International Journal of Radiation Research
Volume:2 Issue: 2, Apr 2004

In radiotherapy, wedge filters are used for optimizing the tumor dose distribution in patients. The attenuation in beam intensity due to the presence of wedge filter is compensated by means of a wedge factor measured at the central axis of the beam. The field size, depth and SSD dependence of wedge factor have been assessed for 9MV radiations of Neptun PC linear accelerator. Wedge factors (WF) at different SSD, field size (FS) and depth (d) in water were measured for 8 steel wedges with different sizes and angles of 15, 30, 45, and 60 degree. Experimental data were obtained using Neptun 10PC, Linac 9MV X-ray machine, a 3D water phantom, dosimeters and an electrometer. To study the effect of field size on WF, the wedge factor was measured for square field sizes from 5 ´ 5 to 20 ´ 20 cm, with 1 cm increment intervals for all wedges; and, at the depth of 10 cm, SSD of 100 cm with monitor unit (MU) of 80. Effects of depth on WF were studied by measurement in various depths from 3-19cm for all wedge angles at SSD of 100cm, field size of 10 ´ 10cm and 80 MU irradiation. Effects of SSD on WF were investigated by a variation of SSD from 90-110cm with 5cm increment intervals; while the dosimeter was set at depth of 10cm and field size of 10 ´ 10cm were irradiated for 80MU. Linear dependence of WF with field size and depth of measurements were confirmed with 95% certainty. Shapiro-Wilk test, showed that the residual data of the regression tests have normal distributions (P>0.05). There was also found no linear relationship between WF and SSD (P>0.05). WF has linear dependence with field size and depth of measurements, but the rate of variations are less than 2.2% per 10cm variation in field size and less than 1.3% per 10 cm variation in depth of measurements, therefore, correction of WF for field size and depth of treatments in clinical trials is negligible.

People who have been administrated radiopharmaceuticals could be a source of radiation to their relatives, medical nurses, and people who are in contact with them. The aim of this work was to estimate radiation dose received by nuclear medicine nurses. In this study, the dose rates at various distances of 5 – 100 cm from 70 patients, who were administered diagnostic amounts of 201Tl-Chloride and 99mTc-MIBI, were measured using an ionization chamber. For determination of external radiation dose to the nurses, three different time intervals were used for measurements. The maximum values of external dose rates of 201Tl and 99mTc-MIBI were 11.2 µSv/h ±2.3 and 43.1µSv/h ±11.9 respectively, at 5cm from the patients. Significant exposure from patients after injection of 99mTc-MIBI was limited to the day of administration. Departure dose rate of 201Tl fell gradually; so, it became significant by 3 days after administration. Maximum and average absorbed dose of nuclear medicine staff from 201Tl, was 4.6 and 2.7 µSv/h, and for 99mTc-MIBI was 18.1 and 9.8 µSv/h in each scan. Significant exposure from the patients is limited to the few hours after administratio n, therefore patients should be recommended to urinate frequently before leaving the nuclear medicine department.

Annual patient effective dose equivalent can be considered as a quantitative physical parameter describing the activities performed in each nuclear medicine department. Annual staff dose equivalent could be also considered as a parameter describing the amount of radiation risk for performing the activities. We calculated the staff to patient dose equivalent ratio to be used as a physical parameter for quantification of ALARA law in nuclear medicine departments. As a part of nationwide study, this paper reports the staff and patient absorbed dose equivalents from diagnostic nuclear medicine examinations performed in four nuclear medicine departments during 1999-2002. The type and frequency of examinations in each department were determined directly from hospital medical reports. Staff absorbed dose equivalents were calculated from regular personal dosimeter reports. The total number of examinations increased by 16.7% during these years. Annual patient collective dose equivalent (EDE) increased about 13.0% and the mean effective dose equivalent per exam was 3.61±0.07 mSv. Annual total staff absorbed dose equivalent (total of 24 radiation workers) in four departments increased from 40.45 mSv to 47.81 mSv during four years that indicates an increase of about 20.6%. The average of annual ratios of staff to patient effective dose equivalents in four departments were 1.83×10-3, 1.04×10-3, 3.28×10-3 and 3.24×10-3, respectively, within a range of 0.9×10-3 – 4.17×10-3. The mean value of ratios in four years was about 2.24×10-3 ± 1.09×10-3 that indicates the staff dose of about two 1000th of patient dose. The mean value of ratios in four years was about 1.89×10-3 ± 0.95×10-3 indicating the staff dose of about one 1000th of the patient dose. The staff to patient absorbed dose equivalent ratio could be used as a quantitative parameter for describing ALARA law in radiation protection and risk-benefit assessments.

Induction of radioadaptive responses in cells pretreated with a low dose radiation before exposure to a high dose is well documented by many investigators. The aim of this study is to determine the frequency of chromosomal aberration in peripheral blood lymphocytes of patients treated by radioiodine (131I) for hyperthyroidism, with or without previous thyroid scan with 99mTc. Venous blood samples were obtained from 35 patients one month after radioiodine therapy and cytogenetically evaluated using analysis of metaphase in two groups. The first group (n = 15, 13 females and 2 males, mean age= 44.7 ±11.5 years and mean weight 74.4±7.9 Kg) received 5 mCi 99mTc for thyroid scanning 38.6±19.9 days before radioiodine therapy with 10.4 ± 3.4 mCi 131I. The second group (n = 20, 14 females and 6 males, mean age = 41.0 ± 10.8 years and mean weight = 68.1±9.2 Kg) didn''t have history of thyroid scanning. We also studied a control group (n = 29, 11 Females and 8 males, mean age = 33.7±7.4 and mean weight = 70.0±8.8 Kg) who didn''t have any history of diagnostic or therapeutic and also occupational exposure. The mean frequency of total chromosomal aberrations in the first and second groups and controls were 1.46 ±1.55, 1.65 ± 1.62 and 0.93 ± 0.92 respectively. Results also showed that the mean frequency of total chromosome aberration in two groups were higher than controls and significantly higher in patients who had not received 99mTc compared those who had undertaken thyroid scan before radioiodine therapy (p=0.03). These findings may indicate the fact that the radiation dose received from 99mTc could induce resistance to subsequent higher radiation dose of 131I in peripheral blood lymphocytes and it might be due to cytogenetic radioadaptive response. Iran. J. Radiat. Res., 2004; 2 (2): 69-7 4

Hot lab is a specially designed room in a nuclear medicine hospital where the radiopharmaceuticals are delivered, stored and prepared for dispensing. 99Mo/99mTc-generator is the major source in the hot lab used for various medical imaging. It is important to maintain a standard for hot lab procedures to optimize the patient care and minimize radiation exposure to all nuclear medicine personnel, patients, public, as well as environment. The radiation doses in the hot lab were measured by GM and NaI Detectors for about 12 months. Package surface doses and generator surface doses were also measured. An increase in the counted rate above background was considered for the study. A constant distance was made in every step. At the receipt date, the 99Mo/99mTc-generator surface dose (450±150 μGy/hr) found to be nearly six times higher than the package surface dose (80±20 μGy/hr). The dose rate at the outer surface of the fume-hood glass found to be 80±15 μGy/hr in the 1st day of generator placement, whereas at the 2nd day it was 70±12 μGy/hr; showing a gradual decline in dose rate during 3rd (50±10 μGy/hr), 4th (40±9 μGy/hr), 5th day (30±6 μGy/hr) and 6th day (25±4 μGy/hr). In the 1st day of a generator storing in the hot lab, the dose rate found to be 3-4 times higher than the 6th days. The dose rate at various places indicated poor performance of the fume-hood glass. The study emphasizes on the need of growing awareness among all the radiation workers and encouraging the safe working practices in nuclear medicine.

Radiosurgery is a focal brain irradiation technique that delivers, usually in a single session, high dose of ionizing radiation. The presence of lateral electronic disequilibrium and steep dose gradients in small fields demands special attention to the selection of a suitable detector with respect to its size, composition and response. Small circular fields were produced by home-made collimators attached to a 9MV Neptun 10 PC linac ranging from 12.5mm to 25 mm at isocenter level in 2.5 mm increment. Stereotactic beam data including percent depth dose, off axis ratio and output factor were measured using p-type silicon chip detector in a water phantom. Beam data were plotted for all available collimator sizes. Percent depth dose values at depth of 100 cm show 10 percent increase with enlargement of the field sizes from 12.5 mm to 25mm. Small overestimation of output factor has been observed using diode detector. It is concluded for stereotactic radiosurgery with higher energy photon diode detector could be a good and reasonable choice to measure percent depth dose and off axis ratio. Regarding the output factor, it is better to compare the results with those obtained by other

Lymphocyte-dicentric assay is the most generally accepted method for biological dosimetry of overexposed individuals. In this study, the frequency of unstable chromosome aberration in blood lymphocytes was used to estimate radiation dose received by individuals. Evaluation of dose using a calibration curve produced elsewhere may have a significant uncertainty; therefore, experiments were performed to produce a dose-response curve using an established protocol of international atomic energy agency. Lymphocytes in whole peripheral blood obtained from healthy individuals, were exposed to various doses of gamma radiation (0.25 – 4 Gy). Then after 1 hour of incubation in 37 oC, were cultured in complete RPMI-1640 medium. 500 mitoses were analysed for the presence or absence of unstable chromosomal aberrations for each radiation dose after the standard metaphase preparation and staining slides. Results and Intercellular distribution of dicentric chromosomes at each radiation dose has been used to contrast a dose-response curve. It seems that dose-effect relationship follows with the linear-quadratic model. There is a good agreement between our dose-response curves with similar published studies by other laboratories.

Recent pre-clinical and clinical studies indicate that irradiation in the dose range of 15 to 30 Gy can reduce rate of restenosis in patients who have undergone an angioplasty. The use of filled balloon with radioactive solution was proposed as one of the possible intravascular irradiation techniques. The Monte Carlo N-particle Transport Code (MCNP4b) was used to calculate the dose rate distribution in the tissue equivalent material around the 188Re and 186Re liquid sources. Schematic of Medical Internal Radiation Dose (MIRD) for homogeneous distribution of radio-nuclide in a lesion was used for mean organ absorbed dose calculation due to the internal distribution. Results indicate that 188Re liquid with 100 mCi/ml and 186Re liquid with 250 mCi/ml can deliver desired dose in the vessel wall to reduce restenosis. The dose ratio in depth of 0.5 mm to surface of vessel wall for 188Re and 186Re were 40% and 18%, respectively. Therefore in case of 186Re, there is a little non-uniformity with respect to the 188Re case. The delivery of form 186Re dose to normal tissue around target tissue is less than 188Re. Use of the Monte Carlo simulation with 188Re-DTPA and 186Re-DTPA for intra-vascular brachytherapy is a feasible method of delivering a desired dose to the vessel walls. Although188Re-DTPA delivers the desired dose to the target tissue with lower radioactive concentration (mCi/ml), but with the use of 186Re-DTPA, the delivery dose to normal tissue around the target tissue is less.