Stopping Power Calibration Research Articles

Detection of range shifts in proton beam therapy using the J-PET scanner: a patient simulation study

Objective. The Jagiellonian positron emission tomography (J-PET) technology, based on plastic scintillators, has been proposed as a cost effective tool for detecting range deviations during proton therapy. This study investigates the feasibility of using J-PET for range monitoring by means of a detailed Monte Carlo simulation study of 95 patients who underwent proton therapy at the Cyclotron Centre Bronowice (CCB) in Krakow, Poland. Approach. Discrepancies between prescribed and delivered treatments were artificially introduced in the simulations by means of shifts in patient positioning and in the Hounsfield unit to the relative proton stopping power calibration curve. A dual-layer, cylindrical J-PET geometry was simulated in an in-room monitoring scenario and a triple-layer, dual-head geometry in an in-beam protocol. The distribution of range shifts in reconstructed PET activity was visualized in the beam’s eye view. Linear prediction models were constructed from all patients in the cohort, using the mean shift in reconstructed PET activity as a predictor of the mean proton range deviation. Main results. Maps of deviations in the range of reconstructed PET distributions showed agreement with those of deviations in dose range in most patients. The linear prediction model showed a good fit, with coefficient of determination r 2 = 0.84 (in-room) and 0.75 (in-beam). Residual standard error was below 1 mm: 0.33 mm (in-room) and 0.23 mm (in-beam). Significance. The precision of the proposed prediction models shows the sensitivity of the proposed J-PET scanners to shifts in proton range for a wide range of clinical treatment plans. Furthermore, it motivates the use of such models as a tool for predicting proton range deviations and opens up new prospects for investigations into the use of intra-treatment PET images for predicting clinical metrics that aid in the assessment of the quality of delivered treatment.

PO-1006 Patient-specific stopping power calibration for proton therapy based on proton radiographic images

PO-1006 Patient-specific stopping power calibration for proton therapy based on proton radiographic images

Regularised patient-specific stopping power calibration for proton therapy planning based on proton radiographic images

Proton transmission imaging has been proposed and investigated as imaging modality complementary to x-ray based techniques in proton beam therapy. In particular, it addresses the issue of range uncertainties due to the conversion of an x-ray patient computed tomography (CT) image expressed in Hounsfield Units (HU) to relative stopping power (RSP) needed as input to the treatment planning system. One approach to exploit a single proton radiographic projection is to perform a patient-specific calibration of the CT to RSP conversion curve by optimising the match between a measured and a numerically integrated proton radiography.In this work, we develop the mathematical tools needed to perform such an optimisation in an efficient and robust way. Our main focus lies on set-ups which combine pencil beam scanning with a range telescope detector, although most of our methods can be employed in combination with other set-ups as well. Proton radiographies are simulated in Monte Carlo using an idealised detector and applying the same data processing chain used with experimental data. This approach allows us to have a ground truth CT-RSP curve to compare the optimisation results with.Our results show that the parameters of the CT-RSP curve are strongly correlated when using a pencil beam based set-up, which leads to unrealistic variation in the optimised CT-RSP curves. To address this issue, we introduce a regularisation procedure which guarantees a plausible degree of smoothness in the optimised CT-RSP curves. We investigate three different methods to perform the numerical projection operation needed to generate a proton digitally reconstructed radiography. We find that the approximate and computationally faster method performs as well as the more accurate but more demanding method. We perform a Monte Carlo experiment based on a head and neck patient to evaluate the range accuracy achievable with the optimised CT-RSP curves and find an agreement with the ground truth expectation of better than . Our results further indicate that the region in the patient in which the proton radiography is acquired does not necessarily have to correspond to the treatment volume to achieve this accuracy. This is important as the imaged region could be freely chosen, e.g. in order to spare organs at risk.

Proton versus photon deep inspiration breath hold technique in patients with hodgkin lymphoma and mediastinal radiation

BackgroundThe benefits of proton therapy in the treatment of patients with Hodgkin lymphoma (HL) are controversially discussed. Therefore we compared intensitiy modulated proton therapy (IMPT) with intensity modulated radiotherapy (IMRT), in the form of volumetric modulated arc therapy (VMAT) in patients with Hodgkin lymphoma (HL), through a comparative treatment planning study.MethodsRadiation plans for 21 patients with Hodgkin Lymphoma (HL) were computed for IMPT and deep inspiration breath hold (DIBH) VMAT. Plans were optimized and computed assuming deep inspiration breath holding conditions. Dosimetric comparison on standard metrics from dose volume histograms was performed to appraise the relative merits of the two techniques, while proton plan robustness was assessed by re-computing the dose distribution of each plan by varying the Hounsfield Units to stopping power calibration by applying a ± 3 and 4% error.ResultsDIBH-VMAT and IMPT both provided excellent coverage, conformity and heterogeneity of the clinical target volume (CTV) and planning target volume (PTV). IMPT reduced mean doses to the breasts, lungs, heart and normal tissue by 38–83%. IMPT significantly reduced mean doses to the heart to < 5 Gy despite bulky mediastinal disease and decreased breast doses in female patients to < 1 Gy. Despite the simulated 3 and 4% miscalibration errors, no remarkable or measurable impact was observed on the organs at risk (OARs).ConclusionsThis is the first comparison between DIBH-VMAT and IMPT in HL treatment. We could demonstrate statistically significant decreases in all dose/volume metrics of the OARs. Regardless of the planning paradigm used, range uncertainties can substantially under dose the PTV, while perhaps not leading to clinically significant deterioration of CTV coverage. With the geometry applied no impact was observed for OARs, suggesting IMPT as a superior technique for potentially reducing future health risks for HL patients.

127 - Improved patient-specific optimization of the stopping power calibration for proton therapy planning using a single optimized proton radiography

Purpose: We present an improved method to calculate patient-specific calibration curves to convert X-ray computed tomography (CT) Hounsfield Unit (HU) to relative stopping powers (RSP) for proton therapy treatment planning. We introduce also an optimization method to improve the spatial resolution and the water equivalent thickness (WET) accuracy of proton radiographies [1]. Materials/Methods: By optimizing the HU-RSP calibration curve, the difference between a proton radiographic image and a digitally reconstructed X-ray radiography (DRR) is minimized. The feasibility of this approach has previously been demonstrated [2,3]. This scenario assumes that all discrepancies between proton radiography and DRR originate from uncertainties in the HU-RSP curve. In reality, external factors cause imperfections in the proton radiography, such as misalignment compared to the DRR and unfaithful representation of geometric structures (“blurring”). We analyze these effects based on synthetic datasets of anthropomorphic phantoms and suggest an extended optimization scheme which explicitly accounts for these effects. Performance of the method is been tested for various simulated irradiation parameters. We also investigate the use of a special optimization method to enhance the spatial resolution and the WET accuracy of proton radiographies prior to the HU-RSP re-calibration. The optimization method is designed for imaging systems measuring only the residual range of protons without relying on tracker detectors to determine the beam trajectory before and after the target. The ultimate purpose of the optimization is to minimize uncertainties in the HU-RSP calibration curve. We therefore suggest and perform a thorough statistical treatment to quantify the accuracy of the optimized HU-RSP curve. Results: We demonstrate that without extending the optimization scheme, spatial blurring (equivalent to FWHM=3mm convolution) in the proton radiographies can cause up to 10% deviation between the optimized and the ground truth HU-RSP calibration curve. Instead, results obtained with our extended method reach 1% or better correspondence, as shown in Figure 1. We have further calculated gamma index maps for different acceptance levels. With DTA=0.5mm and RD=0.5%, a passing ratio of 100% is obtained with the extended method, while an optimization neglecting effects of spatial blurring only reach ~90%. Conclusions: Our contribution underlines the potential of a single optimized proton radiography to generate a patientspecific calibration curve and to improve dose delivery by optimizing the HU-RSP calibration curve as long as all sources of systematic incongruence are properly modeled.

SU‐C‐204‐02: Improved Patient‐Specific Optimization of the Stopping Power Calibration for Proton Therapy Planning Using a Single Proton Radiography

Purpose:We present an improved method to calculate patient‐specific calibration curves to convert X‐ray computed tomography (CT) Hounsfield Unit (HU) to relative stopping powers (RSP) for proton therapy treatment planning.Methods:By optimizing the HU‐RSP calibration curve, the difference between a proton radiographic image and a digitally reconstructed X‐ray radiography (DRR) is minimized. The feasibility of this approach has previously been demonstrated. This scenario assumes that all discrepancies between proton radiography and DRR originate from uncertainties in the HU‐RSP curve. In reality, external factors cause imperfections in the proton radiography, such as misalignment compared to the DRR and unfaithful representation of geometric structures (“blurring”). We analyze these effects based on synthetic datasets of anthropomorphic phantoms and suggest an extended optimization scheme which explicitly accounts for these effects. Performance of the method is been tested for various simulated irradiation parameters. The ultimate purpose of the optimization is to minimize uncertainties in the HU‐RSP calibration curve. We therefore suggest and perform a thorough statistical treatment to quantify the accuracy of the optimized HU‐RSP curve.Results:We demonstrate that without extending the optimization scheme, spatial blurring (equivalent to FWHM=3mm convolution) in the proton radiographies can cause up to 10% deviation between the optimized and the ground truth HU‐RSP calibration curve. Instead, results obtained with our extended method reach 1% or better correspondence. We have further calculated gamma index maps for different acceptance levels. With DTA=0.5mm and RD=0.5%, a passing ratio of 100% is obtained with the extended method, while an optimization neglecting effects of spatial blurring only reach ∼90%.Conclusion:Our contribution underlines the potential of a single proton radiography to generate a patient‐specific calibration curve and to improve dose delivery by optimizing the HU‐RSP calibration curve as long as all sources of systematic incongruence are properly modeled.

SU‐E‐J‐136: Investigation Into Robustness of Stopping Power Calculated by DECT and SECT for Proton Therapy Treatment Planning

Purpose:To investigate the robustness of dual energy CT (DECT) and single energy CT (SECT) proton stopping power calibration techniques and quantify the associated errors when imaging a phantom differing in chemical composition to that used during stopping power calibration.Methods:The CIRS tissue substitute phantom was scanned in a CT‐simulator at 90kV and 140kV. This image set was used to generate a DECT proton SPR calibration based on a relationship between effective atomic number and mean excitation energy. A SECT proton SPR calibration based only on Hounsfield units (HUs) was also generated. DECT and SECT scans of a second phantom of known density and chemical composition were performed. The SPR of the second phantom was calculated with the DECT approach (SPR_DECT),the SECT approach (SPR_SECT) and finally the known density and chemical composition of the phantom (SPR_ref). The DECT and SECT image sets were imported into the Pinnacle3 research release of proton therapy treatment planning. The difference in dose when exposed to a common pencil beam distribution was investigated.Results:SPR_DECT was found to be in better agreement with SPR_ref than SPR_ SECT. The mean difference in SPR for all materials was 0.51% for DECT and 6.89% for SECT. With the exception of Teflon, SPR_DECT was found to agree with SPR_ref to within 1%. Significant differences in calculated dose were found when using the DECT image set or the SECT image set.Conclusion:The DECT calibration technique was found to be more robust to situations in which the physical properties of the test materials differed from the materials used during SPR calibration. Furthermore, it was demonstrated that the DECT and SECT SPR calibration techniques can Result in significantly different calculated dose distributions.

Patient-specific stopping power calibration for proton therapy planning based on single-detector proton radiography

A simple robust optimizer has been developed that can produce patient-specific calibration curves to convert x-ray computed tomography (CT) numbers to relative stopping powers (HU-RSPs) for proton therapy treatment planning. The difference between a digitally reconstructed radiograph water-equivalent path length (DRRWEPL) map through the x-ray CT dataset and a proton radiograph (set as the ground truth) is minimized by optimizing the HU-RSP calibration curve. The function of the optimizer is validated with synthetic datasets that contain no noise and its robustness is shown against CT noise. Application of the procedure is then demonstrated on a plastic and a real tissue phantom, with proton radiographs produced using a single detector. The mean errors using generic/optimized calibration curves between the DRRWEPL map and the proton radiograph were 1.8/0.4% for a plastic phantom and −2.1/ − 0.2% for a real tissue phantom. It was then demonstrated that these optimized calibration curves offer a better prediction of the water equivalent path length at a therapeutic depth. We believe that these promising results are suggestive that a single proton radiograph could be used to generate a patient-specific calibration curve as part of the current proton treatment planning workflow.

WE‐C‐500‐01: Imaging Needs for Proton Therapy

Proton therapy is a rapidly growing treatment modality. Superficially, proton therapy looks identical to x‐ray therapy ‐ the patient receives a treatment planning CT followed by daily image‐guided treatment sessions. However, the specifics of proton therapy impose unique demands on the imaging procedures at each phase of the treatment. In x‐ray therapy the Hounsfield Units (HU) of the CT are converted to relative electron density for megavoltage x‐ray dose calculations. In proton therapy the pertinent value to be determined from HU is proton stopping power for human tissue, usually different from stopping power in the phantom used for measurement. Accurate determination of stopping power from HU requires a unique calibration process and tight controls on CT imaging technique. A number of approaches have emerged in x‐ray therapy for daily localization of soft tissues — the most popular involving placement of fiducial markers or Cone Beam CT (CBCT). However, dose shadowing downstream of fiducials may disallow them from clinical application for proton therapy. CBCT is not yet used for proton therapy, but for some treatment sites daily localization of the tumor may not guarantee dosimetric coverage as variations in the relative positions of tumor and normal tissues in the beam path perturb the proton dose distribution. Direct proton beam imaging — radiography and tomography — are in development. Nuclear activation of tissues by the proton beam presents a unique quality assurance measure in which imaging of the activation could verify the range of the protons in the patient.Learning Objectives:1. Present the unique demands of the treatment simulation process for proton therapy, including HU to stopping power calibration, technique standardization, and artifact reduction.2. Discuss the proton specific challenges in daily target localization including dose shadowing from fiducials, dose perturbation by anatomical changes, and the need for respiratory correlated IGRT.3. Introduce the possibility of proton beam range verification by direct proton beam imaging and post‐treatment activation imaging.Research grant from Varian Medical Systems

A formulation of tissue- and water-equivalent materials using the stoichiometric analysis method for CT-number calibration in radiotherapy treatment planning

Tissue- and water-equivalent materials (TEMs) are widely used in quality assurance and calibration procedures, both in radiodiagnostics and radiotherapy. In radiotherapy, particularly, the TEMs are often used for computed tomography (CT) number calibration in treatment planning systems. However, currently available TEMs may not be very accurate in the determination of the calibration curves due to their limitation in mimicking radiation characteristics of the corresponding real tissues in both low- and high-energy ranges. Therefore, we are proposing a new formulation of TEMs using a stoichiometric analysis method to obtain TEMs for the calibration purposes. We combined the stoichiometric calibration and the basic data method to compose base materials to develop TEMs matching standard real tissues from ICRU Report 44 and 46. First, the CT numbers of six materials with known elemental compositions were measured to get constants for the stoichiometric calibration. The results of the stoichiometric calibration were used together with the basic data method to formulate new TEMs. These new TEMs were scanned to validate their CT numbers. The electron density and the stopping power calibration curves were also generated. The absolute differences of the measured CT numbers of the new TEMs were less than 4 HU for the soft tissues and less than 22 HU for the bone compared to the ICRU real tissues. Furthermore, the calculated relative electron density and electron and proton stopping powers of the new TEMs differed by less than 2% from the corresponding ICRU real tissues. The new TEMs which were formulated using the proposed technique increase the simplicity of the calibration process and preserve the accuracy of the stoichiometric calibration simultaneously.

SU-GG-T-265: Stopping Power for Tissue Equivalent Materials and Hounsfield Numbers for Proton Radiation Treatment Planning: Calculation and Measurements

Purpose: To generate a proton stopping power calibration curve from tissue equivalent materials combined with Hounsfield Unit (HU) values measured by an in house CT scanner for use in proton treatment planning. Method and Materials: Tissue equivalent phantom materials (Gammex Model 467) used for CT calibration were placed at the center of a water tank (40×30×30 cm3) and scanned with the CT scanner to measure their HU values. Image reconstruction was done with 5 mm slice thickness. In addition, the tissue equivalent phantom material relative stopping powers were measured with 197.1 MeV protons whose defining range was 18.28 cm. Ten phantom materials were individually placed in front of a water tank. A PPCO5 parallel plate chamber was used to measure pristine Bragg peaks in water. Results: A stoichiometric calibration method was used to characterize the CT scanner by fitting the measured HU values to the equation HU = ρerel (A Z3.86 + B Z1.86 + C). Parameter A, B, and C are specific to the CT scanner used in this measurement and they correspond to photoelectric, Rayleigh and Compton cross sections. These fit parameters were used to calculate the HU values for the tissues whose compositions are listed in ICRP (1975). In addition, proton relative stopping powers for these materials were calculated by the Bethe-Bloch formula. The relative stopping power for each phantom material was also measured from the difference of the pulled back Bragg peak compared to that of water, and the results were compared. Conclusion: A stopping power versus HU value calibration curve for the proton treatment planning system was generated.

SU‐GG‐T‐323: Verification of the Accuracy of the Proton Range Compensator Scatter Factor and Dose within Inhomogeneity Calculations Using the Eclipse Treatment Planning System

Purpose: To determine the accuracy of the proton range compensator scatter factor (CSF) and dose near inhomogeneity calculations in the Eclipse Treatment Planning System version 8.1 (TPS). Method and Materials: A head phantom was constructed that contains bone‐equivalent structures as well as several channels designed for ion chamber placement. A plan was created in Eclipse using the CT scan of the phantom that calculated dose at several selected reference points. Ion chamber measurements were made with a .125 cc cylindrical Farmer chamber at the same locations in the Eclipse plan. Measurements and calculations in water‐equivalent phantoms were made with and without compensator for clinical patient plans to determine CSF. Results: The measured doses at the selected points in a low dose gradient region were found to be within 1 to 6% of those from TPS. Dose comparisons between measured and Eclipse estimated dose for points lying closer than 1cm from the bone‐equivalent material/water interface, gave dose estimates that were 11% to 14% lower than measured values. The distance to agreement were within the range uncertainties associated with CT number stopping power calibration. In addition, in a homogeneous phantom we calculated the compensator scatter factors (CSF) by taking the ratio of the verification plan dose without the compensator at the samewater to the verification plan dose without the compensator at the same water equivalent depth for 4 clinical patients. We calculated CSF values for these patients between 0.965 and 1.062; compensator and patient scatter factors (CPSF) values from 0.983 to 1.068 and patient scatter factors (PSF) ranging from 0.97 to 1.019. Conclusions: Our measurements indicate that Eclipse reasonably estimates dose within 6% of measurements at regions near the CAX approximately 1cm or further from sharp inhomogeneities.