P. Kindl

Shuttle dose at the Vienna Leksell Gamma Knife

The aim of this study was to determine the shuttle dose for all collimator helmets (4, 8, 14 and 18 mm) of the Gamma Knife, model B, in Vienna, Austria. The additional dose accumulated during the transport of the patient in and out of the treatment position should be considered in the dose planning procedure of multicentre treatment regimens and in fractionated stereotactic Gamma Knife radiotherapy. The GafChromic film study was basically used to determine the shuttle dose of all four collimator helmets. In addition, measurements with an ionization chamber (18 and 14 mm collimator--and, for the 18 mm collimator helmet, TLD dosimetry--were performed in order to confirm the GafChromic film data. The shuttle dose ranged between 99.6 and 183.5 mGy, depending mainly on the size of the collimator and the irradiated isocentres at the half-life activity of Co-60 in a brand new Gamma unit. Our film-generated data were in good correlation with the dose levels obtained with the ionization chamber and the TLD dosimetry, showing a dose difference of less than 0.8%. Since it was possible to verify the shuttle dose even for the 4 and 8 mm collimator helmets, we consider it a non-negligible factor and would advocate the inclusion of the shuttle dose in radiosurgical dose planning.

Dosimetry studies with TLDs for stereotactic radiation techniques for intraocular tumours

Between March 1993 and January 1997, stereotactic radiation techniques were used to irradiate 66 intraocular tumour patients with the Gamma Knife (Leksell Gamma Knife, model B unit) at the University of Vienna, Austria. This study investigates the dosimetry for stereotactic irradiation of ocular structures. For the dosimetry program KULA 4.4, Gamma Knife stereotactic irradiation of the eye represents an extreme frontal skull position. In addition, irradiation of the eye may be performed in the usual supine position in exceptional cases only. With the patient in the prone position, the dose planning program has to calculate with a significantly large number of single-beam extrapolations. In our first experiment we measured the isocentre dose for eight different gamma-angle positions, both in prone and supine positions, using TLD measurements in an Alderson head phantom. We found a maximum deviation of +/- 1.6% using these individually calibrated TLDs. In the second experiment we examined the dose cross profiles for the two most frequently used treatment positions (supine position, gamma = 65 degrees, and prone position, gamma = 140 degrees). For this purpose we implanted a specially designed TLD array into the orbit of a human cadaver head. We found excellent agreement of the dose values measured for the isocentre as well as the posterior part of the eye with orbit with deviations of less than -2.7%. However, for the anterior part of the eye, deviations between computer-generated calculations and the TLD measurements were found to range up to -30%. These differences were noticed both for supine and prone positions. For the Gamma Knife stereotactic irradiation of ocular tumours or pathologies, precautions should be taken to avoid significant underdosage in the anterior part of the radiation field.

A novel 675.2 nm diode laser densitometer for use with GafChromic films

In this article we compare the accuracy of a diode laser densitometer emitting 675.2 nm to that of a commercial He-Ne laser densitometer emitting 632.8 nm for GafChromic MD-55 film readout. A Leksell gamma unit (AB Elekta Stockholm, Sweden) Model B with a 14 and 8 mm collimator at the same isocenter (combined 11 mm collimator) was used to irradiate GafChromic MD-55 films. Dose response curves, dose cross profile and FWHM were measured with a custom-designed diode laser scanning device, emitting light at 675.2 nm. The same data were recorded with a commercial He-Ne laser densitometer (PTW FIPS Plus, Freiburg, Germany), emitting light at 632.8 nm. Both measurements were compared to dose cross profiles of a radiosurgery dose planning program (GammaPlan 5.12, Elekta, Sweden). Compared to the commercial He-Ne laser densitometer, the custom-designed diode laser scanning device showed better agreement with the calculated dose cross profile. For two axes, the full width half maxima (FWHM) of the diode laser scanning device was within 0.1 mm deviation compared to the data calculated by the dose planning program. The FWHM of the commercial He-Ne laser densitometer was less accurate (1.6 and 2.1 mm deviation). Our data show that a diode laser scanning device using a light source emitting 675.2 nm increases the accuracy of a GafChromic MD-55 film readout. This greater accuracy may be related to the diode laser measuring the optical density close to maximum absorption of the GafChromic film MD-55 (671-675 nm).

Investigations on the reliability of a multi-component analysis of positron lifetime spectra, using a new method of producing computer-simulated test spectra

Abstract The reliability of a multi-component analysis of positron lifetime spectra is investigated by the evaluation of computer-simulated test spectra. Because of the fact that most evaluation programs use analytical models for the time-resolution function of the spectrometer, a new method is presented to simulate lifetime spectra without any ‘modelling’ of the prompt curve. The basic idea of this new way of simulation is that instead of an analytical function only a set of discrete values of a prompt spectrum (measured or likewise simulated) is used. In this way one gets more realistic test spectra which can be employed as a quality control for any given evaluation program. In this paper, the analysing program POSGAUSS is extensively tested by such lifetime spectra, and the arising systematical and statistical errors are discussed in detail. The main results of this investigation are: (i) already small problems in modelling the resolution function within the analysing program can produce large errors of the fit results, (ii) the standard deviations of the fitted lifetimes and intensities are strongly determined by the total area (and much less by the peak-to-background ratio) of the lifetime spectra, and (iii) an instability of the time-zero point as it can occur even in well temperature-stabilized spectrometers has no important effect on the fit results, as long as this instability is not excessively high.

Comparison of Readout Between a New Diode Laser Scanner and a Helium-Neon Laser Densitometer for GafChromic MD-55 Films

GafChromic MD-55 films were irradiated by the Vienna γ-knife unit with a 14 and 8 mm collimator helmet (spherical field of close to 11 mm diameter) at the same isocenter to compare the dose cross-profiles along the x and z directions with different scanning systems. Dose-response curves for GafChromic MD-55 film (Nuclear Associates Model No. 37-041) were measured using two different densitometer systems: a commercial He-Ne laser densitometer (PTW FIPS Plus, Freiburg, Germany) and a newly developed analog diode laser densitometer. For this purpose, we chose the diode laser as the light source, emitting at 675.2 nm with a single-mode surface, measured with a Lock-In Amplifier by a solar cell. The highest uniformity was seen in the change of absorption ranging from 4% to 6%, depending on the polarizing direction of the light with the new scanning system. The reproducibility of the reading method increases from 0.14% in the region of 2 Gy to 0.18% near 16 Gy. Measurements of the dose cross-profiles of the 11 mm collimator with the 675.2 nm laser scanner generally show good agreement with the values of the dose planning program. Reference studies with the He-Ne laser reveal a general tendency between 1.5 and 2.2 mm in diameter as compared to computer-simulated dose profiles and the 675.2 nm laser densitometer values.

Localization and System Integration Tests as Quality Assurance for the Leksell γ-Unit

Quality control in stereotactic radiotherapy of brain lesions with the Leksell γ-unit or the LINAC-facility is a must, as missing the target is the most serious error that can occur in that kind of treatment. We developed a quality assurance program to reduce this risk. To evaluate the accuracy of the procedure, which defines the target, including all possible errors of the therapy chain, irradiations of phantoms were performed, using the so-called “known” and “unknown” target method. Accuracy is defined by the deviation of the irradiated target from the calculated target with digital images used for therapy planning. GafChromic MD-55 films, which have been irradiated by means of a special author-developed phantom, were applied for measuring the precision of the radiation unit. The results obtained for isocentric accuracy of the Leksell γ-unit were in good agreement with the measurements of the manufacturing company. The deviation measured with the 4-mm collimator helmet was up to 0.3 mm, including the CT images and the stereotactic frame less than 0.85 mm, respectively. The phantom designed for this purpose is useful in quality assurance measurements of this stereotactic therapy chain.