Determination of the dose rate constant through Monte Carlo simulations with voxel phantoms.

Purpose: The main goal of this study was to determine the dose rate constant of radionuclides used in Nuclear Medicine when they are biodistributed in humans. The secondary goal was to calculate the effective half-life for the same radionuclides in order to determine the realistic dose due to the incorporation of these nuclides for a variety of reasons.

Methods: Anthropomorphic voxel phantoms, with characteristics based on ICRP-110 were considered and the biodistributions of radionuclides were simulated using the Monte Carlo code MCNPX.

A dosimetric study of prostate brachytherapy using Monte Carlo simulations with a voxel phantom, measurements and a comparison with a treatment planning procedure.

In prostate brachytherapy treatments, there is an initial swelling of the prostate of the patient due to an oedema related to the insertion of the seeds. The variation of the prostate volume can lead to variations in the final prescribed dose in treatment planning procedures. As such, it is important to understand their influence for dose optimisation purposes.

Evaluation of the radiotherapy treatment planning in the presence of a magnetic valve tissue expander.

The combination of radiotherapy treatments and breast reconstruction, using temporary tissue expanders, generates several concerns due to the presence of a magnetic valve inside the radiation field. The objective of this work is to evaluate a radiotherapy treatment planning for a patient using a tissue expander. Isodose curve maps, obtained using radiochromic films, were compared to the ones calculated with two different dose calculation algorithms of the Eclipse radiotherapy Treatment Planning System (TPS), considering the presence or absence of the heterogeneity.

Production of neutrons in laminated barriers of radiotherapy rooms: comparison between the analytical methodology and Monte Carlo simulations.

The necessity to build or adapt radiotherapy rooms in reduced areas leads to the search for unconventional solutions for shielding projects. In most cases, adding metals to the primary barriers is the best alternative to shield rooms properly. However, when photons with energies equal or higher than 10 MV interact with high atomic number nuclei, neutrons are ejected and may result in a radioprotec- tion problem for both outside and inside the room.

Influence of the presence of tissue expanders on energy deposition for post-mastectomy radiotherapy.

An increasing number of studies have shown that post-mastectomy radiotherapy presents benefits associated with the patients survival and a significant fraction of the treated patients makes use of tissue expanders for breast reconstruction. Some models of tissue expanders have a magnetic disk on their surface that constitutes heterogeneity in the radiation field, which can affect the dose distribution during the radiotherapy treatment. In this study, the influence of a metallic heterogeneity positioned in a breast tissue expander was evaluated by means of Monte Carlo simulations using the MCNPX code and using Eclipse treatment planning system.

Monte Carlo study of a new I-125 brachytherapy prototype seed with a ceramic radionuclide carrier and radiographic marker.

In prostate cancer treatment, there is an increasing interest in the permanent radioactive seeds implant technique. Currently, in Brazil, the seeds are imported with high prices, which prohibit their use in public hospitals. A ceramic matrix that can be used as a radioisotope carrier and radiographic marker was developed at our institution.

Application of digital image processing for the generation of voxels phantoms for Monte Carlo simulation.

This paper presents the application of a computational methodology for optimizing the conversion of medical tomographic images in voxel anthropomorphic models for simulation of radiation transport using the MCNP code. A computational system was developed for digital image processing that compresses the information from the DICOM medical image before it is converted to the Scan2MCNP software input file for optimization of the image data. In order to validate the computational methodology, a radiosurgery treatment simulation was performed using the Alderson Rando phantom and the acquisition of DICOM images was performed.

Optimal shielding design for bunkers of compact cyclotrons used in the production of medical radionuclides.

Purpose: There are several options to consider in the design of a vault that will house a cyclotron for radioisotopes production with regards to the door entrance. Alternatives are a direct-shielded door, a simple maze, or a double-legged maze. In this work, the impact of the neutron and photon doses at the vault entrance was evaluated for these options.

Monte carlo simulation of bony heterogeneity effects on dose profile for small irradiation field in radiotherapy.

In the radiotherapy treatment planning of a lesion located in the head region with small field radiation beams, the heterogeneity corrections play an important role. In this work, we investigated the influence of a bony heterogeneity on dose profile inside a soft tissue phantom containing a bony material. PDD curves were obtained by simulation using the Monte Carlo code EGSnrc and employing Eclipse(R) treatment planning system algorithms (Batho, Modified Batho, Equivalent TAR and Anisotropic Analytic Algorithm) for a 15 MV photon beam and field sizes of 2x2 and 10x10 cm(2).

Percentage depth dose evaluation in heterogeneous media using thermoluminescent dosimetry.

The purpose of this study is to investigate the influence of lung heterogeneity inside a soft tissue phantom on percentage depth dose (PDD). PDD curves were obtained experimentally using LiF:Mg,Ti (TLD-100) thermoluminescent detectors and applying Eclipse treatment planning system algorithms Batho, modified Batho (M-Batho or BMod), equivalent TAR (E-TAR or EQTAR), and anisotropic analytical algorithm (AAA) for a 15 MV photon beam and field sizes of 1 x 1, 2 x 2, 5 x 5, and 10 x 10 cm 2 . Monte Carlo simulations were performed using the DOSRZnrc user code of EGSnrc.

Diagnostic assessment to estimate and minimize neutron dose rates received by occupationally exposed individuals at cyclotron facilities.

Since 2003, radiopharmaceuticals for medical diagnostic purposes have been produced at the Instituto de Engenharia Nuclear, in Brazil, using two cyclotron accelerators - CV-28 and RDS111. As a result of the ever increasing production, a diagnostic assessment to reduce neutron dose rates received by occupationally exposed individuals during irradiation processes has been developed. The purpose of this work is to present this assessment, which is currently being applied to both the Fluorine and Iodine targets of CV-28 and RDS111 cyclotron accelerators.

Photon doses at the entrance of 60Co and low-energy medical accelerator rooms under unusual irradiation conditions.

In this paper, the general-purpose Monte Carlo code MCNP5 was used to study the dose variance due to the position of medical linear accelerators, under unusual conditions, for shielding design of radiotherapy facilities. It was found that the computational methods generally used to estimate the scattered photon doses at the entrance of radiotherapy unit vaults provide conservative results when compared with the MCNP results, considering the standard condition. On the other hand, for the situations where the axis of gantry rotation is redirected at, for example, 45 degrees with respect to the walls of the room, the photon doses at the entrance can reach values up to seven times higher than those obtained under the standard condition, depending on the energy of the primary beam.

On the production of neutrons in laminated barriers for 10 MV medical accelerator rooms.

When space limitations are primary constraints, laminated barriers with metals can be an option to provide sufficient shielding for a radiotherapy treatment room. However, if a photon clinical beam with end point energy of 10 MeV or higher interacts with the metal inside the barriers neutrons are ejected and can result in an exposure problem inside and outside the vault. The empirical formulae existing in the literature to estimate neutron dose equivalents beyond laminated barriers do not take into account neutron production for spectra below 15 MV.

Neutron scattering in concrete and wood: Part II--Oblique incidence.

The knowledge of neutron reflection coefficients is of practical interest when projecting the shielding of radiotherapy rooms, since it is known that about 75% of the neutrons at the maze entrance of these rooms are scattered neutrons. In a previous paper, the energy spectra of photoneutrons were calculated, when reflected by ordinary, high-density concrete and wood barriers, using the MCNP5 code, considering normal incidence and neutron incident energies varying between 0.1 and 10 MeV.

Multileaf shielding design against neutrons produced by medical linear accelerators.

This work aims at presenting a study using Monte Carlo simulation of a Multileaf Shielding (MLS) System designed to be used for the protection of patients who undergo radiotherapy treatment, against undesired exposure to neutrons produced in the components of the medical linear accelerator heads. The choice of radiotherapy equipment as the subject of study fell on the Varian Clinac 2,100/2,300 with MLC-120 operating at 18 MeV. The general purpose Monte Carlo N-Particle radiation transport code, MCNP5, was used in the computer simulation in order to determine the ambient dose equivalent, H (10), on several points on the patient's plane, with the equipment operation with and without the MLS.

The use of high-density concretes in radiotherapy treatment room design.

With the modernization of radiotherapic centers, medical linear accelerators are largely replacing (60)Co teletherapy units. In many cases, the same vault housing the (60)Co teletherapy unit is reused for the linear accelerator and, when space is at a premium, high-density concrete (3.0-5.

Neutron dose calculation at the maze entrance of medical linear accelerator rooms.

Currently, teletherapy machines of cobalt and caesium are being replaced by linear accelerators. The maximum photon energy in these machines can vary from 4 to 25 MeV, and one of the great advantages of these equipments is that they do not have a radioactive source incorporated. High-energy (E > 10 MV) medical linear accelerators offer several physical advantages over lower energy ones: the skin dose is lower, the beam is more penetrating, and the scattered dose to tissues outside the target volume is smaller.

Monte Carlo simulation of scattered and thermal photoneutron fluences inside a radiotherapy room.

In this work, the behaviour of scattered and thermal photoneutron fluences in a radiotherapy treatment room was investigated by means of Monte Carlo simulation. The MCNP code was used to study the dependence of these neutron fluences on room design, room area and the effect of neutron moderator materials placed on the room walls. The results of the investigation showed a poor agreement between the simulations and empirical approximations, suggesting that the formulae found in the literature can underestimate the neutron flux inside a radiotherapy room.

Neutron scattering in concrete and wood.

In this work, the energy spectra of photoneutrons, scattered by ordinary, high-density concrete and wood barriers, have been evaluated using the MCNP4B code. These spectra were calculated for different scattering angles, and for incident neutron energies varying between 0.1 and 10 MeV.

A study of neutron spectra from medical linear accelerators.

Medical accelerators with photon energies over 10 MeV generate an undesired fast neutron contamination in the therapeutic beam. In this work, the Monte Carlo code MCNP was used to simulate the transport of these photoneutrons across the head of various medical accelerators of high energy. The average and most probable neutron energies were obtained from these spectra, before and after crossing the accelerator shielding.

Neutron dose rate evaluation for medical linear accelerators.

During X-ray therapeutic irradiation with energies above the threshold of (X,n) reactions in the structural materials of medical accelerators, a photoneutron fluence is generated. In Brazil, no measurements of neutron doses in radiotherapy rooms are being done yet, when licensing these equipment. Consequently, it is very important to obtain accurate analytical formulae and/or simulation of these dose rates, in order to estimate the increase in dose received by the patient and staff, as well as to correctly project the additional shielding for the treatment room.

Goiânia: 12 years after the 137Cs radiological accident.

In 1987, in the city of Goiânia, Brazil, occurred one of the worst radiological accidents ever reported. The remains of 137Cs contamination in a terrain where part of a radiotherapy unit had been manipulated in 1987 were measured in 1999-2000, and some of the results are presented here. Using the technique of gamma ray spectrometry in situ and in the laboratory, the ambient dose equivalent rate at 1 m above the ground and 137Cs concentration in soil were determined.

Measurements performed in Goiânia after a new intervention action in 2001.

Brazil's worst radiological accident took place in 1987, in the city of Goiânia. In 1999 and 2000, detailed measurements of 137Cs contamination were performed in junkyard II, one of the places involved in the accident. High values of 137Cs activity per unit mass were found in soil layers at depths between 10 and 40 cm from the surface, reaching values as high as 175 kBq x kg(-1).

Remains of 137Cs contamination in the city of Goiânia, Brazil.

The results of measurements, performed in 1999, of the remaining 137Cs contamination in some of the sites where fragments of a radioactive source of a teletherapy unit had been manipulated in 1987 are presented. This episode occurred in the city of Goiânia, during Brazil's worst radiological accident ever reported. Using the technique of gamma ray spectrometry, analyses of both surface and profile soil and vegetable samples were made.

Ambient dose equivalent rate in Goiânia 12 years after the 137Cs radiological accident.

This paper describes the situation of ambient dose equivalent rates in four of the main foci of 137Cs contamination in the city of Goiânia, Brazil, in 1999, 12 y after one of the worst radiological accidents in the world. During the decontamination, all the buildings of the three highly contaminated sites were demolished and the top soil removed. Afterwards, the soil of two of these lots was covered with concrete, and they remain vacant today.