The lowest possible energy of proton scanning beam in cyclotron proton therapy facilities is typically between 60 and 100 MeV. Treatment of superficial lesions requires a pre-absorber to deliver doses to shallower volumes. In most of the cases a range shifter (RS) is used, but as an alternative solution, a patient-specific 3D printed proton beam compensator (BC) can be applied. A BC enables further reduction of the air gap and consequently reduction of beam scattering. Such pre-absorbers are additional sources of secondary radiation. The aim of this work was the comparison of RS and BC with respect to out-of-field doses for a simulated treatment of superficial paediatric brain tumours. EURADOS WG9 performed comparative measurements of scattered radiation in the Proteus C-235 IBA facility (Cyclotron Centre Bronowice at the Institute of Nuclear Physics, CCB IFJ PAN, Kraków, Poland) using two anthropomorphic phantoms-5 and 10 yr old-for a superficial target in the brain. Both active detectors located inside the therapy room, and passive detectors placed inside the phantoms were used. Measurements were supplemented by Monte Carlo simulation of the radiation transport. For the applied 3D printed pre-absorbers, out-of-field doses from both secondary photons and neutrons were lower than for RS. Measurements with active environmental dosimeters at five positions inside the therapy room indicated that the RS/BC ratio of the out-of-field dose was also higher than one, with a maximum of 1.7. Photon dose inside phantoms leads to higher out-of-field doses for RS than BC to almost all organs with the highest RS/BC ratio 12.5 and 13.2 for breasts for 5 and 10 yr old phantoms, respectively. For organs closest to the isocentre such as the thyroid, neutron doses were lower for BC than RS due to neutrons moderation in the target volume, but for more distant organs like bladder-conversely-lower doses for RS than BC were observed. The use of 3D printed BC as the pre-absorber placed in the near vicinity of patient in the treatment of superficial tumours does not result in the increase of secondary radiation compared to the treatment with RS, placed far from the patient.

Belgium Nuclear Research Centre Boeretang 200 Mol BE 2400 Belgium

Dansk Center for Partikelterapi Palle Juul Jensens Boulevard 25 8200 Aarhus N Denmark

Departament de Física Universitat Autònoma de Barcelona Bellaterra E 08193 Spain

Department of Radiation Dosimetry Nuclear Physics Institute Czech Academy of Sciences Prague CZ 250 68 Řež Czech Republic

Helmholtz Zentrum München Institute of Radiation Medicine Ingolstädter Landstraße 1 Neuherberg 85764 Germany

Institute of Nuclear Physics PAN Radzikowskiego 152 Krakow 31 342 Poland

National Centre for Nuclear Research Otwock Świerk 05 400 Poland

Ruđer Bošković Institute Bijenička c 54 Zagreb 10000 Croatia

Skandionkliniken von Kraemers Allé 26 Uppsala 752 37 Sweden

Technische Universität München Physik Department Garching 85748 Germany

University of Newcastle upon Tyne Tyne and Wear Newcastle upon Tyne NE1 7RU United Kingdom

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