A comparison of carbon ions versus protons for integrated mode ion imaging

BACKGROUND

Incorporating image guidance into ion beam therapy is critical for minimizing beam range uncertainties and realizing the modality's potential. One promising avenue for image guidance is to capture transmission ion radiographs (iRads) before and/or during treatment. iRad image quality is typically maximized using a single-event imaging system, which involves tracking individual ions, albeit the approach is generally not suited to clinical beam settings. An alternative faster and clinically compatible method is integrated mode imaging, where individual pencil beam data is acquired, rather than single ion data. To evaluate the usefulness of transmission ion imaging for image guidance, it is crucial to evaluate the image quality of integrated mode iRad systems.

PURPOSE

We report extensive image quality metrics of integrated mode carbon ion radiographs (cRads) and compare them with proton radiographs (pRads).

METHODS

iRads were obtained at the Marburg Ion Beam Therapy Center using a plastic volumetric scintillator equipped with CCD cameras. The detector captures orthogonal views of the 3D energy deposition in the scintillator from individual pencil beams. Four phantoms were scanned using a15 × 15 cm 2 $15\times 15 \ {\rm cm}^2$ field of view and a beam spacing of 1 mm. First, 9 tissue-substitute inserts were used to evaluate water equivalent thickness (WET) accuracy. Radiographs of those inserts were reconstructed for beam spacings ranging from 1 to 7 mm to evaluate the impact of spacing on quantitative accuracy. For spatial resolution, custom 3D printed line pair (lp) modules ranging from 0.5 to 10 lp/cm were scanned. To evaluate low contrast detectability, a custom 3D printed low contrast module consisting of 20 holes with depths ranging from 1 to 8 mm and diameters from 1 to 10 mm was scanned. iRads of an anthropomorphic head phantom were also obtained.

RESULTS

Spatial resolution and low contrast detection are systematically improved for cRads compared to pRads. Image resolution was 3.7 lp/cm for cRads and 1.7 lp/cm for pRads in the center of the field of view. Spatial resolution was found to vary with the object's location in the field of view. While pRads could mostly resolve low contrast holes of 10 mm in diameter, cRads could resolve holes of up in 4 mm diameter. WET accuracy is similar for both ion species, with a root mean squared error of approximately 1 mm. WET accuracy was stable (maximum of 0.1 mm increase) across beam spacings, although important under-sampling artifacts were observed for iRads reconstructed using large beam spacings, especially for cRads. iRads of the anthropomorphic head phantom showed improved apparent contrast using cRads, especially to identify bony structures.

CONCLUSIONS

This work is the first investigation of image quality metrics such as spatial resolution and low contrast detectability for integrated mode cRads, with a full comparison with pRads. Enhanced image quality is obtained with cRads compared to pRads, although pRads still maintain high WET accuracy and deliver image quality within acceptable bounds.

Integrated-mode proton radiography with 2D lateral projections

Integrated-mode proton radiography leading to water equivalent thickness (WET) maps is an avenue of interest for motion management, patient positioning, andin vivorange verification. Radiographs can be obtained using a pencil beam scanning setup with a large 3D monolithic scintillator coupled with optical cameras. Established reconstruction methods either (1) involve a camera at the distal end of the scintillator, or (2) use a lateral view camera as a range telescope. Both approaches lead to limited image quality. The purpose of this work is to propose a third, novel reconstruction framework that exploits the 2D information provided by two lateral view cameras, to improve image quality achievable using lateral views. The three methods are first compared in a simulated Geant4 Monte Carlo framework using an extended cardiac torso (XCAT) phantom and a slanted edge. The proposed method with 2D lateral views is also compared with the range telescope approach using experimental data acquired with a plastic volumetric scintillator. Scanned phantoms include a Las Vegas (contrast), 9 tissue-substitute inserts (WET accuracy), and a paediatric head phantom. Resolution increases from 0.24 (distal) to 0.33 lp mm-1(proposed method) on the simulated slanted edge phantom, and the mean absolute error on WET maps of the XCAT phantom is reduced from 3.4 to 2.7 mm with the same methods. Experimental data from the proposed 2D lateral views indicate a 36% increase in contrast relative to the range telescope method. High WET accuracy is obtained, with a mean absolute error of 0.4 mm over 9 inserts. Results are presented for various pencil beam spacing ranging from 2 to 6 mm. This work illustrates that high quality proton radiographs can be obtained with clinical beam settings and the proposed reconstruction framework with 2D lateral views, with potential applications in adaptive proton therapy.

Carbon ion radiography with a composite ionization chamber detector

Range uncertainty in carbon ion therapy can diminish treatment efficacy because it may cause deviation from the planned dose distribution. The precise and accurate determination of relative stopping power (RSP) maps of carbon ions in the patient is a direct solution to this problem. To obtain RSP maps in patients undergoing carbon ion radiography, our team developed a preliminary prototype of a composite ionization chamber detection (CICD) system. The CICD prototype employs synchronously gated integral electronics with the ability to measure the depth-to-dose curve and the beam profile simultaneously. Carbon ion radiography experiments were performed on hemispherical, sloped, and stepped phantoms using the Heavy Ion Medical Machine (HIMM) beam. The beam energy was 190.19 MeV/μ and the beam spot full width at half maximum (FWHM) was 7.42 mm. The radiographic image of the sloped phantom, the thickness prediction accuracy of each pixel (2 mm) is 88.25%, its absolute mean error (AME) is 1.07 mm, and the maximum absolute deviation (MAD) is 2.64 mm. The prediction accuracy of the CICD prototype is mainly affected by electronic noise, with a noise-to-signal ratio (NSR) of about 14.36 dB. Carbon ion radiography simulations were performed in this study using Geant4 software to eliminate the effect of the electronic noise. The thickness prediction accuracy is 98.54%, 98.62%, and 99.07% per pixel for hemispherical, sloped and stepped phantoms, respectively, with AME of 0.09 mm, 0.27 mm, and 0.48 mm. Carbon ion radiography utilizing the CICD prototype scheme has the ability to refine the accuracy and resolution of radiographic images, consequently establishing a scientific foundation for diminishing the effects of range uncertainty and fully exploiting the advantages of precision particle therapy.

The impact of path estimates in iterative ion CT reconstructions for clinical-like cases

Ion computed tomography (CT) promises to mitigate range uncertainties inherent in the conversion of x-ray Hounsfield units into ion relative stopping power (RSP) for ion beam therapy treatment planning. To improve accuracy and spatial resolution of ion CT by accounting for statistical multiple Coulomb scattering deflection of the ion trajectories from a straight line path (SLP), the most likely path (MLP) and the cubic spline path (CSP) have been proposed. In this work, we use FLUKA Monte Carlo simulations to investigate the impact of these path estimates in iterative tomographic reconstruction algorithms for proton, helium and carbon ions. To this end the ordered subset simultaneous algebraic reconstruction technique was used and coupled with a total variation superiorization (TVS). We evaluate the image quality and dose calculation accuracy in proton therapy treatment planning of cranial patient anatomies. CSP and MLP generally yielded nearly equal image quality with an average RSP relative error improvement over the SLP of 0.6%, 0.3% and 0.3% for proton, helium and carbon ion CT, respectively. Bone and low density materials have been identified as regions of largest enhancement in RSP accuracy. Nevertheless, only minor differences in dose calculation results were observed between the different models and relative range errors of better than 0.5% were obtained in all cases. Largest improvements were found for proton CT in complex scenarios with strong heterogeneities along the beam path. The additional TVS provided substantially reduced image noise, resulting in improved image quality in particular for soft tissue regions. Employing the CSP and MLP for iterative ion CT reconstructions enabled improved image quality over the SLP even in realistic and heterogeneous patient anatomy. However, only limited benefit in dose calculation accuracy was obtained even though an ideal detector system was simulated.

Deformable image registration of the treatment planning CT with proton radiographies in perspective of adaptive proton therapy

The purpose of this work is to investigate the potentiality of using a limited number of in-room proton radiographies to compensate anatomical changes in adaptive proton therapy. The treatment planning CT is adapted to the treatment delivery scenario relying on 2D-3D deformable image registration (DIR). The proton radiographies, expressed in water equivalent thickness (WET) are simulated for both list-mode and integration-mode detector configurations in pencil beam scanning. Geometrical and analytical simulations of an anthropomorphic phantom in the presence of anatomical changes due to breathing are adopted. A Monte Carlo simulation of proton radiographies based on a clinical CT image in the presence of artificial anatomical changes is also considered. The accuracy of the 2D-3D DIR, calculated as root mean square error, strongly depends on the considered anatomical changes and is considered adequate for promising adaptive proton therapy when comparable to the accuracy of conventional 3D-3D DIR. In geometrical simulation, this is achieved with a minimum of eight/nine radiographies (more than 90% accuracy). Negligible improvement (sim1%) is obtained with the use of 180 radiographies. Comparing different detector configurations, superior accuracy is obtained with list-mode than integration-mode max (WET with maximum occurrence) and mean (average WET weighted by occurrences). Moreover, integration-mode max performs better than integration-mode mean. Results are minimally affected by proton statistics. In analytical simulation, the anatomical changes are approximately compensated (about 60%-70% accuracy) with two proton radiographies and minor improvement is observed with nine proton radiographies. In clinical data, two proton radiographies from list-mode have demonstrated better performance than nine from integration-mode (more than 100% and about 50%-70% accuracy, respectively), even avoiding the finer grid spacing of the last numerical optimization stage. In conclusion, the choice of detector configuration as well as the amount and complexity of the considered anatomical changes determine the minimum number of radiographies to be used.