A novel technique to optimise the length of a linear accelerator treatment room maze without compromising radiation protection.

Simulations with the FLUktuierende KAskade (FLUKA) Monte Carlo code were used to establish the possibility of introducing lead to cover the existing concrete walls of a linear accelerator treatment room maze, in order to reduce the dose of the scattered photons at the maze entrance. In the present work, a pilot study performed at Singleton Hospital in Swansea was used to pioneer the use of lead sheets of various thicknesses to absorb scattered low energy photons in the maze. The dose reduction was considered to be due to the strong effect of the photoelectric interaction in lead resulting in attenuation of the back-scattered photons. Calculations using FLUKA with mono-energetic photons were used to represent the main components of the x-ray spectrum up to 10 MV. Mono-energetic photons were used to enable the study of the behaviour of each energy component from the associated interaction processes. The results showed that adding lead of 1 to 4 mm thickness to the walls and floor of the maze reduced the dose at the maze entrance by up to 80%. Subsequent scatter dose measurements performed at the maze entrance of an existing treatment room with lead sheet of 1.3 mm thickness added to the maze walls and floor supported the results from the simulations. The dose reduction at the maze entrance with the lead in place was up to 50%. The variation between simulation and measurement was attributed to the fact that insufficient lead was available to completely cover the maze walls and floor. This novel proposal of partly, or entirely, covering the maze walls with lead a few millimetres in thickness has implications for the design of linear accelerator treatment rooms since it has the potential to provide savings, in terms of space and costs, when an existing maze requires upgrading in an environment where space is limited and the maze length cannot be extended sufficiently to reduce the dose.

Dose reduction of scattered photons from concrete walls lined with lead: Implications for improvement in design of megavoltage radiation therapy facility mazes.

PURPOSE: This study explores the possibility of using lead to cover part of the radiation therapy facility maze walls in order to absorb low energy photons and reduce the total dose at the maze entrance of radiation therapy rooms. METHODS: Experiments and Monte Carlo simulations were utilized to establish the possibility of using high-Z materials to cover the concrete walls of the maze in order to reduce the dose of the scatteredphotons at the maze entrance. The dose of the backscatteredphotons from a concrete wall was measured for various scattering angles. The dose was also calculated by the FLUKA and EGSnrc Monte Carlo codes. The FLUKA code was also used to simulate an existing radiotherapy room to study the effect of multiple scattering when adding lead to cover the concrete walls of the maze. Monoenergetic photons were used to represent the main components of the x ray spectrum up to 10 MV. RESULTS: It was observed that when the concrete wall was covered with just 2 mm of lead, the measured dose rate at all backscattering angles was reduced by 20% for photons of energy comparable to Co-60 emissions and 70% for Cs-137 emissions. The simulations with FLUKA and EGS showed that the reduction in the dose was potentially even higher when lead was added. One explanation for the reduction is the increased absorption of backscatteredphotons due to the photoelectric interaction in lead. The results also showed that adding 2 mm lead to the concrete walls and floor of the maze reduced the dose at the maze entrance by up to 90%. CONCLUSIONS: This novel proposal of covering part or the entire maze walls with a few millimeters of lead would have a direct implication for the design of radiation therapy facilities and would assist in upgrading the design of some mazes, especially those in facilities with limited space where the maze length cannot be extended to sufficiently reduce the dose.

Dose reduction of scattered photons from concrete walls lined with lead: Implications for improvement in design of megavoltage radiation therapy facility mazes.

PURPOSE: This study explores the possibility of using lead to cover part of the radiation therapy facility maze walls in order to absorb low energy photons and reduce the total dose at the maze entrance of radiation therapy rooms. METHODS: Experiments and Monte Carlo simulations were utilized to establish the possibility of using high-Z materials to cover the concrete walls of the maze in order to reduce the dose of the scattered photons at the maze entrance. The dose of the backscattered photons from a concrete wall was measured for various scattering angles. The dose was also calculated by the FLUKA and EGSnrc Monte Carlo codes. The FLUKA code was also used to simulate an existing radiotherapy room to study the effect of multiple scattering when adding lead to cover the concrete walls of the maze. Monoenergetic photons were used to represent the main components of the x ray spectrum up to 10 MV. RESULTS: It was observed that when the concrete wall was covered with just 2 mm of lead, the measured dose rate at all backscattering angles was reduced by 20% for photons of energy comparable to Co-60 emissions and 70% for Cs-137 emissions. The simulations with FLUKA and EGS showed that the reduction in the dose was potentially even higher when lead was added. One explanation for the reduction is the increased absorption of backscattered photons due to the photoelectric interaction in lead. The results also showed that adding 2 mm lead to the concrete walls and floor of the maze reduced the dose at the maze entrance by up to 90%. CONCLUSIONS: This novel proposal of covering part or the entire maze walls with a few millimeters of lead would have a direct implication for the design of radiation therapy facilities and would assist in upgrading the design of some mazes, especially those in facilities with limited space where the maze length cannot be extended to sufficiently reduce the dose.

Estimation of the dose at the maze entrance for x-rays from radiotherapy linear accelerators.

Recent measurements have shown that the NCRP formula to estimate the x radiation dose at the maze entrance of high energy radiotherapy rooms underestimates the dose by an order of magnitude. In the present work the Monte Carlo Code MCNP was used to model a radiotherapy room and investigate the NCRP formula. The dose of the scattered photons was calculated for 6-MV and 10-MV x-rays for the following situations: primary beam in vacuum, primary beam with air in the room, collimated primary beam (by the jaws) with air in the room and primary beam collimated with air in the room, and phantom at 100-cm SSD. It was found that for 6-MV x-rays the dose, when these materials were present in the beam path, was 1.2, 1.6, 5.3, and 13.1 x 10(-22) Gy photon(-1), respectively. Therefore the presence of all these materials together increased the dose by a factor of 11. The dose due to leakage was calculated separately to be 9.1 x 10(-22) Gy photon(-1). This adds another factor of 8. The 10-MV results were similar to those at 6 MV. There was good agreement between MCNP calculations and the published measurements. The spectrum and average energy of scattered photons at different locations in the radiotherapy room and the maze were also calculated by MCNP.

Comparison of EGS4 and MCNP Monte Carlo codes when calculating radiotherapy depth doses.

The Monte Carlo codes EGS4 and MCNP have been compared when calculating radiotherapy depth doses in water. The aims of the work were to study (i) the differences between calculated depth doses in water for a range of monoenergetic photon energies and (ii) the relative efficiency of the two codes for different electron transport energy cut-offs. The depth doses from the two codes agree with each other within the statistical uncertainties of the calculations (1-2%). The relative depth doses also agree with data tabulated in the British Journal of Radiology Supplement 25. A discrepancy in the dose build-up region may by attributed to the different electron transport algorithims used by EGS4 and MCNP. This discrepancy is considerably reduced when the improved electron transport routines are used in the latest (4B) version of MCNP. Timing calculations show that EGS4 is at least 50% faster than MCNP for the geometries used in the simulations.

A comparison of speeds of personal computers using an x-ray scattering Monte Carlo benchmark.

In recent years personal computers, PCs, have become more popular and competitive with other computers, in terms of price and memory, to run Monte Carlo (MC) programmes. A few years ago some work was performed to test computer speed, mainly other than PCs, using the MC code EGS4. In the present work a Monte Carlo neutron and photon code, MCNP version 4.2 and 4A, was used to test the speed of various PCs. A benchmark was written, with previously checked results, to test the PCs' speed. PCs used in the test were based on the Intel 486 (33, 66 and 100 MHz) and pentium 90. The benchmark was also used on an upgraded SunSparcstation 4/360. The pentium speed was found ot be about 3.5, 2 and 1.5 times faster than that of the 33, 66 and 100 MHz, respectively. The 33 MHz speed was comparable to that of the SunSparcstation. It is concluded that the application of MC code, the cost of the PC and the required speed of results may influence the choice of the PC.

Transformation of lung cells from inhalation of radon daughters in dwellings: a preliminary study.

There is now world-wide concern for the quantification of lung cancer risk due to indoor radon, but the recent estimates are based on epidemiological studies of miners alone. The present attempt is a preliminary study of the alteration of lung stem cells irradiated by alpha particles emitted by radon daughters. Local energy deposited has been calculated for alpha particles, emitted from radon daughters lining the mucous layer in the respiratory tracts. This calculation has then been followed by dose evaluation and estimation of transformation of lung cells as a function of age. Mean life span of the stem cells was varied between 5 and 45 years to simulate living conditions in different environments. The cumulative fraction of transformed cells after 40 and 70 years has been calculated for radon concentration in the range 23-230 Bq/m3. Increase of the fraction of transformed cells with radon concentration was exponential. It has been assumed that causes other than radiation increase the rate of cell death of mature and stem lung cells, and hence the turnover of stem cells to replace them. It is concluded that the rate of transformation of cells is small for low radon concentration even late in age for non-polluted environments. For radon concentrations of 50 and 100 Bq/m3 the fractions of transformed cells are 0.2 and 6 per cent, respectively for an exposure time of 70 years.

Local energy deposited for alpha particles emitted from inhaled radon daughters.

An analytical method has been developed to calculate the local energy deposited by alpha particles emitted from radon daughters deposited on the mucus surface in the lung airways. For the particular cases of 218Po (Ra A) and 214Bi (Ra C'), microdose spectra have been evaluated in test spheres of 1 micron diameter which were taken to lie within airways of diameters 18,000, 3,500 and 600 microns. In each case, the contributions of the near and far wall were computed separately. The average microdosimetric parameters yF and yD have also been calculated. For the two smaller airways, yF and yD values were found to be about 110 and 135 keV microns-1 for 218Po and about 87 and 107 keV microns-1 for 214Bi respectively. The corresponding values were about 10% higher for the largest airway.

Main factors affecting the calculation of radiation dose to the lung from inhalation of radon daughters.

The radiation dose to the lung due to inhalation of radon daughter products has been computed with improved data on lung models, aerosol parameters, deposition and clearance mechanisms. The effects of the following factors on the dose calculated for dwellings have been considered in detail: (a) the diffusion coefficient of the unattached daughter products; (b) the activity median diameter (AMD) of the aerosol size distribution; (c) room ventilation rate; (d) air flow rate in the respiratory tracts; (e) lung morphometry; and (f) effect of phase of the tissue-equivalent medium on the stopping power and range for alpha particles. It has been found that realistic changes in values of some factors change the dose rate by a factor of 2. By adopting likely values, it is calculated that annual dose rates in the fifth bronchial generation are typically 4.3 and 1.8 mGy year-1, at the epithelial surface and 20 microns depth, respectively, for a person exposed to a radon concentration of 23 Bq m-3, the average value for U.K. dwellings.

Influence of the physical state and straggling on the computation of the radiation dose due to radon daughters deposited in the lung.

The effect of the physical state (phase) of the absorbing medium and the energy straggling of the alpha particles on the calculation of the radiation dose due to the daughter products of radon deposited in the lung have been studied in detail. The stopping power data for alpha particles in water and water vapour have been used. It has been found that the effect of straggling on the stopping power calculations is small, and therefore its contribution to dose calculations is negligible. The phase effect has been found to be dependent on the energy of the alpha particles and the depth in the medium. If the stopping power of water vapour is used instead of that for liquid water, the dose may be overestimated by 5-20 and 1-11% for 6 and 7.7 MeV alpha particles, respectively, at the beginning of alpha range, and underestimated by 15 and 40% respectively for the above energies at the end of the range.