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1

The effect of external wedge on the photoneutron dose equivalent at a high energy medical linac

Hashemi , S. M., Raisali G., Taheri M., Majdabadi A., Ghafoori M.

Nukleonika

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2011

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Vol. 56, No. 1

49-51

EN

Medical linacs used in radiotherapy produce bremsstrahlung spectra. In the energy range from 8 to 25 MV medical linacs produce, besides the clinically useful electron and photon beams, secondary neutrons. The aim of this study was to investigate the effect of an external wedge filter on the photoneutron dose equivalent produced by a medical linac at patient plane. Polycarbonate (PC) films were used for the determination of photoneutron dose equivalent produced by a Varian 2100 C/D linac working at 18 MV photon mode. Neutron dose equivalent was measured at distances 0, 10, 20 and 50 cm from the center of the X-ray beam for open field and after inserting a wedge filter. It was noted that by inserting the external wedge in the path of the X-ray beam, the photoneutron dose equivalent was increased compared to open field. It can be concluded that an external wedge, made from heavy materials may act like the other components of linac head, producing undesired photoneutrons and thus increasing patient dose.

2

Cyclotron production of technetium radionuclides using a natural metallic molybdenum thick target and consequent preparation of [Tc]-BRIDA as a radio-labelled kit sample

Targholizadeh H., Raisali G., Jalilian A. R., Rostampour N., Ensaf M., Dehghan M. K.

Nukleonika

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2010

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Vol. 55, No. 1

113-118

EN

Numerous studies have focused on the use of accelerators for production of 99mTc, but all of these investigations have been performed at low-level currents. In this research, for the first time, we have constructed a high power level natural Mo target for production of 99mTc radioisotope using cyclotrons. A high purity natural molybdenum target (130 mg/cm2), suitable for proton beam power level of several kilowatts, has been constructed using a thermal spray coating method. The target was irradiated in a Cyclone30 accelerator using 160 mi A of 25 MeV proton beam energy for 1000 mi A-h. The activity of produced 99mTc was measured as 2.75 Ci. The technetium radionuclides produced were extracted using an MEK organic phase, followed by preparation of Tc-BRIDA as a radio-labelled kit sample. Animal biodistribution studies have been performed in rats. After administration of the radio-labelled Tc-BRIDA in rats, we observed most of the radioactivity accumulated in intestine as expected for IDA derivatives. The results of measurements show a successful production of Tc radionuclides (including 99mTc) in the bombarded target and subsequent labelling of the kit with Tc. It is anticipated that the developed coating method for the production of high power Mo targets using enriched 100Mo and a proton beam of about 1 mA is capable of producing about 100 Ci of 99mTc per irradiated target. The developed high power Mo target, if constructed using enriched 100Mo, could be a practical method for a large-scale production of 99mTc for local applications near cyclotron facilities.

3

Determination of dosimetry parameters of ADVANTAGETM 103Pd brachytherapy seed using MCNP4C computer code

Ataeinia V., Raisali G., Sadeghi M.

Nukleonika

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2009

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Vol. 54, No. 3

181-187

EN

The IsoAid LLC Inc. has been introduced ADVANTAGETM 103Pd brachytherapy seed in 2006. The aim of this work is to determine the dosimetric characteristics of this seed according to AAPM’s recommendation in TG43-U1 using MCNP4C computer code. The dose rate constant has been determined to be 0.694š0.001 cGy.h–1.U–1. The radial dose function has been calculated at distances from 0.25 to 7 cm. Two-dimensional anisotropy function have been calculated at distances from 0.25 to 7 cm and at angles from 0 to 90 degrees centigrade at 10 degrees centigrade increments. The one-dimensional anisotropy function and anisotropy constant have been also calculated. The anisotropy constant in water has been calculated as 0.872š0.001. The results of this investigation are compared with the results of Meigooni et al. obtained by PTRAN code in 2006 and Sowards results obtained by PTRAN code in 2007. The comparison of the dose rate constant and the one-dimensional anisotropy function obtained from the two codes shows good agreement; also the radial dose function at distances lower than 3 cm and the two-dimensional anisotropy function at angles greater than 20 degrees centigrade are in good agreement. But, for the calculated radial dose function at distances beyond 3 cm, we observed differences between our values and Meigooni et al. and Sowards results. Also, differences between the calculated two-dimensional anisotropy function using the two codes for angles smaller than 20 degrees centigrade are considerable. The differences between the results of MCNP4C and PTRAN codes could be related to the different cross-section data libraries used in these two codes.

4

Determination of the geometry function for a brachytherapy seed, comparing MCNP results with TG-43U1 analytical approximations

Raisali G., Ghonchehnazi M. G., Shokrani P., Sadeghi M.

Nukleonika

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2008

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Vol. 53, No. 2

45-49

EN

Geometry function is the only dosimetry parameter of a brachytherapy source seed, introduced in TG-43U1 protocol which is determined using calculational methods rather than physical measurement. In order to evaluate the accuracy of point and line source approximations, for calculation of the geometry function, the MCNP computer code has been used for a typical brachytherapy seed and the results have been compared. The MCNP has been used to simulate the geometry and activity distribution of a Pd-103 seed in order to calculate the geometry function for various angles and distances from the source. The comparison of results shows that at distances close to the source, the values predicted with different methods are not in agreement. The difference between the MCNP calculations and line approximation for small angles from ? = 0 to 15° is about 27% at 0.25 cm from the seed center. This difference is so much higher for point source approximation (up to a factor of 3) even up to distances of 0.5 cm from the source. As ? increases, the difference between MCNP and approximate methods is reduced. Therefore, for small distances from brachytherapy seeds, it is recommended to calculate the geometry function using more detailed methods instead of point and linear source approximations. This will provide more accurate results for other TG-43U1 dosimetry parameters such as radial dose function or anisotropy function which for some points are calculated via interpolation or extrapolation of the available discrete dosimetry data.

5

A study of the photoneutron dose equivalent resulting from a Saturne 20 medical linac using Monte Carlo method

Hashemi F., Hashemi-Malayeri B., Raisali G., Shokrani P., Sharafi A.

Nukleonika

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2007

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Vol. 52, nr 1

39-43

EN

High energy linacs have several advantages including lower skin dose and higher dose rate at deep sighted tumors. But, at higher energies photonuclear reactions produce neutron contamination. Photoneutron contamination has been investigated from the early days of modern linacs. However, more studies have become possible using Monte Carlo codes developed in recent years. The aim of this study was to investigate the photoneutron spectrum and dose equivalent produced by an 18 MV Saturne linac at different points of a treatment room and its maze. The MCNP4C code was used to simulate the transport of photoneutrons produced by a typical 18 MV Saturne linac. The treatment room of a radiotherapy facility in which a Saturne 20 linac is installed was modeled. Neutron dose equivalent was calculated and its variations at various distances from the center of the X-ray beam was studied. It was noted that by increasing the distance from the center of the beam, fast neutrons decrease rapidly, but thermal neutrons do not change significantly. In addition, the photoneutron dose equivalent was lower for smaller fields. The fast photoneutrons were not recorded in the maze. It can be concluded that the fast photoneutrons are highly attenuated by concrete barrier, while the slow photoneutrons are increased. In addition, increasing the X-ray field size increases the photoneutron dose equivalent around the treatment room and maze. It seems that the walls play an effective role in increasing the photoneutron dose equivalent.