M.J. van Goethem

Geant4 simulations of proton beam transport through a carbon or beryllium degrader and following a beam line

Monte Carlo simulations based on the Geant4 simulation toolkit were performed for the carbon wedge degrader used in the beam line at the Center of Proton Therapy of the Paul Scherrer Institute (PSI). The simulations are part of the beam line studies for the development and understanding of the GANTRY2 and OPTIS2 treatment facilities at PSI, but can also be applied to other beam lines. The simulated stopping power, momentum distributions at the degrader exit and beam line transmission have been compared to accurate benchmark measurements. Because the beam transport through magnetic elements is not easily modeled using Geant4a connection to the TURTLE beam line simulation program was made. After adjusting the mean ionization potential of the carbon degrader material from 78 eV to 95 eV, we found an accurate match between simulations and benchmark measurements, so that the simulation model could be validated. We found that the degrader does not completely erase the initial beam phase space even at low degraded beam energies. Using the validation results, we present a study of the usability of beryllium as a degrader material (mean ionization potential 63.7 eV). We found an improvement in the transmission of 30-45%, depending on the degraded beam energy, the higher value for the lower energies.

Spectra of clinical CT scanners using a portable Compton spectrometer

PURPOSEnSpectral information of the output of x-ray tubes in (dual source) computer tomography (CT) scanners can be used to improve the conversion of CT numbers to proton stopping power and can be used to advantage in CT scanner quality assurance. The purpose of this study is to design, validate, and apply a compact portable Compton spectrometer that was constructed to accurately measure x-ray spectra of CT scanners.nnnMETHODSnIn the design of the Compton spectrometer, the shielding materials were carefully chosen and positioned to reduce background by x-ray fluorescence from the materials used. The spectrum of Compton scattered x-rays alters from the original source spectrum due to various physical processes. Reconstruction of the original x-ray spectrum from the Compton scattered spectrum is based on Monte Carlo simulations of the processes involved. This reconstruction is validated by comparing directly and indirectly measured spectra of a mobile x-ray tube. The Compton spectrometer is assessed in a clinical setting by measuring x-ray spectra at various tube voltages of three different medical CT scanner x-ray tubes.nnnRESULTSnThe directly and indirectly measured spectra are in good agreement (their ratio being 0.99) thereby validating the reconstruction method. The measured spectra of the medical CT scanners are consistent with theoretical spectra and spectra obtained from the x-ray tube manufacturer.nnnCONCLUSIONSnA Compton spectrometer has been successfully designed, constructed, validated, and applied in the measurement of x-ray spectra of CT scanners. These measurements show that our compact Compton spectrometer can be rapidly set-up using the alignment lasers of the CT scanner, thereby enabling its use in commissioning, troubleshooting, and, e.g., annual performance check-ups of CT scanners.

Proton radiography to improve proton therapy treatment

The quality of cancer treatment with protons critically depends on an accurate prediction of the proton stopping powers for the tissues traversed by the protons. Today, treatment planning in proton radiotherapy is based on stopping power calculations from densities of X-ray Computed Tomography (CT) images. This causes systematic uncertainties in the calculated proton range in a patient of typically 3-4%, but can become even 10% in bone regions [1-8]. This may lead to no dose in parts of the tumor and too high dose in healthy tissues [9]. A direct measurement of proton stopping powers with high-energy protons will allow reducing these uncertainties and will improve the quality of the treatment. Several studies have shown that a sufficiently accurate radiograph can be obtained by tracking individual protons traversing a phantom (patient) [4, 6, 10]. Our studies benefit from the gas-filled time projection chambers based on GridPix technology [11], developed at Nikhef, capable of tracking a single proton. A BaF2 crystal measuring the residual energy of protons was used. Proton radiographs of phantom consisting of different tissue-like materials were measured with a 30 x 30 mm(2) 150 MeV proton beam. Measurements were simulated with the Geant4 toolkit. First experimental and simulated energy radiographs are in very good agreement [12]. In this paper we focus on simulation studies of the proton scattering angle as it affects the position resolution of the proton energy loss radiograph. By selecting protons with a small scattering angle, the image quality can be improved significantly.

55: TOF-PET scanner configurations for quality assurance in proton therapy: a patient case study

In order to determine the clinical benefit of positron emission tomography (PET) for dose delivery verification in proton therapy, we performed a patient case study comparing in-situ with in-room time-of-flight (TOF) PET. For the in-situ option, we consider both a (limited-angle) clinical scanner and a dual-head scanner placed close to the patient.

PV-0564: Experimental validation of proton stopping power calculations based on dual energy CT imaging

Purpose or Objective: The interest in particle therapy, with light and heavy ion beams, has grown worldwide, due to their beneficial physical and biological properties. At the Heidelberg Ion beam Therapy Center, four ions are available for irradiation with an active scanning beam delivery system: 1H, 4He, 12C and 16O. While most of the actual studies comparing different characteristics of the ions are based on Monte Carlo or analytical dose calculations, we present here an experimental based comparison for spread-out Bragg peaks (SOBP) and a first clinical-like scenario study, experimentally validated.

Short-lived Positron Emitters in Beam-on PET Imaging During Proton Therapy

Positron emission tomography is so far the only method for in-vivo dose delivery verification in hadron therapy that is in clinical use. PET imaging during irradiation maximizes the number of detected counts and minimizes washout. In such a scenario, also short-lived positron emitters will be observed. As very little is known on the production of these nuclides, we determined which ones are relevant for proton therapy treatment verification. In order to be relevant, nuclides have to be produced close to the distal edge and thus at rather low proton energy. Therefore we measured the integral production of short-lived positron emitters in the stopping of 55 MeV protons in carbon, oxygen, phosphorus and calcium. The experiments were performed at the irradiation facility of the AGOR cyclotron at KVI-Center for Advanced Radiation Technology, University of Groningen. The positron emitters were identified based on their half-life. In order to do this, the proton beam was pulsed, i.e. delivered as a succession of beam-on and beam-off periods, and the time evolution of the 511 keV positron annihilation photons was recorded. A half-life analysis of the beam-off period allowed to determine the production rates of separate nuclides. The 511 keV photons were detected by a germanium clover detector [1]. A correction for the escape of positrons from the target, determined via Monte Carlo simulations, was applied. In the stopping of 55 MeV protons, the most copiously produced short-lived nuclides and their production rates relative to the relevant long-lived nuclides are: 12N (T1/2 = 11 ms) on carbon (9% of the 11C production), 29P (T1/2 = 4.1 s) on phosphorus (20% of the 30P production) and 38mK (T1/2 = 0.92 s) on calcium (113% of the 38gK production). No short-lived nuclides are produced on water (i.e. oxygen). The experimental production rates are used to calculate the production on PMMA and a representative set of 4 tissue materials. [fig. 1] The number of decays per 55 MeV proton stopped in these materials, integrated over an irradiation, is calculated as function of the duration of the irradiation. The most noticeable result is that for an irradiation in (carbon-rich) adipose tissue, 12N will dominate the PET image up to an irradiation duration of 70 s. On bone tissue, 15O dominates over 12N after 8-15 s (depending on the carbon-to-oxygen ratio). Considering nuclides created on phosphorus and calcium, the short-lived ones provide 2.5 times more decays than the long-lived ones during a 70 s irradiation. Bone tissue will thus be better visible in beam-on PET compared to PET imaging after an irradiation. The results warrant detailed investigations into the energy-dependent production of 12N, 29P and 38mK and their effect on PET imaging during proton irradiations.

PET Scanning Protocols for In-Situ Dose Delivery Verification of Proton Therapy

Positron emission tomography is so far the only method for in-vivo dose delivery verification in hadron therapy that is in clinical use. A PET scanner placed in the treatment position (in-situ) will be able to obtain the highest number of counts, as it minimizes the decay of the positron emitting nuclei before the scan is started as well as reduces the effect of biological washout. We investigated the influence of the scan protocol, i.e. the moment when a scan in done in relation to the treatment delivery, on the ability to measure unacceptable deviations from the treatment plan. We developed a Geant4-based Monte-Carlo framework for proton therapy simulations. Four patient cases are studied: two head-and-neck, one sarcoma near the spine, and one breast cancer case. For each irradiation field, the production of the following PET isotopes is calculated: 15O, 11C, 10C, 14O, 30P, 38K, and 13N. The time sequence of the pencil beam scanning irradiation, the decay of the PET isotopes during the irradiation, and biological washout are included in the simulation. The production of these nuclei is then used to calculate a PET image for two scan protocols: a scan of 120 seconds after the first field, or a scan of 120 seconds after the last field. To mimic a typical scanner spatial resolution, the images are blurred using a Gaussian blurring function with 4 mm FWHM. Deviations from the treatment plan are simulated by shifting the patient 4 mm perpendicular to the field angle, or by increasing the patient density by 3%. The PET image is then simulated again for each scanning protocol. The ability of each protocol to detect these deviations from the treatment plan is investigated by comparing the planned and the modified PET image. Several analysis methods are used: line profiles coupled to the field-directions, structural similarity index analysis [1], and gamma index analysis. [2] Preliminary data shows that a difference in density is best detected by starting the scan directly after the first field. However, shifts perpendicular to the field directions are better detected when the scan is done after the last field, due to the increased activity and counting-rate.

From 2D to 3D: Proton radiography and proton CT in proton therapy: A simulation study

(1) Purpose In order to reduce the uncertainty in translation of the X-ray Computed Tomography (CT) image into a map of proton stopping powers (3-4% and even up to 10% in regions containing bones [1-8]), proton radiography is being studied as an alternative imaging technique in proton therapy. We performed Geant4 Monte Carlo simulations for a 2-dimentional (2D) proton radiography system to obtain directly proton stopping powers of the imaged object. In the next step, the object was rotated every 10 degrees to obtain the 3D proton CT, and the iterative reconstruction method was used to reproduce the image. (2) Materials/methods In our proton radiography simulation setup we used two ideal (100% efficiency) position sensitive detectors (red squares), with the size of 10x10 cm2 each, to track a single proton entering and exiting a phantom under study. The residual energy of a proton was detected by a BaF2 crystal (yellow cylinder), with a diameter of 15 cm, placed after the second position sensitive detector. A cylindrical phantom with a 2.5 cm diameter and 2.5 cm height was made of CT solid water (Gammex 357, ρ=1.015 g/cm2) and filled with different materials: PMMA (ρ=1.18 g/cm2, red insert), air (ρ=1.21•10-3 g/cm2, below and/or above each inserts), and tissue-like materials: adipose (Gammex 453, ρ=0.92 g/cm2, yellow insert) and cortical bone (Gammex 450, ρ=1.82 g/cm2, blue insert) [9]. The phantom was irradiated with 3x3 cm2 scattered proton beam with an energy of 150 MeV. It was irradiated with 2•105 protons at each of the 36 rotation angles. The phantom was placed perpendicularly to the beam direction allowing a proton to pass through a number of materials with different densities. (3) Results First, the energy loss radiographs (a difference between proton beam energy and residual energy deposited in the energy detector) at each of the 36 phantom rotation angles were created. For the iterative reconstruction algorithm, a reference image of the phantom was created in two ways: (1) based on the energy loss in different phantom materials simulated with Geant4, and (2) using a simple back projection algorithm. The reconstruction agrees well with the actual phantom. A maximum of 50 iterations were used showing the smallest mean squared error already after 5 iterations. (4) Conclusion First attempt to iteratively reconstruct the cylindrical phantom with more materials on the proton beam shows a satisfactory result. To improve the reconstruction at the material boundaries, additional local iterations will be applied.

PO-0871: Relative electron density determination for radiotherapy and proton therapy treatment planning

Purpose/Objective: In volumetric-modulated arc (VMAT) prostate stereotactic body radiotherapy (SBRT), dose coverage of the PTV becomes challenging when the sparing of the organs at risk (OAR) is strictly pursued. Our current 35Gyin-five-fractions prostate SBRT VMAT plans assure PTV33.2Gy ≥95% only. Looking for an improved PTV33.2Gy, the dosimetric impact of a slightly increased near-maximum target dose (D2%), and of a prostate-rectum interface spacer were here tested. Materials and Methods: For eleven patients two CT studies, before (NoSpc) or after (Spc) the insertion of SpaceOAR (Augmenix Inc., Waltham, MA) prostate-rectum hydrogel spacer, were acquired. On each CT study two VMAT plans, Hom-plans (D2% ≤37.5Gy), and Het-plans (D2% ≤40.2Gy), were computed. All plans assured D1cc<35Gy for rectum, bladder, and urethral-PRV (3mm isotropic expansion), together with PTV33.2Gy ≥95%. From the four groups of plans (Hom-NoSpc, Hom-Spc, Het-NoSpc, Het-Spc) metrics for target dose coverage (D98%, D50%, PTV33.2Gy, PTV35Gy), and rectal dose sparing (V18Gy, V28Gy, V32Gy), were then compared by hypothesis testing (t-test, Wilcoxon). Linear correlation and ANOVA analyses between the variations from spacer insertion in the fractional overlap with PTV of the rectum (ΔVovl), and the corresponding variations in PTV33.2Gy, and in rectal VX, were also performed. Results: According to hypothesis testing, by comparing Spc vs. NoSpc plans reductions in rectal V18Gy, V28Gy, and V32Gy, and improvements in target D98%, and PTV33.2Gy significantly resulted. By comparing Het vs. Hom plans, significant improvements in target D50%, PTV33.2Gy, and PTV35Gy, whereas no significant reductions in rectal VX, were computed. By directly comparing Het-Spc vs. Hom-NoSpc plans, all the conceived metrics were significantly improved: PTV33.2Gy, in particular, increased from 96.1% (±1.1%) to 98.7% (±1.2%). In the Table the mean values (1 sd) of all computed metrics for the four types of plans are reported. In the Figure, the mean DVHs (± sd) for rectum and PTV when comparing Het-Spc vs. Hom-NoSpc plans are shown: the enlargement of the therapeutic window is evident. For spacer insertion, ΔVovl significantly correlated with, and was the effective source of variation for the observed decrease in rectal V32Gy, and V28Gy. By contrast, ΔVovl neither linearly correlated with, nor was the effective source of variation for the observed increase in PTV33.2Gy. Conclusions: Spacer insertion was causative for improvements in rectal dose sparing, but not in target dose coverage, whereas increased near maximum dose (D2%) was associated with improved target dose coverage. The combined use of both spacer insertion and increased D2% improved both rectal dose sparing and PTV33.2Gy. PO-0871 Relative electron density determination for radiotherapy and proton therapy treatment planning J.K. Van Abbema, M. Van Goethem, M.J.W. Greuter, A. Van der Schaaf, S. Brandenburg, E.R. Van der Graaf University of Groningen Kernfysisch Versneller Instituut Center for Advanced Radiation Technology, Department of Medical Physics, Groningen, The Netherlands University of Groningen University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands University of Groningen University Medical Center Groningen, Department of Radiology, Groningen, The Netherlands

SU‐E‐T‐60: Evaluation of a Two‐Dimensional Ionization Chamber Array for Proton Beam Dosimetry

PURPOSEnThe application of the two-dimensional ionization chamber array Octavius 729xdr (PTW-Freiburg, Germany) in proton beam dosimetry, consisting of 729 vented ionization chambers arranged in a 27 × 27 cm2 matrix, equivalent to the 2D-Array 729 used for photon beams1 , is investigated.nnnMETHODSnMeasurements were carried out with a double scattered 150 MeV proton beam at the AGOR cyclotron at the KVI (Rijksuniversiteit, Groningen - Netherlands). A water bellow with adjustable depth ranging from 10 to 300 mm was placed in the beam line directly behind the collimator for built-up. The stability, linearity and output factors of the array for circular fields produced using collimators with sizes of 3, 5,7 and 10 cm in diameter were investigated. PDD profiles with a Farmer chamber (type 30010, PTW-Freiburg, Germany) and the array were measured by stepwise increasing the water bellows depth.nnnRESULTSnThe variation in measurement signal of the 729xdr array was less than 0.2 % after preirradiation with 2 Gy. The linearity of the array was found to be excellent. Output factors showed the expected small variations for circular field sizes. PDDs measured with the array and a Farmer chamber showed good agreement in the region of the Bragg peak with the array slightly underestimating the dose in the built-up region.nnnCONCLUSIONnThe first measurements with the Octavius 729xdr array show the potential of a two-dimensional ionization chamber array in proton beam dosimetry. The dosimetric properties and the applications of the array in clinical dosimetry will be investigated in more detail in future studies.References1B. Poppe, A. Blechschmidt, A. Djouguela, Two-dimensional ionization chamber arrays for IMRT plan verification, Med. Phys. 33, 1005-1015 (2008).