An anthropomorphic breathing phantom of the thorax for testing new motion mitigation techniques for pencil beam scanning proton therapy

Abstract

Motion-induced range changes and incorrectly placed dose spots strongly affect the quality of pencil-beam-scanned (PBS) proton therapy, especially in thoracic tumour sites, where density changes are large. Thus motion-mitigation techniques are necessary, which must be validated in a realistic patient-like geometry. We report on the development and characterisation of a dynamic, anthropomorphic, thorax phantom that can realistically mimic thoracic motions and anatomical features for verifications of proton and photon 4D treatments. The presented phantom is of an average thorax size, and consists of inflatable, deformable lungs surrounded by a skeleton and skin. A mobile 'tumour' is embedded in the lungs in which dosimetry devices (such as radiochromic films) can be inserted. Motion of the tumour and deformation of the thorax is controlled via a custom made pump system driving air into and out of the lungs. Comprehensive commissioning tests have been performed to evaluate the mechanical performance of the phantom, its visibility on CT and MR imaging and its feasibility for dosimetric validation of 4D proton treatments. The phantom performed well on both regular and irregular pre-programmed breathing curves, reaching peak-to-peak amplitudes in the tumour of <20 mm. Some hysteresis in the inflation versus deflation phases was seen. All materials were clearly visualised in CT scans, and all, except the bone and lung components, were MRI visible. Radiochromic film measurements in the phantom showed that imaging for repositioning was required (as for a patient treatment). Dosimetry was feasible with Gamma Index agreements (4%/4 mm) between film dose and planned dose >90% in the central planes of the target. The results of this study demonstrate that this anthropomorphic thorax phantom is suitable for imaging and dosimetric studies in a thoracic geometry closely-matched to lung cancer patients under realistic motion conditions.

Real-time intrafraction motion monitoring in external beam radiotherapy

“…Malinowski et al ( 2007 ) proposed a motorized platform which can be used to move a rigid phantom or dosimeter with high reproducibility (table 3 ). Anthropomorphic phantoms which provide a more realistic representation of patient anatomy during end-to-end tests were also developed (Biederer et al 2006 , Nioutsikou et al 2006 , Kashani et al 2007 , Remmert et al 2007 , Serban et al 2008 , Steidl et al 2012 , Haas et al 2014 , Cheung and Sawant 2015 , Perrin et al 2017 ) with some representative examples summarized in table 3 . The representation of ribs is particularly important in thoracic particle therapy since the presence (or absence) of a rib on the particle beam path may result in under (or over-) shoot of the particle beam’s Bragg peak.…”

Required transition from research to clinical application: Report on the 4D treatment planning workshops 2014 and 2015

“…Some ingenious and complex phantoms have been manufactured and reported in the literature, reproducing the dynamics commonly present in the irradiation of abdominal or thoracic tumours. Phantoms produced by Kashani et al [Kashani 2007], Vinogradskiy et al [Vinogradsky 2009], Serban et al [Serban 2008] and Perrin et al [Perrin 2014] included a deformable lung, even more closely mirroring realistic patient anatomy. Others have attempted to use ex-vivo organs [Markel 2015], one of which was reported at the 4D workshop itself [Mann 2015].…”

“…During the workshops it was discussed that for anthropomorphic phantoms there is an inherent trade-off between anthropomorphic behavior, on the one hand, and dosimetric precision and reproducibility on the other. As a case in point, compare the dynamic, anthropomorphic phantom, LuCa, developed at the Paul Scherrer Institute [Perrin 2014], with the thorax phantom, PULMONE, developed at the Institute of Cancer Research [Nioutsikou 2006]. The former, LuCa, is constructed of an inflatable lung within a deformable rib cage frame; the tumor moves passively with inflation and deflation of the lung.…”

Evaluation of the accuracy of deformable image registration on MRI with a physical phantom

“…The density of the 4.0% agarose gel is similar to that of prostate tissue (derived density of 1.036 g/mL), and its visualization characteristics on MRI are similar to those of prostate tissue . Although many other materials can be used to simulate human tissues under MRI, we focused here on Mobil DTE oil and agarose gel to facilitate the study of low‐contrast subjects.…”