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Characterization of Extrafocal Dose Influence on the Out-of-Field Dose Distribution by Monte Carlo Simulations and Dose Measurements

Ghareeb, Firass1; Lencart, Joana1,2; Oliveira, Jorge1; Santos, João A.M.1,2,3

doi: 10.1097/HP.0000000000001079

Out-of-field scattered and transmitted extrafocal radiation may induce secondary cancer in long-term survivors of external radiotherapy. Pediatric patients have higher life expectancy and tend to receive higher secondary radiation damage due to geometric and biological factors. The goal of this study is to characterize the location and the magnitude of extrafocal dose regions in the case of three-dimensional conformal radiotherapy and volumetric arc therapy, to apply this information to clinical treatment cases, and to provide mitigation strategies. Extrafocal dose has been investigated in a Varian TrueBeam linac equipped with a high-definition 120 multileaf collimator using different physical and virtual phantoms, dose calculation (including Monte Carlo techniques), and dose measurement methods. All Monte Carlo calculations showed excellent agreement with measurements. Treatment planning system calculations failed to provide reliable results out of the treatment field. Both Monte Carlo calculations and dose measurements showed regions with higher dose (extrafocal dose areas) when compared to the background. These areas start to be noticeable beyond 11 cm from the isocenter in the direction perpendicular to the multileaf collimator leaves’ travel direction. Out-of-field extrafocal doses up to 160% of the mean dose transmitted through the closed multileaf collimator were registered. Two overlapping components were observed in the extrafocal distribution: the first is an almost elliptical blurred dose distribution, and the second is a well-defined rectangular dose distribution. Extra precautions should be taken into consideration when treating pediatric patients with a high-definition 120 multileaf collimator to avoid directing the extrafocal radiation into a radiosensitive organ during external beam therapy.

1Medical Physics, Radiobiology and Radiation Protection Group, IPO Porto Research Center (CI-IPOP), Portuguese Oncology Institute of Porto (IPO Porto), Porto, Portugal

2Medical Physics Department, Portuguese Oncology Institute of Porto (IPO Porto), Porto, Portugal

3Instituto de Ciências Biomédicas Abel Salazar (ICBAS), University of Porto, Porto, Portugal.

The authors declare no conflicts of interest.

For correspondence contact João A.M. Santos, Instituto Português de Oncologia do Porto Francisco Gentil, EPE, Serviço de Física Médica, Rua Dr. António Bernardino de Almeida 4200-072 Porto, Portugal, or email at

(Manuscript accepted 17 January 2019)

Online date: April 30, 2019

© 2019 by the Health Physics Society