Influence of radiotherapy room shielding on ambient dose equivalent due to photons H*(10)p and neutrons H*(10)n in the patient's plane.

This study discusses a computer simulation for the equivalent ambient dose due to photons, H*(10)p, and neutrons, H*(10)n, in the patient's plane undergoing radiation therapy. A standard radiotherapy room with an additional shielding made by one lead or steel tenth-value layer was considered. A Varian 2100/2300 C/D linear accelerator head operating at 18 MV was modeled. Jaw openings of 5 cm × 5 cm, 10 cm × 10 cm, 20 cm × 20 cm, and 30 cm × 30 cm, as well as the multileaf collimator under eight different angles of gantry inclination, were also modeled. The use of steel in the shield generates a slightly raised average value of H*(10)p (0.527%) compared to when using lead. This finding can be interpreted as that the use of lead or steel coating makes no difference to the additional shield calculations when only photons are considered. When considering the contribution to H*(10)n, there is a significant difference (11.7% increase) for using lead compared to steel shielding.

Modeling of a neutron irradiator using Monte Carlo.

The Nuclear Engineering Department of the Military Institute of Engineering (SE/7-IME) is designing and simulation a neutron irradiator with 1 Ci of 241Am-9Be source. The objective of this irradiator is to generate a flux of neutrons to be used in research and teaching maintaining, for purposes of radiological protection, the rate of ambient dose equivalent, H*(10), below 10 µSv/h at 30 cm from the surface. This paper presents the proposed irradiator, values of H*(10) at different distances from the irradiator and the neutron flux in different points of the beam irradiation, all calculated using the MCNPX code.

Shielding implications on secondary radiation doses in prostate cancer treatment.

Medical linear accelerators (linacs) require a physical structure designed to provide adequate structural support which ensures the safety of patients, operators and the general public. During a radiotherapy session, healthy tissues are exposed to radiation, even with these safety guarantees. This unwanted exposure may increase the likelihood of developing secondary cancer. This work uses the MCNP-5 code to computationally simulate a conformational 3D radiotherapy protocol for prostate cancer. Also, it investigates the potential effects of radiotherapy room shielding composition on equivalent and effective doses in the patient's body. A computational model of an actual room was developed considering a Varian Trilogy linac operating at 10 MeV. This model enabled dose calculations for an anthropomorphic phantom called REX to be performed. This phantom has sufficient details of all relevant organs and tissues needed to estimate the effective dose of the patient. The treatment protocol modeled in this study came from the database of patients treated by the Brazilian National Cancer Institute (Inca). For this protocol, the total dose to be applied to the patient is equally distributed over the four gantry inclination angles (0°, 90°, 180° and 270°). The simulated results suggested that the equivalent dose on different organs and tissues has been increased by concrete shielding. Regarding the effective dose due to the presence of additional shielding (steel or lead), the simulation suggests that such variations can be considered small. Overall the results allowed quantifying the specific contribution of concrete, lead, and steel as part of shielding on the equivalent and effective doses in the patient.

Brazilian regulatory authority contribution to the shielding dimensioning model of radiotherapy rooms proposed by the NCRP 151.

The National Council on Radiation Protection and Measurements (NCRP) Report No. 151 is an essential document for bunker design commonly applied for radiotherapy treatment rooms. This document is used as a reference by several countries, including Brazil. The objective of this study is to evaluate the shielding dimensioning methodology recommended by NCRP 151, and compare it with the one adopted by the Brazilian regulatory authority. Radiotherapy rooms and respective doors were designed to use linear accelerators operating at 6, 10, 15, and 18 MeV under two different ways: (a) applying exclusively the methodology recommended by the NCRP 151, and (b) taking into consideration the complementary recommendations from the Brazilian authorities. The results suggest that designers in Brazil can count on at least 4 and 11% safety margin for dimensioning primary barriers in controlled and free areas respectively. Also 8% for secondary barriers in controlled areas, 9.7% for secondary barriers adjacent to the primary belt of free areas, and 6.6% for the lead of the doors.