International Journal of Radiation Research
Volume:5 Issue: 1, Jan 2007

To analyze the dosimetric and radio biologic advantages between intensity modulated radiotherapy (IMRT) and 3 dimensional conformal radiotherapy (3DCRT) and selection of optimal photon energy for IMRT treatments. 24 patients with localized prostate carcinoma were planned for 3DCRT and IMRT techniques. Radiation dose of 54 Gy with 2 Gy/fraction, was planned to Planning target volume (PTV1) (prostate + seminal vesicle + 1 cm margin) and 72 Gy to PTV2 (prostate + 1 cm margin) respectively. 3DCRT planning was done using 15 MV photon beam while IMRT plans were created using 6 MV and 15MV photons. Treatment plans were analyzed using mean, median, dose maximum and cumulative dose volume histogram for PTV1, PTV2, bladder, and rectum. Tumor control probability (TCP) was calculated for prostate. Normal tissue complication probability (NTCP) was calculated for bladder, rectum, and head of femur. Mean dose to prostate was 72.79 ± 0.18 Gy for IMRT 15 MV, 72.16 ± 0.27 Gy for 3DCRT and 72.48 ± 0.19 Gy for IMRT 6 MV. TCP was greater for IMRT 15 MV followed by IMRT 6 MV. The mean value of NTCP was significantly lower (p = 0.0015) for IMRT 6 MV compared to 3DCRT for rectum while for bladder all were comparable. IMRT techniques shows superiority in sparing surrounding critical organs, thus reducing normal tissue complication rates while maintaining the same or higher tumor control probability. No significant difference was observed between IMRT 6 MV and IMRT 15 MV techniques.

A radioprotective effect of amifostine as well as its ability to modulate the level of spontaneous and gamma-irradiation-induced genetic changes on human peripheral blood lymphocytes has been investigated. Amifostine, known as a potent radical scavenger, has been introduced as the most effective radioprotector, yet it is not completely approved for the clinical use. However, further in vitro and clinical studies are needed to clarify its mechanisms of action. Whole blood samples from healthy donors were exposed to various doses of gamma-rays. Lymphocytes in cultures were treated with amifostine at different concentrations (2, 4 and 6 mM) in the presence or in the absence of 1 IU/ml alkaline phosphatase before or after gamma-irradiation. Standard procedure for the cytokinesis-block micronucleus (CBMN) assay was used to assess the effect of amifostine on radiation induced micronucleus in binucleate lymphocytes. Irradiated blood samples showed an increase in the total number of micronuclei (MN) significantly different from controls (p<0.05). However, pre-treatment of lymphocytes with amifostine in the presence of alkaline phosphatase, 15 minutes before irradiation, led to a significant decrease in the frequencies of MN and cells with more than one MN (p<0.05). Amifostine, in its own, produced little or no protection. However, the addition of amifostine with alkaline phosphatase to the cell cultures 15 minutes after irradiation produced substantial radioprotection significantly different from the frequencies of MN induced by radiation alone (p<0.05). Results clearly indicated that gamma-rays induced MN in lymphocytes in a dose dependent manner. The highest protective effect was achieved when amifostine was phosphorilated by alkaline phosphatase and present before irradiation in the cellular environment, indicating its radical scavenging mechanism of radioprotection. Since the administration of amifostime after irradiation also led to a considerable decrease in the frequency of radiation induced MN, which might be possible for other mechanisms such as induction of cell cycle delay and hence influencing DNA repair, are involved in radioprotection by amifostine.

The presence of radionuclides in the soil, measures need to be taken to minimize the radiation dose to humans. It is mostly gamma-emitting radionuclides that contribute to the radiation dose in long term behavior of these radionuclides in soil may determine the extent to which such countermeasures need to be applied. The activity concentration of these radionuclides in the soil samples that collected from the Longwood forest at western Ghats have been analyzed using Gamma spectrometry. The mean activities of 238U, 232Th and 40K were observed 26.261Bq/kg, 53.614 Bq/kg, 204.084 Bq/kg, dry weight, respectively. The average value of dose rate was calculated 55.48 nGy/h by applying the conversion factor where as the environmental dosimeter shows the absorbed dose rate at 1m high is 96.96 nGy/h. It was observed that the activity concentration of primordial radionuclides and the gamma dose rate measurements by ERD and from soil is relatively higher than world average.

Monte Carlo method (MC) has played an important role in design and optimization of medical linacs head and beam modeling. The purpose of this study was to compare photon beam features of two commercial linacs, Varian 21EX and Elekta SL-25 using MCNP4C MC code. The 6MV photon beams of Varian 21EX and Elekta Sl-25 linacs were simulated based on manufacturers provided information. Photon energy spectra and absolute absorbed dose values were calculated for field sizes of 10×10 and 20×20 cm2. Also, contamination electron spectra for field size of 20 ×20 cm2 were scored for both linacs. Our results showed that the relative absorbed dose values and contamination electron spectrum were similar and comparable, but photon fluence and absolute absorbed dose values were 17% and 13% higher for Varian linac respectively for the field size of 10×10 cm2. Despite the differences in head components of two commercial linacs, their relative depth dose values were very close to each other. The absolute dose per incident electron showed some discrepancy, as well. Thus, this study suggests the use of absolute absorbed dose values as an invaluable factor when different linacs head are compared using Monte Carlo Method.

Stereotactic radiosurgery is an important clinical tool for the treatment of small lesions in the brain, including benign conditions, malignant and localized metastatic tumors. A dosimetry study was performed for Elekta ‘Synergy S ’ as a dedicated Stereotactic radiosurgery unit, capable of generating circular radiation fields with diameters of 1-5 cm at isocentre using the BEAM/EGS4 Monte Carlo code. The linear accelerator Elekta “Synergy S” equipped with a set of 5 circular collimators from 10 mm to 50 mm in diameter at isocentre distance was used. The cones were inserted in a base plate mounted on the collimator linac head. A PinPoint chamber and Wellhofer water tank chamber were selected for clinical dosimetry of 6 MV photon beams. The results of simulations using the Monte Carlo system BEAM/EGS4 to model the beam geometry were compared with dose measurements. An excellent agreement was found between Monte Carlo calculated and measured percentage depth dose and lateral dose profiles which were performed in water phantom for circular cones with 1, 2, 3, 4 and 5 cm in diameter. The comparison between calculation and measurements showed up to 0.5 % or 1mm difference for all field sizes. The penumbra (80-20%) results at 5 cm depth in water phantom and SSD=95 ranged from 1.5 to 2.1 mm for circular collimators with diameter 1 to 5 cm. This study showed that BEAMnrc code has been accurate in modeling Synergy S linear accelerator equipped with circular collimators.

The rejected films in radiography may be responsible for an unnecessary increase in the radiation dose to the population. The effective dose to population is a very important factor in estimation of stochastic risk in radiology. The main goal of this study was to determine the effective dose to members of the public due to rejected films in diagnostic radiology departments in the Northern province Mazandaran- of Iran. A repeat analysis program was set in all radiology departments in Mazandaran province (population = 2976219 person) to determine the total number and type of rejected films. All repeat and discarded films were collected and separated into types according to the etiology leading to their being discarded. Considering technical data about various radiological procedures and using the standard dosimetry tables, the annual effective dose per caput as well as annual gonadal dose per caput due to image retake was estimated. The total number of rejected radiographs in one year period was 73857 (overall reject rate ~ 11.15%) which led to 34.91 µSv and 37.17 µGy as annual average effective dose to a member of the public and annual average gonadal dose per caput respectively. The main reason of retaking the images was improper exposure factors. The reject rate was in the middle range of similar values in other studies; whereas in the present study the main reason for rejection was improper exposure factors (67.11%), the main reason for radiography repeat film was different in various countries. Comparing to the estimated 2.4 mSv from natural background radiation, the average annual effective dose and annual average gonadal dose per caput due to repeat/retake films are negligible. However, reducing the reject films is economically rewarding. Further national studies are suggested.

Miniature neutron sources with high neutron flux have abundant applications in medicine, industry and researches. The most important general characteristic of miniature neutron sources is their diameter which is 3mm in average. In this research, we have surveyed and designed an Am-Be miniature neutron source fabrication. This investigation resulted in creation of an Am-Be neutron source, using beryllium metal powder with 98% carat and 100-200 |µm mesh and Americium source with activity of about 200 |µCi. Neutron source designing was performed under safety and protective factors. The system was designed in two different forms based on the fluent yield of neutron or cut off neutron yield. The mean neutron flux of miniature neutron source was measured as 1.14 (n/sec.cm2), and it was calculated as 2.56 (n/sec.cm2) by MCNP (4C) code. Due to purity and mesh of beryllium, which were not calculated by MCNP code, the calculated flux via Monte Carlo method was approximately 2 times larger than neutron flux from fabricated miniature neutron sources. In order to fabricate the miniature neutron sources Am- Be with high efficiency, the americium sources with high activity and the target material (Be) in different forms are required.

Prompt gamma neutron activation analysis (PGNAA) is known as a non-invasive technique capable of measuring elemental concentration in voluminous samples in a short period of time. Also it is a valuable diagnostic tool for total body elemental measurements. 252Cf and 241Am-Be sources which are usually used in this method, generate not only neutrons, but also emit high-energy and unwanted gamma-rays. Because the patient must be located against the neutron source, patient dose during an analysis is an important concern when using this technique. Gamma-rays were attenuated without losing the neutron flux or significant alteration in the neutron spectrum. A relatively safe body chemical composition analyzer was designed with an optimal spherical gamma-ray shield, enclosed to the neutron source. Effects of gamma-ray shielding and optimum radius of spherical Pb shield was investigated and compared with the unfiltered bare source, using MCNP4C code. Then, the gamma ray dose equivalent per source neutron rate (user defined parameter) in the soft tissue is calculated for different radiuses of spherical Pb shield, for both neutron sources. A decreasing flux of gamma-ray was observed when the radius of the spherical Pb shields increased. The value of this reduction was about 94% for 252Cf source when a lead spherical shield of radius 4 cm was used; while the reduction was about 50% for 241Am-Be source with the same spherical shield. For a spherical Pb shield of radius 4 cm, reduction of the gamma dose equivalent per source neutron rate was about 8.44×10-17 Sv when the neutron source was 252Cf and about 1.24×10-16 Sv when the neutron source was 241Am-Be. Results show using optimum gamma-ray shield geometry can reduce the patient absorbed dose per incident neutron in a body chemical composition analyzer.