Photoneutron Dose Estimation in GRID Therapy Using an Anthropomorphic Phantom: A Monte Carlo Study

Nahid Chegeni, Amir Hossein Karimi, Iraj Jabbari, Shole Arvandi


Background: In the past, GRID therapy was used as a treatment modality for the treatment of bulky
and deeply seated tumors with orthovoltage beams. Now and with the introduction of megavoltage
beams to radiotherapy, some of the radiotherapy institutes use GRID therapy with megavoltage
photons for the palliative treatment of bulky tumors. Since GRID can be a barrier for weakening the
photoneutrons produced in the head of medical linear accelerators (LINAC), as well as a secondary
source for producing photoneutrons, therefore, in terms of radiation protection, it is important to
evaluate the GRID effect on photoneutron dose to the patients. Methods: In this study, using the
Monte Carlo code MCNPX, a full model of a LINAC was simulated and verifed. The neutron source
strength of the LINAC (Q), the distributions of flux (φ), and ambient dose equivalent (H*[10]) of
neutrons were calculated on the treatment table in both cases of with/without the GRID. Finally,
absorbed dose and dose equivalent of neutrons in some of the tissues/organs of MIRD phantom
were computed with/without the GRID. Results: Our results indicate that the GRID increases
the production of the photoneutrons in the LINAC head only by 0.3%. The calculations in the
MIRD phantom show that neutron dose in the organs/tissues covered by the GRID is on average
by 48% lower than in conventional radiotherapy. In addition, in the uncovered organs by the grid,
this amount is reduced to 25%. Conclusion: Based on the fndings of this study, in GRID therapy
technique compared to conventional radiotherapy, the neutron dose in the tissues/organs of the body
is dramatically reduced. Therefore, there will be no concern about the GRID effect on the increase of
unwanted neutron dose, and consequently the risk of secondary cancer.

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