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A Monte Carlo Study for Photoneutron Dose Estimations around the High-Energy Linacs.

Mohammadi N, Miri-Hakimabad SH, Rafat-Motavalli L - J Biomed Phys Eng (2014)

Bottom Line: The results of dose calculations at the isocenter and maze showed that photoneutron dose decreases as the function of distance from the isocenter and increases with increasing the distance from the entrance maze.It is concluded that the simplified model of linac head is a useful and reliable method in dosimetry calculations.Calculations illustrated that the photoneutron dose is not negligible and duo to its harmful biological effects on body, it should be considered in the treatment plans.

View Article: PubMed Central - PubMed

Affiliation: Physics Department, Faculty of Sciences, Ferdowsi University of Mashhad, Iran.

ABSTRACT

Background: High-energy linear accelerator (linac) is a valuable tool and the most commonly used device for external beam radiation treatments in cancer patients. In the linac head, high-energy photons with energies above the threshold of (γ,n) interaction produce photoneutrons. These photoneutrons deliver the extra dose to the patients undergoing radiation treatment and increase the risk of secondary cancer.

Objective: In this study, a simplified model of the linac head was simulated and photoneutron dose equivalent was calculated at the isocenter and maze in the sphere detector. In addition, the absorbed and equivalent dose of photoneutron were estimated in the some organs of the phantom.

Methods: The simulations were made using the Monte Carlo code. The ICRP reference adult male voxel phantom was used as the human body model for dosimetry calculations.

Results: The results of dose calculations at the isocenter and maze showed that photoneutron dose decreases as the function of distance from the isocenter and increases with increasing the distance from the entrance maze.

Conclusion: It is concluded that the simplified model of linac head is a useful and reliable method in dosimetry calculations. Calculations illustrated that the photoneutron dose is not negligible and duo to its harmful biological effects on body, it should be considered in the treatment plans.

No MeSH data available.


Related in: MedlinePlus

Photoneutron dose equivalent at different distances from the isocenter for 15 MV accelerator
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Figure 6: Photoneutron dose equivalent at different distances from the isocenter for 15 MV accelerator

Mentions: At irradiation fields of 20×20 cm2, 30×30 cm2, and 40×40 cm2 thermal neutron fluence increase 38%, 45%, and 50% compared to that of 10×10 cm2, respectively. While the rate of increase for fast neutrons at irradiation fields of 20×20 cm2, 30×30 cm2, and 40×40 cm2 were 66%, 72%, and 76%, more than that of the irradiation fields of 10×10 cm2,separately. These significant discrepancies between irradiation field of 10×10cm2 and the larger ones are due to the size of spherical detector (radius of 10 cm). The photoneutron dose equivalent in the air as a function of distance from the isocenter for different field sizes and energy of 15 MV, was shown in the figure 6. For all irradiation field sizes the amount of dose equivalent at the isocenter was larger than other positions. In addition, by dose equivalent at isocenter of larger field size were more than dose equivalent at irradiation field of 10×10 cm2, so that this quantity at field size of 20×20cm2, 30×30 cm2, and 40×40 cm2 was 47%, 48%, and 48.5%, higher than at 10×10 cm2, respectively. The difference between dose equivalent of field sizes of 10×10 cm2 and 20×20 cm2 were considerable but the effect of field size of 20×20 cm2 and 40×40 cm2 on dose was almost the same. By increasing the distance from the isocenter(at 20 cm away from isocenter), a sharp drop was seen in the dose equivalent rate and for greater distances this reduction occurred with smoother slop. In the field size of 10×10 cm2 the amount of dose reduction at 20 cm away from the isocenter was 50%, while dose equivalent at distances of 40 cm and 140 cm from isocenter were 9% and 56% less than that at 20 cm from the isocenter, respectively. This trend was similar for the other irradiation field sizes so that the amount of dose reduction relative to isocenter at distance of 20 cm were 73%, 63%, and 41% at field size of 20×20 cm2, 30×30 cm2, and 40×40 cm2, separately. To compare and validate the results with the other published data, a neutron strength of 1.2×1012 n/Gy for 18 MV linac was used as Zabihzadeh et.al [19] and then the results compared by those of Sohrabi [20]. Table 2 indicates the results of this study and the corresponding values measured by Sohrabi et.al for 18 MV. Comparisons confirm that there is a good agreement between the outcomes of these two studies.


A Monte Carlo Study for Photoneutron Dose Estimations around the High-Energy Linacs.

Mohammadi N, Miri-Hakimabad SH, Rafat-Motavalli L - J Biomed Phys Eng (2014)

Photoneutron dose equivalent at different distances from the isocenter for 15 MV accelerator
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4289520&req=5

Figure 6: Photoneutron dose equivalent at different distances from the isocenter for 15 MV accelerator
Mentions: At irradiation fields of 20×20 cm2, 30×30 cm2, and 40×40 cm2 thermal neutron fluence increase 38%, 45%, and 50% compared to that of 10×10 cm2, respectively. While the rate of increase for fast neutrons at irradiation fields of 20×20 cm2, 30×30 cm2, and 40×40 cm2 were 66%, 72%, and 76%, more than that of the irradiation fields of 10×10 cm2,separately. These significant discrepancies between irradiation field of 10×10cm2 and the larger ones are due to the size of spherical detector (radius of 10 cm). The photoneutron dose equivalent in the air as a function of distance from the isocenter for different field sizes and energy of 15 MV, was shown in the figure 6. For all irradiation field sizes the amount of dose equivalent at the isocenter was larger than other positions. In addition, by dose equivalent at isocenter of larger field size were more than dose equivalent at irradiation field of 10×10 cm2, so that this quantity at field size of 20×20cm2, 30×30 cm2, and 40×40 cm2 was 47%, 48%, and 48.5%, higher than at 10×10 cm2, respectively. The difference between dose equivalent of field sizes of 10×10 cm2 and 20×20 cm2 were considerable but the effect of field size of 20×20 cm2 and 40×40 cm2 on dose was almost the same. By increasing the distance from the isocenter(at 20 cm away from isocenter), a sharp drop was seen in the dose equivalent rate and for greater distances this reduction occurred with smoother slop. In the field size of 10×10 cm2 the amount of dose reduction at 20 cm away from the isocenter was 50%, while dose equivalent at distances of 40 cm and 140 cm from isocenter were 9% and 56% less than that at 20 cm from the isocenter, respectively. This trend was similar for the other irradiation field sizes so that the amount of dose reduction relative to isocenter at distance of 20 cm were 73%, 63%, and 41% at field size of 20×20 cm2, 30×30 cm2, and 40×40 cm2, separately. To compare and validate the results with the other published data, a neutron strength of 1.2×1012 n/Gy for 18 MV linac was used as Zabihzadeh et.al [19] and then the results compared by those of Sohrabi [20]. Table 2 indicates the results of this study and the corresponding values measured by Sohrabi et.al for 18 MV. Comparisons confirm that there is a good agreement between the outcomes of these two studies.

Bottom Line: The results of dose calculations at the isocenter and maze showed that photoneutron dose decreases as the function of distance from the isocenter and increases with increasing the distance from the entrance maze.It is concluded that the simplified model of linac head is a useful and reliable method in dosimetry calculations.Calculations illustrated that the photoneutron dose is not negligible and duo to its harmful biological effects on body, it should be considered in the treatment plans.

View Article: PubMed Central - PubMed

Affiliation: Physics Department, Faculty of Sciences, Ferdowsi University of Mashhad, Iran.

ABSTRACT

Background: High-energy linear accelerator (linac) is a valuable tool and the most commonly used device for external beam radiation treatments in cancer patients. In the linac head, high-energy photons with energies above the threshold of (γ,n) interaction produce photoneutrons. These photoneutrons deliver the extra dose to the patients undergoing radiation treatment and increase the risk of secondary cancer.

Objective: In this study, a simplified model of the linac head was simulated and photoneutron dose equivalent was calculated at the isocenter and maze in the sphere detector. In addition, the absorbed and equivalent dose of photoneutron were estimated in the some organs of the phantom.

Methods: The simulations were made using the Monte Carlo code. The ICRP reference adult male voxel phantom was used as the human body model for dosimetry calculations.

Results: The results of dose calculations at the isocenter and maze showed that photoneutron dose decreases as the function of distance from the isocenter and increases with increasing the distance from the entrance maze.

Conclusion: It is concluded that the simplified model of linac head is a useful and reliable method in dosimetry calculations. Calculations illustrated that the photoneutron dose is not negligible and duo to its harmful biological effects on body, it should be considered in the treatment plans.

No MeSH data available.


Related in: MedlinePlus