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. 2020 Mar-Apr;25(2):187-192.
doi: 10.1016/j.rpor.2019.12.029. Epub 2020 Jan 10.

Monte Carlo study on the secondary cancer risk estimations for patients undergoing prostate radiotherapy: A humanoid phantom study

Amir Ghasemi-Jangjoo  1   2 , Hosein Ghiasi  1
Affiliations

Monte Carlo study on the secondary cancer risk estimations for patients undergoing prostate radiotherapy: A humanoid phantom study

Amir Ghasemi-Jangjoo et al. Rep Pract Oncol Radiother. 2020 Mar-Apr.

Abstract

Aim: The aim of this study was to estimate the secondary malignancy risk from the radiation in FFB prostate linac-based radiotherapy for different organs of the patient.

Background: Radiation therapy is one of the main procedures of cancer treatment. However, the application the radiation may impose dose to organs of the patient which can be the cause of some malignancies.

Materials and methods: Monte Carlo (MC) simulation was used to calculate radiation doses to patient organs in 18 MV linear accelerator (linac) based radiotherapy. A humanoid MC phantom was used to calculate the equivalent dose s for different organs and probability of secondary cancer, fatal and nonfatal risk, and other risks and parameters related to megavoltage radiation therapy. In out-of-field radiation calculation, it could be seen that neutrons imparted a higher dose to distant organs, and the dose to surrounding organs was mainly due to absorbed scattered photons and electron contamination.

Results: Our results showed that the bladder and skin with 54.89 × 10-3 mSv/Gy and 46.09 × 10-3 mSv/Gy, respectively, absorbed the highest equivalent dose s from photoneutrons, while a lower dose was absorbed by the lung at 3.42 × 10-3 mSv/Gy. The large intestine and bladder absorbed 55.00 × 10-3 mSv/Gy and 49.08 × 10-3, respectively, which were the highest equivalent dose s due to photons. The brain absorbed the lowest out-of-field dose, at 1.87 × 10-3 mSv/Gy.

Conclusions: We concluded that secondary neutron portion was higher than other radiation. Then, we recommended more attention to neutrons in the radiation protection in linac based high energy radiotherapy.

Keywords: Equivalent dose; Monte Carlo simulation; Prostate radiotherapy; Radiation contamination; Secondary malignancy.

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Figures

Fig. 1
Fig. 1
(a) The room layout and dimensions simulated in this study. The 3.65 m high room with the 55 cm thick concrete ceiling was simulated. Additionally, 110 cm concrete bulk simulated on top of the linac head in linac rotation direction for shielding the upper space from the primary radiation while the linac irradiates below the patient. The bulk performs as primary barrier with thickness of 1.65 m. (b) Axial view of the prostate and the treatment plan by which a patient is treated and our MC simulation was carried out. Isodoses curves can be seen in the plan provided by CorePlan TPS and used field sizes (two fields of 8 × 8 cm2 and two fields of 7 × 8 cm2) applied for simulations. Isocenter relative dose was shown in red color as 100.6%.
Fig. 2
Fig. 2
Neutron equivalent dose equivalent to different organs in the prostate radiotherapy from out-of-field radiation scored by MC simulation.
Fig. 3
Fig. 3
Scattered photon equivalent dose in the prostate radiotherapy from out-of-field radiation scored by MC simulation.
Fig. 4
Fig. 4
Capture gamma ray equivalent dose to different organs in the prostate radiotherapy from out-of-field radiation scored by MC simulation.
Fig. 5
Fig. 5
Electron contamination spectra derived by MC simulation for 40 × 40 cm2 field size and two linac configurations; solid red line is under condition with flattening filter and dot black line shows the contamination spectra without flattening filter. The flattening filter is a main part of a linac that contaminant electron production is attributed to. The figure shows the considerable effect of flattening filter on the electron contamination fluence.
See this image and copyright information in PMC

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