Volume 13 (2007): Issue 3 (January 2007)

Preliminary results of research to devise a method allowing spatial alignment of BEAM maps obtained from EEG examinations with SPECT data are presented. The main concept of the method presented lies in simultaneous recording of multi-channel EEGs during SPECT examination, and also in visualizing location of EEG electrodes on SPECT images that provide spatial three dimensional coordinates assignment. The proposed methodology of simultaneous SPECT and EEG examinations could be a significant complement to results of epileptic focus localisation obtained with the ISAS method used for the last few years. The ISAS method allows localisation of focuses with 80% confidence, but it requires carrying out MRI examinations for alignment of compared anatomical structures on two SPECT images. Complementing these results with a BEAM map analysis would improve significantly the effectiveness of the examinations. This work presents results of experiments carried out on the Jaszczak phantom.

Boron Neutron Capture Therapy is a very promising form of cancer therapy, consisting in irradiating a stable isotope of boron (¹⁰B) concentrated in tumor cells with a low energy neutron beam. This technique makes it possible to destroy tumor cells, leaving healthy tissues practically unaffected. In order to carry out the therapy in the proper way, the proper range of the neutron beam energy has to be chosen.

In this paper we present the results of the calculations, using the MCNP code, aiming at studying the energetic dependence of the absorbed dose from the neutron capture reaction on boron (the therapeutic dose), and hydrogen and nitrogen (the injuring dose).

To estimate the skin dose to the patient from the treatment planning, the knowledge about exit dose is essential, which is calculated from the percentage depth dose. In this study 6 MV and 18 MV beams from linear accelerator and cobalt-60 beams were used. The ionometric measurements were carried out with parallel plate chamber of sensitive volume 0.16 cc. Parallel plate chamber was fitted in to 30 x 30 cm² polystyrene phantom at a fixed FSD with the measuring entrance window facing farther from the source. The field size for this measuring condition was maintained at 10 x 10 cm². The ionization measurements were also carried out by changing the thickness of the polystyrene phantom at the entrance side of the point of measurement. In order to find out the variation of relative exit dose (RED) with field size the measurements were carried out without and with the full back-scattering material (27.2 gm/cm²) placed beyond the entrance window of the chamber. The measurements were also done for the entrance polystyrene phantom thicknesses of 10, 20 and 30 cm for the field size ranging from 5 x 5 cm² to 30 x 30 cm². The dose at the exit surface with no backscatter material is about 4.4%, 3.7% and 5.8% less than the dose with the full backscatter material present beyond the point of measurement for 6 MV, 18 MV X-rays and cobalt-60 gamma rays. The reduction in exit dose does not depend much of the phantom thickness through which the beam traverses before exiting at the chamber side. Dose enhancements of about 1.03 times were observed for a field size of 5 x 5 cm² for 6 MV, 18 MV X-rays and cobalt-60 gamma rays. The dose enhancement factor (DEF) values were noticed to vary with field size beyond 15 x 15 cm² for all the energies studied. Also it can be observed that the dose enhancement factor (DEF) values do not depend on the thickness of the phantom material through which the beam has traversed. The DEF values were found to vary marginally for different phantom material thickness for the particular field size. The study indicates that a reduction of 4.4% and 3.7% in relative exit dose when there is no backscatter material present for 6 and 18 MV X-rays for most of the clinically used radiotherapy portals. The measured exit dose was found to be mostly independent of field size and the thickness of the phantom material through which the beam gets transmitted at the entrance side. An addition of backscatter material of thickness equal to two-thirds of the d_max depth of the radiation beam concerned results in full dose at the exit side.

Coronary artery disease is due to atheromatous narrowing and subsequent occlusion of the coronary vessel. Application of optimised feed forward multi-layer back propagation neural network (MLBP) for detection of narrowing in coronary artery vessels is presented in this paper. The research was performed using 580 data records from traditional ECG exercise test confirmed by coronary arteriography results. Each record of training database included description of the state of a patient providing input data for the neural network. Level and slope of ST segment of a 12 lead ECG signal recorded at rest and after effort (48 floating point values) was the main component of input data for neural network was. Coronary arteriography results (verified the existence or absence of more than 50% stenosis of the particular coronary vessels) were used as a correct neural network training output pattern. More than 96% of cases were correctly recognised by especially optimised and a thoroughly verified neural network. Leave one out method was used for neural network verification so 580 data records could be used for training as well as for verification of neural network.

A ionization chamber, filled with ¹⁰BF₃ and operating in a pulse mode, was investigated as a possible sensitive pocket detector for detection of neutrons at low flux densities. The chamber is 85 mm long and 19 mm in diameter. The measured sensitivity of the chamber placed near the human body (in the belt), without any additional moderator was of the order of 1 pulse/n cm^-2 in the radiation field of a ²⁵²Cf source.

The Polish Secondary Standard Dosimetry Laboratory is part of the IAEA/WHO network of such laboratories. The SSDLs are usually not equipped with accelerators generating high energy electron beams for calibration of dosimeters. The access to medical accelerators is seriously limited due to the heavy patient load. Therefore attempts are made to use Co-60 beams for calibration of plane parallel chambers and calculate the calibration coefficients for other radiation quality, the high energy electron beam.

The Markus ionization chambers, most frequently used in Poland, were analyzed in this study. The material was composed of 36 plane parallel chambers, from 20 radiotherapy centers in Poland, calibrated at the Polish SSDL during the period of 2003-2006. Before actual calibration, a number of chamber parameters were tested: long term stability, dark current, chamber sensitivity, non-linearity of dosimeter readings. Each chamber was calibrated in two different radiation beams: a) Varian 2300 accelerator 22 MeV electron beams, beam output 1.2 cGy/MU at 300 MU/min; b) Co-60 Theratron 780/403 unit with a Cobalt-60 source of 155700 GBq (4208 Ci) activity as of 6.01.2006. A reference dosimeter Keithley Instruments Inc. 6517-A with cylindrical ionization chambers Nuclear Enterprises Technology Limited type 2571 was used as the reference standard. The methods of IAEA Code of Practice for Dosimetry TRS 398 were adopted. The long term stability was analyzed on basis of calibration coefficients of 23 Markus chambers calibrated several times during the period 1994-2002.

Very small differences in calibration coefficients were detected between the two calibration methods used. They ranged between -0.3 to +0.5%, the mean value being 0.1%. A very good long term stability of calibration coefficients of Markus chambers, related to the mean value over the 7 year period, ranging between -0.5 to +0.3%, was recorded.

Very small differences in the results for the two calibration methods, confirmed by small standard deviations observed, indicate that these two calibration methods in the case of Markus-type plane parallel chambers may be used alternatively.