Out-of-Field Doses Produced by a Proton Scanning Beam Inside Pediatric Anthropomorphic Phantoms and Their Comparison With Different Photon Modalities

Abstract

Since 2010, EURADOS Working Group 9 (Radiation Dosimetry in Radiotherapy) has been involved in the investigation of secondary and scattered radiation doses in X-ray and proton therapy, especially in the case of pediatric patients. The main goal of this paper is to analyze and compare out-of-field neutron and non-neutron organ doses inside 5- and 10-year-old pediatric anthropomorphic phantoms for the treatment of a 5-cm-diameter brain tumor. Proton irradiations were carried out at the Cyclotron Centre Bronowice in IFJ PAN Krakow Poland using a pencil beam scanning technique (PBS) at a gantry with a dedicated scanning nozzle (IBA Proton Therapy System, Proteus 235). Thermoluminescent and radiophotoluminescent dosimeters were used for non-neutron dose measurements while secondary neutrons were measured with track-etched detectors. Out-of-field doses measured using intensity-modulated proton therapy (IMPT) were compared with previous measurements performed within a WG9 for three different photon radiotherapy techniques: 1) intensity-modulated radiation therapy (IMRT), 2) three-dimensional conformal radiation therapy (3D CDRT) performed on a Varian Clinac 2300 linear accelerator (LINAC) in the Centre of Oncology, Krakow, Poland, and 3) Gamma Knife surgery performed on the Leksell Gamma Knife (GK) at the University Hospital Centre Zagreb, Croatia. Phantoms and detectors used in experiments as well as the target location were the same for both photon and proton modalities. The total organ dose equivalent expressed as the sum of neutron and non-neutron components in IMPT was found to be significantly lower (two to three orders of magnitude) in comparison with the different photon radiotherapy techniques for the same delivered tumor dose. For IMPT, neutron doses are lower than non-neutron doses close to the target but become larger than non-neutron doses further away from the target. Results of WG9 studies have provided out-of-field dose levels required for an extensive set of radiotherapy techniques, including proton therapy, and involving a complete description of organ doses of pediatric patients. Such studies are needed for validating mathematical models and Monte Carlo simulation tools for out-of-field dosimetry which is essential for dedicated epidemiological studies which evaluate the risk of second cancers and other late effects for pediatric patients treated with radiotherapy.

Out-of-field effects: lessons learned from partial body exposure

“…In all cases investigated by WG9 out-of-field organ doses were assessed inside anthropomorphic phantoms for the same target (6 cm diameter spherical PTV with the center on the left anterior side of the head). This work was recently summarized by Knezevic et al ( 2022 ). Table 1 presents example data for out-of-field dose levels estimated by EURADOS WG9 for 3D-CRT, IMRT (Majer et al 2017 ) and Intensity Modulated Proton Therapy (IMPT) with PBS technique (Knezevic et al 2018 ).…”

“…For 3D-CRT, IMRT and proton active scanning radiotherapy data are adapted from Knezevic et al ( 2022 ). For passive scattering proton radiotherapy data are adapted from Sayah et al ( 2014 ) assuming 10% contribution from secondary photons to total organ equivalent dose.…”

Measurements of fetal dose with Mevion S250i proton therapy system with HYPERSCAN

“…14 The study found a decreased total organ dose equivalent for intensity modulated proton therapy (IMPT) with pencil beam scanning compared to photon therapy. 14 Compared to phantom measurements taken with bubble detectors at the Indiana University Cyclotron double scattering system, reported by Mesoloras et al, the fetal dose is reduced from the 25-871 µSv per Gy to the maximum 64.7 µSv per 2 Gy(RBE) reported here. 15 With previous literature demonstrating that passively scattered proton therapy is not associated with a significant increase in secondary malignancies when compared with photon therapy, 3,16 our work suggests that this risk is further reduced when using pencil beam scanning systems, such as the Mevion S250i system.…”

“…Additional work by Knežević et al. demonstrated the differences in dose for proton therapy compared to 3D conformal radiation therapy, IMRT, and Gamma Knife (Elekta, Stockholm, Sweden) with the proton therapy plan containing two coplanar beams treating a brain lesion in a pediatric phantom 14 . The study found a decreased total organ dose equivalent for intensity modulated proton therapy (IMPT) with pencil beam scanning compared to photon therapy 14 .…”

“…demonstrated the differences in dose for proton therapy compared to 3D conformal radiation therapy, IMRT, and Gamma Knife (Elekta, Stockholm, Sweden) with the proton therapy plan containing two coplanar beams treating a brain lesion in a pediatric phantom 14 . The study found a decreased total organ dose equivalent for intensity modulated proton therapy (IMPT) with pencil beam scanning compared to photon therapy 14 . Compared to phantom measurements taken with bubble detectors at the Indiana University Cyclotron double scattering system, reported by Mesoloras et al., the fetal dose is reduced from the 25–871 μSv per Gy to the maximum 64.7 μSv per 2 Gy(RBE) reported here 15 .…”