Proton radiography and proton computed tomography based on time-resolved dose measurements

Abstract

We present a proof of principle study of proton radiography and proton computed tomography (pCT) based on time-resolved dose measurements. We used a prototype, two-dimensional, diode-array detector capable of fast dose rate measurements, to acquire proton radiographic images expressed directly in water equivalent path length (WEPL). The technique is based on the time dependence of the dose distribution delivered by a proton beam traversing a range modulator wheel in passive scattering proton therapy systems. The dose rate produced in the medium by such a system is periodic and has a unique pattern in time at each point along the beam path and thus encodes the WEPL. By measuring the time dose pattern at the point of interest, the WEPL to this point can be decoded. If one measures the time–dose patterns at points on a plane behind the patient for a beam with sufficient energy to penetrate the patient, the obtained 2D distribution of the WEPL forms an image. The technique requires only a 2D dosimeter array and it uses only the clinical beam for a fraction of second with negligible dose to patient. We first evaluated the accuracy of the technique in determining the WEPL for static phantoms aiming at beam range verification of the brain fields of medulloblastoma patients. Accurate beam ranges for these fields can significantly reduce the dose to the cranial skin of the patient and thus the risk of permanent alopecia. Second, we investigated the potential features of the technique for real-time imaging of a moving phantom. Real-time tumor tracking by proton radiography could provide more accurate validations of tumor motion models due to the more sensitive dependence of proton beam on tissue density compared to x-rays. Our radiographic technique is rapid (~100 ms) and simultaneous over the whole field, it can image mobile tumors without the problem of interplay effect inherently challenging for methods based on pencil beams. Third, we present the reconstructed pCT images of a cylindrical phantom containing inserts of different materials. As for all conventional pCT systems, the method illustrated in this work produces tomographic images that are potentially more accurate than x-ray CT in providing maps of proton relative stopping power (RSP) in the patient without the need for converting x-ray Hounsfield units to proton RSP. All phantom tests produced reasonable results, given the currently limited spatial and time resolution of the prototype detector. The dose required to produce one radiographic image, with the current settings, is ~0.7 cGy. Finally, we discuss a series of techniques to improve the resolution and accuracy of radiographic and tomographic images for the future development of a full-scale detector.

A novel design of proton computed tomography detected by multiple‐layer ionization chamber with strip chambers: A feasibility study with Monte Carlo simulation

“…Thus, images acquired by proton integrating system suffered from edge artifacts at the interfaces between materials, 15 overall low spatial resolution, and relatively large SPR error. 22 In 2018, Zhang et al developed a proton integrating radiography system that measured the so-called time-resolved dose rate functions (DRFs), and derived water equivalent path length based on intensity weighted root-mean-square of DRFs. 23 The error of SPR was claimed to be within AE 1%, and the detector pixel pitch was 0.388 mm, which was promising for clinical use.…”

Proof of principle of helium‐beam radiography using silicon pixel detectors for energy deposition measurement, identification, and tracking of single ions

“…Most of these works focus on the usage of carbon ions due to the less pronounced MCS, which is especially important for nontracking systems. The reported applied doses for these systems are rather high, for example, 7 mGy for one radiograph or 8 Gy for one CT …”

“…Integrating systems measure the integrated signal of the particles downstream of the object being imaged (pRad: 14,15 ; iRad: 16 ; pCT: 17 ; iCT: 16,[18][19][20]. Most of these works focus on the usage of carbon ions due to the less pronounced MCS, which is especially important for nontracking systems.…”

Dose ratio proton radiography using the proximal side of the Bragg peak

“…Previously, it has been shown that the patientspecific calibration curves (converting the x-ray CT dataset to units of RSP) can be generated through an optimization procedure together with the original x-ray CT. 8 In addition to reducing the proton beam range uncertainty in proton treatment planning, proton radiography has a number of other potential applications, including convenient patient setup, 9 passive range verification, 8 and active range verification. 10 One of the major disadvantages of proton radiography is due to the fact that protons undergo multiple Coulomb scattering (MCS) in the patient, which limits the spatial resolution compared to x-ray imaging. 11 This physical process has driven the classical approach to proton radiography in which attempts are made to correct for MCS by tracking protons before and after the patient and by measuring the residual proton energy at the patient exit.…”

“…3,17,35 The measurement of dose offers a simple approach to proton radiography. A range of such techniques have been tested, including using pairs of sloped spread-out Bragg peaks (SOBPs), 36,37 measuring the time-resolved dose from a proton beam passively scattered by a range modulator wheel, 10,38,39 and by measuring the ratio of doses of two pristine Bragg peaks, 40 which is the subject of this work. The technique works as follows: (1) measure depth dose curves in water for two pristine Bragg peaks; (2) determine the "dose ratio curve" for the energy pair by dividing the lower energy by the higher energy; (3) record the dose map beyond an object of unknown thickness/material for each of the two energies and calculate the dose ratio map; and (4) convert to a WET using the dose ratio curve.…”