Václav Spěváček

Quality control of the stereotactic radiosurgery procedure with the polymer-gel dosimetry

PURPOSE To assess the entire geometric and dosimetric (relative) uncertainties of the radiosurgery procedure with the Leksell gamma knife. MATERIALS AND METHODS The entire Leksell gamma knife stereotactic radiosurgery treatment procedure was simulated with the use of a special water filled head phantom and polymer-gel dosimeter evaluated by nuclear magnetic resonance (NMR). A test vessel filled with the polymer-gel dosimeter was fixed in the head phantom. The phantom underwent stereotactic NMR imaging, treatment planning and then irradiation according to the treatment plan prepared exactly the same way as in the ordinary treatment procedure for a patient. The treatment plan was represented by one isocenter positioned approximately centrally in the head phantom. This procedure was subsequently repeated for all four collimators (4, 8, 14, 18mm) used on the Leksell gamma knife. Evaluation of dosimeters was performed on a Siemens EXPERT 1T NMR scanner. Dose profiles in X, Y and Z axes through the ellipsoidal shaped dose distribution were obtained to compare experimental results from the irradiated phantom with the treatment planning system calculations. RESULTS Reasonable agreement was observed between the treatment planning system calculations of relative dose distribution and the measured data. The maximum observed deviation in the spatial position between the center of the measured and calculated dose profiles was 0.6mm. The maximum observed difference in full width of half maximum between calculated and measured profiles was 1.2mm. CONCLUSIONS The use of polymer-gel dosimetry for a verification of stereotactic procedures has some unique advantages that can be summarized as follows: the dosimeter itself is tissue equivalent, three-dimensional dose distributions can be measured and the dosimeter allows simulation of the patients procedures without any limitations.

Population-based biomonitoring in the Czech Republic: urinary arsenic

The method of Guo et aL (AnaL Chim. Acta, 1997, 349, 313-318) for the determination of the toxicologically relevant arsenic in urine was verified and then used for the determination of arsenic in urine of the Czech population for monitoring purposes. Statistical evaluation at the level alpha = 0.05 did not prove any significant differences between industrial and agricultural regions, between males and females and smokers and nonsmokers. Likewise no differences were found among children in all the regions monitored. In the adult population small differences were found between some regions but these differences were not dependent on industrial pollution. The values of toxicologically relevant arsenic are low for all regions. The summarised value of the median for all groups together is 3.5 microg (g creatinine)(-1).

New radiochromic gel for 3D dosimetry based on Turnbull blue: basic properties

The recently developed new radiochromic gel dosimeter based on Turnbull blue dye formed by irradiation (the TBG dosimeter) does not exhibit dose pattern degradation due to diffusion effects as observed in the Fricke-gel dosimeter with xylenol orange incorporated into the gel matrix (the FXG dosimeter). The TBG dosimeter can be easily prepared and its optical properties enable evaluation of the gels response using the cone-beam optical computed tomography technique. The preparation procedure is described in the paper along with the basic characteristics of the gel, including dose response, dose sensitivity, ageing under different storage conditions, diffusion rates of Turnbull blue and gel density. The measurement of diffusion is described in more detail. The same method was applied to the FXG dosimeter for direct comparison. It was found that the diffusion coefficient of the TBG dosimeter stored at 24 degrees C is less than 4 x 10(-3) mm(2) h(-1) (1sigma confidence level), compared to the value of 7.3 x 10(-1) mm(2) h(-1) (1sigma) of the FXG dosimeter measured at the same temperature. Although the TBG dosimeter is less sensitive than the FXG dosimeter, its diffusion coefficient is practically negligible and, therefore, it offers large potential as a three-dimensional dosimeter for applications encompassing sharp dose gradients such as high-dose-rate brachytherapy.

Electron spectra and structure of dithizone and its ions

Abstract Energies and intensities of electron transition of different forms of dithizone were calculated by the method of molecular orbitals (LCAO MO SCF LCI) and compared with the measured spectra of dithizone and its ions. It has been found that the spectrum of nondissociated dithizone corresponds to a compound which is close to the structure of the enol form. The spectrum of dithizone in moderate alkaline solutions corresponds to the enol form with the SH- group dissociated and in strongly alkaline solutions (approx. 2N NaOH) to the enol form with both hydrogen atoms dissociated. The spectrum of dithizone in 60% H2SO4 corresponds to the enol form with the imine group protonated.

Evaluation of Geometric and Dosimetric Inaccuracies of Stereotactic Irradiation in the Rat Brain

Objective: To evaluate geometric and dosimetric inaccuracies in the irradiation of the rat brain with the Leksell Gamma Knife. Materials and Methods: Altogether three types of dosimeters were employed for these measurements: (a) a thermoluminescent dosimeter, (b) a semiconductor detector and (c) a polymer gel dosimeter. The thermoluminescent dosimeter and the semiconductor detector were calibrated using an ion chamber and then implanted in the brain of a rat cadaver and used for absolute dose determination. A special glass phantom mimicking exactly the shape of the rat body filled with the polymer gel was used for measurements of the relative dose distribution and evaluation of geometric inaccuracies during the stereotactic irradiation in the rat brain. Results: Both thermoluminescent and semiconductor detectors, due to their size, measured mean doses. The observed results demonstrated that the Leksell GammaPlan can be employed for the calculation of absorbed doses in irradiation of experimental animals. In our case, it was necessary to apply a correction factor of 1.078 for the absolute absorbed dose to obtain reliable results. A comparison of calculated dose profiles using the treatment planning system in all three axes with those measured by the polymer gel dosimeter demonstrated a very good geometric agreement with the mean deviation in profile position of 0.5 mm. Conclusion: The results indicate that this technique can effectively check the geometric and dosimetric accuracy of stereotactic irradiation in the rat brain. The Leksell GammaPlan can be employed for the calculation of absorbed doses, but the correction factor of 1.078 had to be applied for the absolute dose calculations in our irradiation geometry.

Designing phantom for head-and-neck treatment verification: Feasibility tests with bone and bone equivalent material incorporated into polymer gel

This work presents different approaches to manufacture of polyacrylamide gel (PAG) with bone inhomogeneity in an attempt to construct an authentic phantom for verification of head-and-neck irradiation specially for quantifying the absorbed dose in the spine. A special phantom was designed which includes air tube simulating thorax and accommodates model of spinal canal. Several methods of bone implantation into PAG were tested. The results indicate that proper mechanical surface cleaning of the bones eliminates the detrimental effects of chemicals used for bone conservation, while sufficient nitrogen flushing during the manufacture procedure effectively lowers the amount of oxygen present in the bone pores.

The influence of antioxidant THPC on the properties of polymer gel dosimeter

In order to decrease the negative influence of oxygen to the response of PAG dosimeters THPC has been added to the gel in the role of scavenger. Apart from the decreased influence of oxygen, THPC also influences other properties of gel dosimeters. This study examines these influences and their quantification. Previous studies have shown that increasing the concentration of THPC causes a decreasing response of the dosimeter (as measured in the relaxation rate R2). Evaluation of the IR spectrum of gels irradiated by a variety of doses has shown that it is caused mostly by the changed structure of the arising polymer, not due to the decreased polymerization. THPC also changes the kinetics of the subsequent reactions in the gel after the end of irradiation. THPC has its influence also on the size of the dose response overshoot that happens in the areas of steep dose gradients. An easy model of action in the gel was suggested, which allows one to estimate the size and kinetics of the changed response of the dosimeter after the end of irradiation depending on the content of THPC, the size of the dose and the dose gradient.

Influence of preparation and calibration method of PAGAT dosimeter on TSE MR readout results

In this study PAGAT dosimeter evaluation by TSE sequence was tested. PAGAT dosimeter preparation procedure was modified to increase the dosimeter sensitivity. Because THPC reacts with gelatin, adding THPC to monomer solution prior to dissolved gelatine helps exploit THPC as an antioxidant. Turbo spin echo sequence enables to evaluate gel dosimeter with 3D equidistant resolution in a reasonable scanning time. Glass walls of the phantom cause problems both by computing inaccuracies and MR imaging artefacts. The inner dosimeter volume is not affected by these inaccuracies and should be used for radiotherapy plan verification.

New 3D radiochromic gel dosimeters with inhibited diffusion

Two new radiochromic gels for 3D dosimetry, both with inhibited diffusion, are described. The first dosimeter is based on the radiation-induced creation of a dye Turnbull blue. The dose sensitivity of the gel measured in terms of spectrophotometric absorbance is (5.0 ± 0.1)×10−3 Gy−1cm−1 (1σ) and the background value 24 h after gel preparation is 0.14 cm−1. The diffusion coefficient of the gel stored at 24°C is less than 4x10−3 mm2h−1 (1σ). The second dosimeter makes use of the radiation-induced formation of insoluble mercurous chloride which creates a fog inside an originally bright gel. The dose sensitivity of the dosimeter is (3.8 ± 0.2)x10−2 Gy−1cm−1 (1σ) and its background value 24 h after preparation is 0.18 cm−1. The diffusion coefficient is less than 2x10−4 mm2h−1 (1σ).

Influence of gelling agents on the dosimetric performance of the Turnbull Blue gel dosimeter

Gelling agents such as agarose, phytagel, and several types of gelatin were used for preparation of Turnbull Blue radiochromic gel dosimeter. Their influence on gel dose response and background value was assessed. It was found that all gelatins cause significant increase of background in a short period of time after gel preparation therefore gelatin is not a suitable gelling agent for this dosimeter. Phytagel and agarose gels exhibit low and stable background and higher dose sensitivity than gelatin gels; however, the disadvantage is increased scattered light intensity in the gel in comparison to gelatin gels. A simple measurement was done demonstrating that the scattered light intensity significantly increases in phytagel and agarose gel in comparison to gelatin gels.