and irradiation process. METHODS: A thorax phantom with inhomogeneities was used, in which thermoluminescence dosimeter (TLD) and ionization chamber measurements were performed. Additionally, absolute dosimetry of the applied beams has been checked. Altogether, 30 plan-measurement combinations have been used in the comparison study. The results have been grouped according to dose calculation algorithms, classified as "type a" or "type b," as proposed by Kntis et al. ["Comparison of dose calculation algorithms for treatment planning in external photon beam therapy for clinical situations," Phys. Med. Biol. 51, 5785-5807 (2006)]. RESULTS: Absolute dosimetry check under standard conditions: The mean ratio between the dose derived from the single field measurement and the stated dose, calculated with the treatment planning system, was 1.007 +/- 0.010 for the ionization chamber and 1.002 +/- 0.014 (mean+/- standard deviation) for the TLD measurements. IMRT Plan Check: In the lung tissue of the planning target volume, a significantly better agreement between measurements (TLD, ionization chamber) and calculations is shown for type b algorithms than for type a (p <0.001). In regions outside the lungs, the absolute differences between TLD measured and stated dose values, relative to the prescribed dose, [(Dm-Ds)/Dprescribed], are 1.9 +/- 0.4% and 1.4 +/- 0.3%, respectively. These data show the same degree of accuracy between the two algorithm types if low-density medium is not present. CONCLUSIONS: The results demonstrate that the performed intercomparison is feasible and confirm the calculation accuracies of type a and type b algorithms in a water equivalent and low-density environment. It is now planned to offer the intercomparison on a regular basis to all Swiss institutions using IMRT techniques.

Topological methods for the comparison of structures using LDR-brachytherapy of the prostate as an example.

The dose coverage of low dose rate (LDR)-brachytherapy for localized prostate cancer is monitored 4-6 weeks after intervention by contouring the prostate on computed tomography and/or magnetic resonance imaging sets. Dose parameters for the prostate (V100, D90 and D80) provide information on the treatment quality. Those depend strongly on the delineation of the prostate contours. We therefore systematically investigated the contouring process for 21 patients with five examiners. The prostate structures were compared with one another using topological procedures based on Boolean algebra. The coincidence number C(V) measures the agreement between a set of structures. The mutual coincidence C(i, j) measures the agreement between two structures i and j, and the mean coincidence C(i) compares a selected structure i with the remaining structures in a set. All coincidence parameters have a value of 1 for complete coincidence of contouring and 0 for complete absence. The five patients with the lowest C(V) values were discussed, and rules for contouring the prostate have been formulated. The contouring and assessment were repeated after 3 months for the same five patients. All coincidence parameters have been improved after instruction. This shows objectively that training resulted in more consistent contouring across examiners.

131-Iodine capsules in thyroid therapy: an individually controlled study of their uptake kinetics as compared to liquid 131-iodine.

We investigated the uptake of therapeutic doses of 131-Iodine in capsular form which were given to 16 patients with benign thyroid disease, and compared it to the uptake of a diagnostic dose of liquid 131-Iodine given to the same patients. The aim of this study was to determine the additional radiation dose sustained by the gastric mucosa, and thus, to establish the safety of this galenic form of 131I. It was found that the average capsule-dissolution time was about 12 min, with a large standard deviation of about 7 min. Using these data and a theoretical radiation-dose calculation, we estimated that the maximum dose to the gastric mucosa was approximately 250 rad (250 cGy) for a therapeutic activity of 5 mCi (185 MBq), which is the maximum dose which may be given as single application to out-patients in Switzerland. Thus, 131I administered in capsular form is a safe galenic form for therapeutic use in patients with thyroid disease.

Stripping of X-ray bremsstrahlung spectra up to 300 kVp on a desk type computer.

The direct result of a spectrometric measurement is a pulse height distribution. In the energy region up to 300 keV three corrections in particular need to be applied to get the photon spectrum: corrections for K-escape, Compton scattering and inefficient photon absorption. A simple 'stripping' procedure is described which may be implemented on a desk type computer. All data necessary are either available in the literature or may be derived from the measurement of very heavily filtered X-ray spectra. The accuracy of the procedure is better than +/- 5% of the peak value. Results are compared with a more detailed stripping procedure, based on Monte Carlo calculated data.

Prototype ionographic imaging chamber. Construction and clinical use.

The design and construction of an experimental ionography chamber are described and the principles of this new electrostatic imaging technique are discussed. Examples of medical ionographic images taken with this simple prototype chamber are presented.

[Developments in ionography (author's transl)]. / Enwicklungstendenzen in der Ionographie.

Common aspects and differences of xeroradiography and ionography are reviewed briefly. Tendencies connected with latent image formation in ionography, and the development of electrostatic latent images are set out. Apart from gaseous absorbers, liquid or solid absorbers may be employed for latent image formation. A sensitivity increase in gaseous inography may be achieved by charge amplification, although with reduced resolution. As an alternative to aerosol and liquid development methods based on the distortion of deformable layers are described. A procedure yielding multiple copies with different image characteristics (edge contrast) via copies of the latent image is outlined. Finally possible closed ionography systems are mentioned which are necessary for real time imaging.

Saturation curve in gases of high atomic number at pressures up to 8 atm. Part II: Freon 13-B1, and mixtures of Freon with xenon and krypton.

The saturation curve has been studied in Freon 13-B1 (CF3Br) and in mixtures of Freon with xenon and krypton up to a pressure of 8 atm. The enhanced initial recombination due to the electron affinity of Freon has been evaluated and an empirical formula constructed that describes the fraction of current which escapes initial recombination over a wide range of voltages, pressures, and electrode spacings. After correction for this initial recombination, the general saturation curve for Freon and for mixtures of this gas with krypton and xenon has been derived and, again, convenient empirical formulae established which allow the current collection efficiency to be calculated for any given parameters within the range investigated. These formulae are of practical value in the design of image-forming ionization chambers.

Saturation curve in gases of high atomic number at pressures up to 8 atm. I. Krypton and xenon.

The saturation curve has been studied in xenon and in krypton up to a pressure of 8 atm. An empirical formula has been found that describes the fraction of current collected over a wide range of voltages, pressures, ionization intensities, and electrode spacings. This is of practical value in the design of ionography chambers. For krypton the collection fraction fKr = (1 + 0.25eta-1.74)-1, and for xenon fXe = (1 + 0.16eta-1.88)-1, where eta = Fp-0.7Vd-2q-1/2 with F = 3.61 X 10(-7) and 2.50 X 10(-7) for krypton and xenon, respectively. The ranges of the variables covered in the experiments were p = 1-8 atm, V = 5-25000 V, d = 0.3-1.3 cm, and q = 4 X 10(-9)-6 X 10(-8) A/cm3.