Fast in situ image reconstruction for proton radiography

Abstract

Objective Proton beam therapy is an emerging modality for cancer treatment that, compared to X-ray radiation therapy, promises to provide better dose delivery to clinical targets with lower doses to normal tissues. Crucial to accurate treatment planning and dose delivery is knowledge of the water equivalent path length (WEPL) of each ray, or pencil beam, from the skin to every point in the target. For protons, this length is estimated from relative stopping power based on X-ray Hounsfield units. Unfortunately, such estimates lead to 3 to 4% uncertainties in the proton range prediction. Therefore, protons in the Bragg peak may overshoot (or undershoot) the desired stopping depth in the target causing tissue damage beyond the target volume. Recent studies indicate that tomographic imaging using protons has the potential to provide directly more accurate measurement of RSPs with significantly lower radiation dose than X-rays. We are currently working on a proton radiography system that promises to provide accurate two-dimensional (2D) images of WEPL values for protons that pass through the body. These will be suitable for positioning and range verification in daily treatments. In this study, we demonstrate that this system is capable of rapidly achieving such accurate images in clinically meaningful times. Methods We have developed a software platform to characterize the potential performance of the prototype proton radiography system. We use Geant4 to simulate raw data detected by the device. An especially written software -pRadwas written to process these data as they are received and uses iterative methods to generate radiographs. The software has been designed to generate a radiograph from a few million protons in under a minute after receiving the first proton from the device. We used a head phantom with known chemical compositions that could be modeled quite accurately in Geant4 simulations of proton radiographs. The radiographs are displayed as pixelated WEPL values displayed on a 2D gray scale image of WEPL values. Results Rapid radiograph reconstruction of 3D phantoms using simulated proton pencil beams have been achieved with our software platform. On a modest desktop computer with a single central processing unit (CPU) and a single graphics processing unit (GPU), it takes about 11 s to reconstruct images using iterative linear algorithms to reconstruct a radiograph from 7.6 million protons. For the radiographic reconstructions of the head phantom described here, the mean WEPL errors, in the proton radiograph using a large majority of the pixels in the complete image were less than 1 mm when compared to images obtained without proton scattering and without detector resolution included.

Technical Note: A fast and monolithic prototype clinical proton radiography system optimized for pencil beam scanning

“…8. The reconstruction algorithm, described in Ordoñez, et al, identifies the appropriate energy (energies) for each pixel and eliminates protons with other energies 11 . As long as the patient‐specific scan has adequate coverage, the resulting radiograph will be identical to those created with full‐field uniform scans.…”

“…7(b) and 7(c). The radiograph was reconstructed using the image reconstruction techniques described in Ordoñez, et al 11 The reconstruction algorithm automatically removes protons that did not stop in the energy detector. This means that the proton radiograph created with the full‐field uniform scan will be identical.…”

“…On the other hand, the use of lower‐energy protons will result in more extensive multiple Coulomb scattering and a further reduction of spatial resolution. As described previously, the use of an image reconstruction method for proton radiography that employs a most likely path algorithm where the exit angle is unknown can partially compensate for this loss of spatial resolution 11,14 . Simulations comparing the use of one downstream tracker plane to two downstream tracker planes indicate the uncertainty of the path estimation is typically 0.3 mm larger for the single plane case.…”

“…The water equivalent thickness (WET) is calculated as the most likely WEPL value in each pixel and displayed as a grey‐scale proton radiographic image. The image reconstruction methods used to produce a proton radiographic image from the WEPL data produced by this system are described in detail by Ordoñez, et al 11 …”

Analysis of characteristics of images acquired with a prototype clinical proton radiography system

“…ProtonVDA LLC developed a clinical prototype proton radiography system, shown in Fig. 1, with design considerations to easily transition into a proton treatment room as described by DeJongh, et al 9 The associated image reconstruction software, as described by Ordoñez, et al, 10 produces clinical proton radiographic images with less than 1 minute of reconstruction time. In order for the clinical implementation to be successful, the proton radiographic images need to possess good and useful image quality while maintaining a low dose to the patient.…”