Out‐of‐field doses in pediatric craniospinal irradiations with 3D‐CRT, VMAT, and scanning proton radiotherapy: A phantom study

Abstract

Craniospinal irradiation (CSI) has greatly increased survival rates for patients with a diagnosis of medulloblastoma and other primitive neuroectodermal tumors. However, as it includes exposure of a large volume of healthy tissue to unwanted doses, there is a strong concern about the complications of the treatment, especially for the children. To estimate the risk of second cancers and other unwanted effects,out-of -field dose assessment is necessary.The purpose of this study is to evaluate and compare out-of -field doses in pediatric CSI treatment using conventional and advanced photon radiotherapy (RT) and advanced proton therapy. To our knowledge, it is the first such comparison based on in-phantom measurements. Additionally, for out-of -field doses during photon RT in this and other studies, comparisons were made using analytical modeling. Methods: In order to describe the out-of -field doses absorbed in a pediatric patient during actual clinical treatment, an anthropomorphic phantom, which mimics the 10-year-old child, was used. Photon 3D-conformal RT (3D-CRT) and two advanced, highly conformal techniques: photon volumetric-modulated arc therapy (VMAT) and active pencil beam scanning (PBS) proton RT were used for CSI treatment. Radiophotoluminescent and poly-allyl-diglycol-carbonate nuclear track detectors were used for photon and neutron dosimetry in the phantom, respectively. Out-of -field doses from neutrons were expressed in terms of dose equivalent. A two-Gaussian model was implemented for out-of -field doses during photon RT. Results: The mean VMAT photon doses per target dose to all organs in this study were under 50% of the target dose (i.e., <500 mGy/Gy), while the mean 3D-CRT photon dose to oesophagus, gall bladder, and thyroid, exceeded that value. However, for 3D-CRT, better sparing was achieved for eyes and lungs. The mean PBS photon doses for all organs were up to three orders of magnitude lower compared to VMAT and 3D-CRT and exceeded 10 mGy/Gy only for the oesophagus, intestine, and lungs. The mean neutron dose equivalent during PBS for eight organs of interest (thyroid, breasts, lungs, liver, stomach, gall bladder, bladder, prostate) ranged from 1.2 mSv/Gy for bladder to 23.1 mSv/Gy for breasts. Comparison of out-of -field doses in this and other phantom studies found in the literature showed that a simple and fast two-Gaussian model for out-of -field doses as a function of distance from the field edge can be applied in a CSI using photon RT techniques.

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

“…Consequently, the total out-of-field dose is considered to be In all cases, for both photon and proton radiotherapy, higher organ doses were measured for the 5-year-old phantom in comparison to the 10-year-old phantom, as expected due to the smaller distance from healthy organs to the irradiated target. As shown in the previous study, organ dose equivalents were on average 1.1, 1.6, and 3.0 times higher for the 5-year-old than for the 10-year-old phantom for GK, IMRT, and 3D CRT, respectively (32,33). Non-neutron organ dose comparisons performed in this study for IMPT show on average 1.8 times higher doses for the 5year-old phantom then for the 10-year-old phantom.…”

Experimental Validation of an Analytical Program and a Monte Carlo Simulation for the Computation of the Far Out-of-Field Dose in External Beam Photon Therapy Applied to Pediatric Patients

“…Indeed results are not only technique dependent but also center dependent, as data might be different if different objectives and constraints are used in the dose optimization algorithms. Previously published experimental data, performed in the same anthropomorphic phantom, also reported on reduced out-of-field doses for VMAT when compared to 3D-CRT ( 13 ) for CSI. During brain treatment, the difference between 3D-CRT and IMRT has been shown to be small when not using a mechanical wedge ( 12 ).…”

“…Nevertheless, for healthy organs further away from the field, only a limited amount of out-of-field dose data evaluating the long-term side effects of these advanced techniques are available. A recent publication has shown that the use of VMAT during craniospinal irradiation (CSI) indicates a reduction of out-of-field doses in most organs ( 13 ). Another experimental study from pediatric CSI revealed that the conventional radiotherapy technique, three-dimensional conformal radiotherapy (3D-CRT), resulted in very high doses to a limited number of organs while it was able to spare organs such as the lungs and breast when compared to IMRT and helical tomotherapy (HT).…”

Determining Out-of-Field Doses and Second Cancer Risk From Proton Therapy in Young Patients—An Overview

“…The work of Majer et al. ( 45 ) was the only experimental work and considered a PBS-PBT facility. They evaluated photon and neutron equivalent doses in selected positions inside a 10-year-old anthropomorphic phantom.…”

“…Majer et al. ( 45 ) compared a PBS-PBT CSI with 3D conformal RT and VMAT treatments. As photon treatments were planned with energies lower than 10 MeV, the contributions of neutrons could be neglected, and stray photons are the only contributors to out-of-field doses.…”