Beam characterisation studies of the 62 MeV proton therapy beamline at the Clatterbridge Cancer Centre

The Clatterbridge Cancer Centre (CCC) in the United Kingdom is the world’s first hospital proton beam therapy facility, providing treatment for ocular cancers since 1989. A 62 MeV beam of protons is produced by a Scanditronix cyclotron and transported through a passive delivery system. In addition to the long history of clinical use, the facility supports a wide programme of experimental work and as such, an accurate and reliable simulation model of the treatment beamline is highly valuable. However, as the facility has seen several changes to the accelerator and beamline over the years, a comprehensive study of the CCC beam dynamics is needed to firstly examine the beam optics. An extensive analysis was required to overcome facility related constraints to determine fundamental beamline parameters and define an optical lattice written with the Methodical Accelerator Design (MAD-X) and the particle tracking Beam Delivery Simulation (BDSIM) code. An optimised case is presented and simulated results of the optical functions, beam distribution, losses and the transverse rms beam sizes along the beamline are discussed. Corresponding optical and beam information was used in TOPAS to simulate transverse beam profiles and compared to EBT3 film measurements. We provide an overview of the magnetic components, beam transport, cyclotron, beam and treatment related parameters necessary for the development of a present day optical model of the facility. This work represents the first comprehensive study of the CCC facility to date, as a basis to determine input beam parameters to accurately simulate and completely characterise the beamline.     

The comparison of doses measured by radiochromic films and semiconductor detector in a 175 MeV proton beam

We wanted to verify the response of radiochromic films in a 175 MeV clinical proton beam used at the Joint Institute for Nuclear Research in Dubna against doses measured using semiconductor detectors and compare the results with published data from other centres. Radiochromic films (RCFs) MD-55 and a Vidar VXR-16 scanner were used. The films were irradiated in an unmodulated proton beam and with a beam modulated with a bolus and a ridge filter. Obtained dose distributions were compared with dose distributions measured with a Si-semiconductor detector.For the unmodulated beam the difference between the RCF and the semiconductor detector was 12% in the Bragg peak top. For the modulated beam the difference inside the spread-out Bragg peak region was 4%. Observed deviations between doses measured with RCF and Si-detector outside the Bragg peak were caused by the inhomogeneity of radiochromic emulsion. In the Bragg peak region the RCF doses were lower than those measured by semiconductors. The results were in agreement with published data from other proton therapy centres.     

GEANT4 models for the secondary radiation flux in the collimation system of a 300 MeV proton microbeam

In Harbin, we are developing a 300 MeV proton microbeam for many applications in space science including upset studies in microelectronic devices, radiation hardness of materials for satellites and radiation effects in human tissues. There are also applications of this facility proposed for proton therapy. The microbeam system will employ a purpose-built proton synchrotron to provide the beam. However there are many challenges to be addressed in the design, construction and operation of this facility. Here we address two important design aspects for which we apply GEANT4 modeling. First, the high energy proton beam interacts strongly with beam line materials, especially the collimation slits, to produce showers of secondary particles which could introduce significant background signals and degrade the resolution of the proton microbeam. Second, the beam transport within the residual vacuum of the beam line may also introduce undesirable background radiation. In both cases mitigation strategies need to be incorporated during the design phase of the new system. We study the use of a dipole magnet following the aperture collimator to reduce the flux of secondary particles incident on the analysis chamber. Monte Carlo simulations are performed using GEANT4 and SRIM. By inserting the dipole magnet, we find as expected a significant reduction in the scattering of protons and other particles, such as neutrons and gamma rays, at the collimation system exit position. Secondary radiation from the residual gas pressure within the beam line vacuum system are also modelled and found to be negligible under the standard operating conditions.     

Evaluations of a flat-panel based compact daily quality assurance device for proton pencil beam scanning (PBS) system

PurposeTo evaluate the flat-panel detector quenching effect and clinical usability of a flat-panel based compact QA device for PBS daily constancy measurements.Materials & MethodThe QA device, named Sphinx Compact, is composed of a 20x20 cm² flat-panel imager mounted on a portable frame with removable plastic modules for constancy checks of proton energy (100 MeV, 150 MeV, 200 MeV), Spread-Out-Bragg-Peak (SOBP) profile, and machine output. The potential quenching effect of the flat-panel detector was evaluated. Daily PBS QA tests of X-ray/proton isocenter coincidence, the constancy of proton spot position and sigma as well as the energy of pristine proton beam, and the flatness of SOBP proton beam through the 'transformed' profile were performed and analyzed. Furthermore, the sensitivity of detecting energy changes of pristine proton beam was also evaluated.ResultsThe quenching effect was observed at depths near the pristine peak regions. The flat-panel measured range of the distal 80% is within 0.9 mm to the defined ranges of the delivered proton beams. X-ray/proton isocenter coincidence tests demonstrated maximum mismatch of 0.3 mm between the two isocenters. The device can detect 0.1 mm change of spot position and 0.1 MeV energy changes of pristine proton beams. The measured transformed SOBP beam profile through the wedge module rendered as flat.ConclusionsEven though the flat-panel detector exhibited quenching effect at the Bragg peak region, the proton range can still be accurately measured. The device can fulfill the requirements of the daily QA tests recommended by the AAPM TG224 Report.     

Analysis of the EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons using reflective mode scanners

We explore in our study the effects of electrons and X-rays irradiations on the newest version of the Gafchromic EBT3 film. Experiments are performed using the Varian “TrueBeam 1.6” medical accelerator delivering 6 MV X-ray photons and 6 MeV electron beams as desired. The main interest is to compare the responses of EBT3 films exposed to two separate beams of electrons and photons, for radiation doses ranging up to 500 cGy. The analysis is done on a flatbed EPSON 10000 XL scanner and cross checked on a HP Scanjet 4850 scanner. Both scanners are used in reflection mode taking into account landscape and portrait scanning positions. After thorough verifications, the reflective scanning method can be used on EBT3 as an economic alternative to the transmission method which was also one of the goals of this study. A comparison is also done between single scan configuration including all samples in a single A4 (HP) or A3 (EPSON) format area and multiple scan procedure where each sample is scanned separately on its own. The images analyses are done using the ImageJ software. Results show significant influence of the scanning configuration but no significant differences between electron and photon irradiations for both single and multiple scan configurations. In conclusion, the film provides a reliable relative dose measurement method for electrons and photons irradiations in the medical field applications.     

Wax boluses and accuracy of EBT and RTQA radiochromic film detectors in radiotherapy with the JINR Phasotron proton beam

Aim

To present the results obtained using radiochromic films EBT and RTQA 1010P for the reconstruction the dose distributions for targets irradiated by proton beam and modified by wax boluses.

Background

In Medico-Technical Complex at the Joint Institute for Nuclear Research in Dubna implemented technology of wax boluses.

Materials and methods

Wax boluses are easier to make and they give better dose distributions than boluses made from modeling clay previously used at our center. We irradiated two imaginary targets, one shaped as a cylinder and the other one as two cuboids. The evaluated calibration curve was used for calculation of the dose distributions measured by the EBT and RTQA radiochromic film. In both cases, the measured dose distributions were compared to the dose distributions calculated by the treatment planning system (TPS). We also compared dose distributions using three different conformity indices at a 95% isodose.

Results

Better target coverage and better compliance of measurements (semiconductor detectors and radiochromic films) with calculated doses was obtained for cylindrical target than for cuboidal target. The 95% isodose covered well the tumor for both target shapes, while for cuboidal target larger volume around the target received therapeutic dose, due to the complicated target shape. The use wax boluses provided to be effective tool in modifying proton beam to achieve appropriate shape of isodose distribution.

Conclusion

EBT film yielded the best visual matching. Both EBT and RTQA films confirmed good conformity between calculated and measured doses, thus confirming that wax boluses used to modify the proton beam resulted in good dose distributions.     

Clinical implementations of 4D pencil beam scanned particle therapy: Report on the 4D treatment planning workshop 2016 and 2017

In 2016 and 2017, the 8th and 9th 4D treatment planning workshop took place in Groningen (the Netherlands) and Vienna (Austria), respectively. This annual workshop brings together international experts to discuss research, advances in clinical implementation as well as problems and challenges in 4D treatment planning, mainly in spot scanned proton therapy. In the last two years several aspects like treatment planning, beam delivery, Monte Carlo simulations, motion modeling and monitoring, QA phantoms as well as 4D imaging were thoroughly discussed.This report provides an overview of discussed topics, recent findings and literature review from the last two years. Its main focus is to highlight translation of 4D research into clinical practice and to discuss remaining challenges and pitfalls that still need to be addressed and to be overcome.     

The FLUKA Monte Carlo code coupled with an OER model for biologically weighted dose calculations in proton therapy of hypoxic tumors

IntroductionThe increased radioresistance of hypoxic cells compared to well-oxygenated cells is quantified by the oxygen enhancement ratio (OER). In this study we created a FLUKA Monte Carlo based tool for inclusion of both OER and relative biological effectiveness (RBE) in biologically weighted dose (ROWD) calculations in proton therapy and applied this to explore the impact of hypoxia.MethodsThe RBE-weighted dose was adapted for hypoxia by making RBE model parameters dependent on the OER, in addition to the linear energy transfer (LET). The OER depends on the partial oxygen pressure (pO₂) and LET. To demonstrate model performance, calculations were done with spread-out Bragg peaks (SOBP) in water phantoms with pO₂ ranging from strongly hypoxic to normoxic (0.01–30 mmHg) and with a head and neck cancer proton plan optimized with an RBE of 1.1 and pO₂ estimated voxel-by-voxel using [¹⁸F]-EF5 PET. An RBE of 1.1 and the Rørvik RBE model were used for the ROWD calculations.ResultsThe SOBP in water had decreasing ROWD with decreasing pO₂. In the plans accounting for oxygenation, the median target doses were approximately a factor 1.1 lower than the corresponding plans which did not consider the OER. Hypoxia adapted target ROWDs were considerably more heterogeneous than the RBE_1.1-weighted doses.ConclusionWe realized a Monte Carlo based tool for calculating the ROWD. Read-in of patient pO₂ and estimation of ROWD with flexibility in choice of RBE model was achieved, giving a tool that may be useful in future clinical applications of hypoxia-guided particle therapy.     

Measurement of stray neutron doses inside the treatment room from a proton pencil beam scanning system

PurposeTo measure the environmental doses from stray neutrons in the vicinity of a solid slab phantom as a function of beam energy, field size and modulation width, using the proton pencil beam scanning (PBS) technique.MethodMeasurements were carried out using two extended range WENDI-II rem-counters and three tissue equivalent proportional counters. Detectors were suitably placed at different distances around the RW3 slab phantom. Beam irradiation parameters were varied to cover the clinical ranges of proton beam energies (100–220 MeV), field sizes ((2 × 2)–(20 × 20) cm²) and modulation widths (0–15 cm).ResultsFor pristine proton peak irradiations, large variations of neutron H¹(10)/D were observed with changes in beam energy and field size, while these were less dependent on modulation widths. H¹(10)/D for pristine proton pencil beams varied between 0.04 μSv Gy⁻¹ at beam energy 100 MeV and a (2 × 2) cm² field at 2.25 m distance and 90° angle with respect to the beam axis, and 72.3 μSv Gy⁻¹ at beam energy 200 MeV and a (20 × 20) cm² field at 1 m distance along the beam axis.ConclusionsThe obtained results will be useful in benchmarking Monte Carlo calculations of proton radiotherapy in PBS mode and in estimating the exposure to stray radiation of the patient. Such estimates may be facilitated by the obtained best-fitted simple analytical formulae relating the stray neutron doses at points of interest with beam irradiation parameters.     

Dosimetric characterization of a small-scale (Zn,Cd)S:Ag inorganic scintillating detector to be used in radiotherapy

PurposeIn modern radiotherapy techniques, to ensure an accurate beam modeling process, dosimeters with high accuracy and spatial resolution are required. Therefore, this work aims to propose a simple, robust, and a small-scale fiber-integrated X-ray inorganic detector and investigate the dosimetric characteristics used in radiotherapy.MethodsThe detector is based on red-emitting silver-activated zinc-cadmium sulfide (Zn,Cd)S:Ag nanoclusters and the proposed system has been tested under 6 MV photons with standard dose rate used in the patient treatment protocol. The article presents the performances of the detector in terms of dose linearity, repeatability, reproducibility, percentage depth dose distribution, and field output factor. A comparative study is shown using a microdiamond dosimeter and considering data from recent literature.ResultsWe accurately measured a small field beam profile of 0.5 × 0.5 cm² at a spatial resolution of 100 µm using a LINAC system. The dose linearity at 400 MU/min has shown less than 0.53% and 1.10% deviations from perfect linearity for the regular and smallest field. Percentage depth dose measurement agrees with microdiamond measurements within 1.30% and 2.94%, respectively for regular to small field beams. Besides, the stem effect analysis shows a negligible contribution in the measurements for fields smaller than 3x3 cm². This study highlights the drastic decrease of the convolution effect using a point-like detector, especially in small dimension beam characterization. Field output factor has shown a good agreement while comparing it with the microdiamond dosimeter.ConclusionAll the results presented here anticipated that the developed detector can accurately measure delivered dose to the region of interest, claim accurate depth dose distribution hence it can be a suitable candidate for beam characterization and quality assurance of LINAC system.     

Characterization of prompt gamma-ray emission with respect to the Bragg peak for proton beam range verification: A Monte Carlo study

In this paper we report a Geant4 simulation study to investigate the characteristic prompt gamma (PG) emission in a water phantom for real-time monitoring of the Bragg peak (BP) during proton beam irradiation. The PG production, emission spatial correlation with the BP, and position preference for detection with respect to the BP have been quantified in different PG energy windows as a function of proton pencil-beam energy from 100 to 200 MeV. The PG response to small BP shifts was evaluated using a 2 cm-thick slab with different human body materials embedded in a water phantom. Our results show that the prominent characteristic PG emissions of 4.44, 5.21 and 6.13 MeV exhibit distinctive correlation with the dose deposition curve. The accuracy in BP position identification using these characteristic PG rays is highly consistent as the beam energy increases from 100 to 200 MeV. There exists a position preference for PG detection with respect to the BP position, which has a strong dependence on the proton beam energy and PG energies. It was also observed that a submillimeter shift of the BP position can be realized by using PG signals. These results indicate that the characteristic PG signal is sensitive and reliable for BP tracking. Although the maximization of the PG measurement associated with the BP is difficult, it can be optimized with energy and detection position preferences.     

Multicenter evaluation of a synthetic single-crystal diamond detector for CyberKnife small field size output factors

PurposeThe aim of the present work was to evaluate small field size output factors (OFs) using the latest diamond detector commercially available, PTW-60019 microDiamond, over different CyberKnife systems. OFs were measured also by silicon detectors routinely used by each center, considered as reference.MethodsFive Italian CyberKnife centers performed OFs measurements for field sizes ranging from 5 to 60 mm, defined by fixed circular collimators (5 centers) and by Iris^™ variable aperture collimator (4 centers). Setup conditions were: 80 cm source to detector distance, and 1.5 cm depth in water. To speed up measurements two diamond detectors were used and their equivalence was evaluated. MonteCarlo (MC) correction factors for silicon detectors were used for comparing the OF measurements.ResultsConsidering OFs values averaged over all centers, diamond data resulted lower than uncorrected silicon diode ones. The agreement between diamond and MC corrected silicon values was within 0.6% for all fixed circular collimators. Relative differences between microDiamond and MC corrected silicon diodes data for Iris^™ collimator were lower than 1.0% for all apertures in the totality of centers. The two microDiamond detectors showed similar characteristics, in agreement with the technical specifications.ConclusionsExcellent agreement between microDiamond and MC corrected silicon diode detectors OFs was obtained for both collimation systems fixed cones and Iris^™, demonstrating the microDiamond could be a suitable detector for CyberKnife commissioning and routine checks. These results obtained in five centers suggest that for CyberKnife systems microDiamond can be used without corrections even at the smallest field size.     

On the parametrization of lateral dose profiles in proton radiation therapy

PurposeThe accurate evaluation of the lateral dose profile is an important issue in the field of proton radiation therapy. The beam spread, due to Multiple Coulomb Scattering (MCS), is described by the Molière's theory. To take into account also the contribution of nuclear interactions, modern Treatment Planning Systems (TPSs) generally approximate the dose profiles by a sum of Gaussian functions. In this paper we have compared different parametrizations for the lateral dose profile of protons in water for therapeutical energies: the goal is to improve the performances of the actual treatment planning.MethodsWe have simulated typical dose profiles at the CNAO (Centro Nazionale di Adroterapia Oncologica) beamline with the FLUKA code and validated them with data taken at CNAO considering different energies and depths. We then performed best fits of the lateral dose profiles for different functions using ROOT and MINUIT.ResultsThe accuracy of the best fits was analyzed by evaluating the reduced χ², the number of free parameters of the functions and the calculation time. The best results were obtained with the triple Gaussian and double Gaussian Lorentz–Cauchy functions which have 6 parameters, but good results were also obtained with the so called Gauss–Rutherford function which has only 4 parameters.ConclusionsThe comparison of the studied functions with accurate and validated Monte Carlo calculations and with experimental data from CNAO lead us to propose an original parametrization, the Gauss–Rutherford function, to describe the lateral dose profiles of proton beams.