Design study of anin situPET scanner for use in proton beam therapy

Abstract

Proton beam therapy can deliver high radiation dose to a tumor without significant damage to surrounding healthy tissue or organs. One way of verifying the delivered dose distribution is to image the short-lived positron emitters produced by the proton beam as it travels through the patient. A potential solution to the limitations of PET imaging in proton beam therapy is the development of a high sensitivity, in-situ PET scanner that starts PET imaging almost immediately after patient irradiation while the patient is still lying on the treatment bed. A partial ring PET design is needed for this application in order to avoid interference between the PET detectors and the proton beam, as well restrictions on patient positioning on the couch. A partial ring also allows us to optimize the detector separation (and hence the sensitivity) for different patient sizes. Our goal in this investigation is to evaluate an in-situ PET scanner design for use in proton therapy that provides tomographic imaging in a partial ring scanner design using time-of-flight (TOF) information and an iterative reconstruction algorithm. A GEANT4 simulation of an incident proton beam was used to produce a positron emitter distribution, which was parameterized and then used as the source distribution inside a water-filled cylinder for EGS4 simulations of a PET system. Design optimization studies were performed as a function of crystal type and size, system timing resolution, scanner angular coverage, and number of positron emitter decays. Data analysis was performed to measure the accuracy of the reconstructed positron emitter distribution as well as the range of the positron emitter distribution. We simulated scanners with varying crystal size (2 to 4 mm) and type (LYSO and LaBr3) and our results indicate that 4-mm wide LYSO or LaBr3 crystals (resulting in 4-5-mm spatial resolution) are adequate; for a full-ring, non-TOF scanner we predict a low bias (< 0.6-mm) and a good precision (< 1-mm) in the estimated range relative to the simulated positron distribution. We then varied the angular acceptance of the scanner ranging from 1/2 to 2/3 of 2π; with a partial ring TOF imaging with good timing resolution (≤ 600ps) is necessary to produce accurate tomographic images. A two-third ring scanner with 300ps timing resolution leads to a bias of 1.0-mm and a precision of 1.4-mm in the range estimate. With a timing resolution of 600ps, the bias increases to 2.0-mm while the precision in the range estimate is similar. For a half ring scanner design, more distortions are present in the image, which is characterized by the increased error in the profile difference estimate. We varied the number of positron decays imaged by the PET scanner by an order of magnitude and we observe some decrease in the precision of the range estimate for lower number of decays, but all partial ring scanner designs studied have a precision ≤ 1.5-mm. The largest number tested, 150M total positron decays, is considered realistic for a clinical fraction of delivered dose, while the...

First full-beam PET acquisitions in proton therapy with a modular dual-head dedicated system

“…A way to evaluate the precision of this estimation is to repeat this measurement for each 1 mm 2 thin profile along the beam direction drawn in a defined region of interest (ROI) within the activated region. For irradiations along the axial direction of the scanner, the profiles are naturally defined by lines of voxels in the image space (Helmbrecht et al 2012, Surti et al 2011.…”

A simulation study of a C-shaped in-beam PET system for dose verification in carbon ion therapy

“…The goal of radiation therapy is to deliver the maximum dose of radiation to the tumor while minimizing the exposure of normal tissue [1]. To achieve this goal in cancer treatment, hadron radiotherapy using carbon ion beams has evolved into a practical modality due to its unique energy deposition at the Bragg peak and its high relative biological effectiveness (RBE) [2].…”

“…Due to practical limitations of the off-line and in-room techniques, dedicated in-beam PET combined with therapy beam-line is the most clinically useful technique for on-line investigation of the positron emitters induced by hadron irradiation. To avoid interference between the PET detectors and the hadron beam, various in-beam PET scanners have been devised, such as a partial-ring PET with limited angular coverage [1] and an OpenPET design with gaps between detector rings [13].…”

Monitoring proton therapy with PET

“…18 Furthermore, image resolution could be improved by in-beam time-of-flight PET detectors. 19 Time of flight also offers the possibility of partial ring in-beam PET detectors without artefacts. In-beam detectors are able to collect data earlier, before significant radiological or biological decay.…”

“…In particular, a kinetic modelling approach is being developed to derive washout-free PET activity production maps using built-in time-activity information contained in dynamic PET data. 19 A possible alternative approach for in vivo range verification is using prompt g-rays. This technique is based on the detection of prompt g-radiation emitted after nuclear excitation by the proton beam.…”