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Shielding design is necessary for brachytherapy treatment room in order to protect the general public and employees. The main objective of this study was to investigate whether the protective unit of our Brachytherapy Centre provided adequate protection to the health and safety assessment of radiobiological impact. In this study, we estimated the effect of radiobiological protection from a single Ir-192 brachytherapy source in Brachytherapy center by using MCNP5 Monte Carlo measurements. The room was based on the design specifications for the HDR 192 Ir treatment was modeled. The estimated dose rate range for HDR 192 Ir and public buildings is (0.45 - 0.64): (micro sivert) μSv/hour. Dose rates measured data for the current setup Brachytherapy HDR unit was approved and agreed quiet well with recommendation of International Atomic Energy Agency. The measured dose rate for public areas and controlled areas, compared with the reference value of 7.5 μSv/h and 0.5 μSv/h and concluded that we have enough shielding to the source but an over estimate with experimental measurements had been seen.

Monte Carlo N-Particle Transport Code (MCNP) is a software package for simulating nuclear processes. It is developed by Los Alamos National Laboratory since at least 1957 with several further major improvements [

The study was carried out at the Mahdieh hamadan brachytherapy center and using recommendation of IAEA- 47 (International Atomic Energy Agency protocol number 47) report for HDR brachytherapy units shielding [

The exposure rate from a gamma point source can be approximated from the following expression

where X is the dose rate; A is activity in mCi; Γ is the gamma factor; and r distance in cm

Monoenergetic x or gamma rays collimated into a narrow beam are attenuated exponentially through a shield according to the following equation:

where I is the intensity outside of a shield of thickness x I_{0} is the unshielded intensity m is the linear attenuation coefficient of the shielding material x is the thickness of shielding material. The linear attenuation coefficient is the sum of the probabilities of interaction per unit path length by each of the three scattering and absorption processes―photoelectric effect, and pair production. Note that m has dimensions of inverse length (1/cm). The reciprocal of m is defined as the mean free path, which is the average distance the photon travels in an absorber before an interaction takes place.

Therefore, to confirm the calculations and validation of the calculation we used both experimental and Monte Carlo methods.

The Mahdieh Radiotherapy Centre has a brachytherapy treatment rooms, a diagnostic Ct scan X-ray equipment and administrative setup. The brachytherapy treatment room shown in

The brachytherapy treatment room of the Centre was modeled using the visual editor of the MCNPX code to the design specification of the facility. The concrete with a density of 2.35 g/cm^{3} was used with material composition of ^{1}H 0.005558, ^{8}O 0.498076, ^{11}Na 0.017101, ^{12}Mg 0.002565, ^{13}Al 0.045746 ^{14}Si 0.315092, ^{16}S 0.001283, ^{19}K 0.019239, ^{20}Ca 0.082941, ^{26}Fe_{54} 0.000707, ^{26}Fe_{56} 0.01139 ^{26}Fe_{57} 0.000265, ^{26}Fe_{58} 3.6 × 10^{−}^{5}. The entrance door of the facility is modeled as a mdf door with a density of 0.830 g/cm^{3} [

11.35 g/cm^{3} with material of ^{82}Pb_{206} 0.242902, ^{82}Pb_{207} 0.223827, ^{82}Pb_{208} 0.53327 use in mdf door [

It has been in existence till date using the decay equation:

where A is the present activity, A_{0} is the initial activity at a known time, γ is the decay constant and t is the time to the date of exposure. The Ir-192 HDR with activities (11.5, 6.1, 5.7, 4.4 and 3.2) Ci, was used for the modeling respectively.

This type of tally makes use of what some might call a variance reduction technique, namely, use of the “next event estimator.” For each source particle and each collision event, a deterministic estimate is made of the fluence contribution at the detector point (or ring in an axisymmetric problem). To simplify description of this type

of tally, assume that calculations are being performed in a uniform medium. Suppose a particle of energy E and weight W from an isotropic source is released at distance r from the detector point. Ray theory methodology, as used in the point-kernel method, dictates that the contribution δΦ to the fluence at the detector point is given by:

in which µ(E) is the linear interaction coefficient for the particle of energy E. Note that 1/4π per steradian is the angular distribution of a point isotropic source. Now suppose that a collision takes place at distance r from the detector point and that, to reach the detector point, a scattering angle of θ_{s} would be required. Here, E is the energy of the particle after the collision and W is its weight. If µ(E, θ_{s}) is the linear interaction coefficient per steradian for scattering at angle θ_{s}, then µ(E, θ_{s})/µ(E) is the probability per steradian for scattering at angle θ_{s}. Geometric attenuation remains as 1/r^{2}, and the contribution δΦ to the fluence at the detector point is given by

Locations where dose rate had been estimated in thereby using MCNP5 were first identified and then measured the corresponding distances from the source. Dose rate monitoring was carried out for the selected locations A, B, C, D, E, and F in meter. Measuring doses must be in the range of 0.5 μSv/h - 7.5 μSv/h. for experimental measurement we used the STEP RDG Detector with Measuring range of 0 - 2000 μSv/h and 1.06 correction factor that shown in

Result in Tables 1-5 and Figures 5-9 shows experimental measurement for a Ir-192 HDR source with apparent activity. All of measurement dose rate are under the reference dose rate recommendation of International Atomic Energy Agency.

Location | |||||
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Distance from source (m) | Measured dose rate (μSv/h) | Reference dose rate (μSv/h) | Activity (Ci) | Simulated dose Rate (μSv/h) | |

Point A | 1.5 | 0.71 | 7.5 | 11.5 | 0.94 |

Point B | 2 | 0.68 | 7.5 | 11.5 | 0.33 |

Point C | 3 | 0.66 | 7.5 | 11.5 | 0.166 |

Point D | 3.5 | 0.5 | 0.5 | 11.5 | 0.32 |

Point E | 4 | 0.45 | 0.5 | 11.5 | 0.16 |

Point F | 5 | 0.45 | 7.5 | 11.5 | 0.89 |

STD | - | 0.121119775 | - | - | 0.354367982 |

avg | - | 0.575 | - | - | 0.467666667 |

St.Dev.% | - | 0.210643088 | - | - | 0.757736241 |

Location | |||||
---|---|---|---|---|---|

Distance from source (m) | Measured dose rate (μSv/h) | Reference dose rate (μSv/h) | Activity (Ci) | Simulated dose Rate (μSv/h) | |

Point A | 1.5 | 0.68 | 7.5 | 6.1 | 0.85 |

Point B | 2 | 0.66 | 7.5 | 6.1 | 0.33 |

Point C | 3 | 0.66 | 7.5 | 6.1 | 0.11 |

Point D | 3.5 | 0.48 | 0.5 | 6.1 | 0.28 |

Point E | 4 | 0.45 | 0.5 | 6.1 | 0.15 |

Point F | 5 | 0.45 | 7.5 | 6.1 | 0.668 |

STD | - | 0.113959057 | - | - | 0.296715352 |

avg | - | 0.563333333 | - | - | 0.398 |

St.Dev.% | - | 0.202294184 | - | - | 0.745515959 |

Location | |||||
---|---|---|---|---|---|

Distance from source (m) | Measured dose rate (μSv/h) | Reference dose rate (μSv/h) | Activity (Ci) | Simulated dose Rate (μSv/h) | |

Point A | 1.5 | 0.54 | 7.5 | 5.7 | 0.82 |

Point B | 2 | 0.54 | 7.5 | 5.7 | 0.32 |

Point C | 3 | 0.51 | 7.5 | 5.7 | 0.105 |

Point D | 3.5 | 0.45 | 0.5 | 5.7 | 0.28 |

Point E | 4 | 0.45 | 0.5 | 5.7 | 0.15 |

Point F | 5 | 0.45 | 7.5 | 5.7 | 0.66 |

STD | - | 0.045166359 | - | - | 0.287618092 |

avg | - | 0.49 | - | - | 0.389166667 |

St.Dev.% | - | 0.092176243 | - | - | 0.739061478 |

Location | |||||
---|---|---|---|---|---|

Distance from source (m) | Measured dose rate (μSv/h) | Reference dose rate (μSv/h) | Activity (Ci) | Simulated dose rate (μSv/h) | |

Point A | 1.5 | 0.61 | 7.5 | 4.4 | 0.802 |

Point B | 2 | 0.64 | 7.5 | 4.4 | 0.32 |

Point C | 3 | 0.43 | 7.5 | 4.4 | 0.1 |

Point D | 3.5 | 0.44 | 0.5 | 4.4 | 0.27 |

Point E | 4 | 0.42 | 0.5 | 4.4 | 0.15 |

Point F | 5 | 0.49 | 7.5 | 4.4 | 0.61 |

STD | - | 0.096488341 | - | - | 0.274901194 |

avg | - | 0.505 | - | - | 0.375333333 |

St.Dev.% | - | 0.191066022 | - | - | 0.732418813 |

Location | |||||
---|---|---|---|---|---|

Distance from source (m) | Measured dose rate (μSv/h) | Reference dose rate (μSv/h) | Activity (Ci) | Simulated dose rate (μSv/h) | |

Point A | 1.5 | 0.68 | 7.5 | 3.2 | 0.79 |

Point B | 2 | 0.66 | 7.5 | 3.2 | 0.3 |

Point C | 3 | 0.66 | 7.5 | 3.2 | 0.097 |

Point D | 3.5 | 0.51 | 0.5 | 3.2 | 0.267 |

Point E | 4 | 0.45 | 0.5 | 3.2 | 0.12 |

Point F | 5 | 0.45 | 7.5 | 3.2 | 0.58 |

STD | - | 0.110166541 | - | - | 0.272877995 |

avg | - | 0.568333333 | - | - | 0.359 |

St.Dev.% | - | 0.19384142 | - | - | 0.760105835 |

The measured dose rates at the selected locations representing supervised areas are all below the recommended values for public areas of 0.5 μSv/h. The controlled areas are also below the value recommended for controlled areas of 7.5 μSv/h. This implies that the biological shielding design of the facility is adequate to attenuate the gamma radiations from the brachytherapy sources used for treatment and hence the general public and staff are adequately protected for the existing HDR Ir-192 brachytherapy system. But unfortunately we saw an over estimate in calculation of room shielding design.