M. Morrocchi

INSIDE in-beam positron emission tomography system for particle range monitoring in hadrontherapy

Abstract. The quality assurance of particle therapy treatment is a fundamental issue that can be addressed by developing reliable monitoring techniques and indicators of the treatment plan correctness. Among the available imaging techniques, positron emission tomography (PET) has long been investigated and then clinically applied to proton and carbon beams. In 2013, the Innovative Solutions for Dosimetry in Hadrontherapy (INSIDE) collaboration proposed an innovative bimodal imaging concept that combines an in-beam PET scanner with a tracking system for charged particle imaging. This paper presents the general architecture of the INSIDE project but focuses on the in-beam PET scanner that has been designed to reconstruct the particles range with millimetric resolution within a fraction of the dose delivered in a treatment of head and neck tumors. The in-beam PET scanner has been recently installed at the Italian National Center of Oncologic Hadrontherapy (CNAO) in Pavia, Italy, and the commissioning phase has just started. The results of the first beam test with clinical proton beams on phantoms clearly show the capability of the in-beam PET to operate during the irradiation delivery and to reconstruct on-line the beam-induced activity map. The accuracy in the activity distal fall-off determination is millimetric for therapeutic doses.

Full-beam performances of a PET detector with synchrotron therapeutic proton beams

Treatment quality assessment is a crucial feature for both present and next-generation ion therapy facilities. Several approaches are being explored, based on prompt radiation emission or on PET signals by [Formula: see text]-decaying isotopes generated by beam interactions with the body. In-beam PET monitoring at synchrotron-based ion therapy facilities has already been performed, either based on inter-spill data only, to avoid the influence of the prompt radiation, or including both in-spill and inter-spill data. However, the PET images either suffer of poor statistics (inter-spill) or are more influenced by the background induced by prompt radiation (in-spill). Both those problems are expected to worsen for accelerators with improved duty cycle where the inter-spill interval is reduced to shorten the treatment time. With the aim of assessing the detector performance and developing techniques for background reduction, a test of an in-beam PET detector prototype was performed at the CNAO synchrotron-based ion therapy facility in full-beam acquisition modality. Data taken with proton beams impinging on PMMA phantoms showed the system acquisition capability and the resulting activity distribution, separately reconstructed for the in-spill and the inter-spill data. The coincidence time resolution for in-spill and inter-spill data shows a good agreement, with a slight deterioration during the spill. The data selection technique allows the identification and rejection of most of the background originated during the beam delivery. The activity range difference between two different proton beam energies (68 and 72 MeV) was measured and found to be in sub-millimeter agreement with the expected result. However, a slightly longer (2 mm) absolute profile length is obtained for in-spill data when compared to inter-spill data.

Online proton therapy monitoring: clinical test of a Silicon-photodetector-based in-beam PET

Particle therapy exploits the energy deposition pattern of hadron beams. The narrow Bragg Peak at the end of range is a major advantage but range uncertainties can cause severe damage and require online verification to maximise the effectiveness in clinics. In-beam Positron Emission Tomography (PET) is a non-invasive, promising in-vivo technique, which consists in the measurement of the β+ activity induced by beam-tissue interactions during treatment, and presents the highest correlation of the measured activity distribution with the deposited dose, since it is not much influenced by biological washout. Here we report the first clinical results obtained with a state-of-the-art in-beam PET scanner, with on-the-fly reconstruction of the activity distribution during irradiation. An automated time-resolved quantitative analysis was tested on a lacrimal gland carcinoma case, monitored during two consecutive treatment sessions. The 3D activity map was reconstructed every 10 s, with an average delay between beam delivery and image availability of about 6 s. The correlation coefficient of 3D activity maps for the two sessions (above 0.9 after 120 s) and the range agreement (within 1 mm) prove the suitability of in-beam PET for online range verification during treatment, a crucial step towards adaptive strategies in particle therapy.

First results of the INSIDE in-beam PET scanner for the on-line monitoring of particle therapy treatments

Quality assessment of particle therapy treatments by means of PET systems has been carried out since late `90 and it is one of the most promising in-vivo non invasive monitoring techniques employed clinically. It can be performed with a diagnostic PET scanners installed outside the treatment room (off-line monitoring) or inside the treatment room (in-room monitoring). However the most efficient way is by integrating a PET scanner with the treatment delivery system (on-line monitoring) so that the biological wash out and the patient repositioning errors are minimized. In this work we present the performance of the in-beam PET scanner developed within the INSIDE project. The INSIDE PET scanner is made of two planar heads, 10 cm wide (transaxially) and 25 cm long (axially), composed of pixellated LFS crystals coupled to Hamamatsu MPPCs. Custom designed Front-End Electronics (FE) and Data AcQuisition (DAQ) systems allow an on-line reconstruction of PET images from separated in-spill and inter-spill data sets. The INSIDE PET scanner has been recently delivered at the CNAO (Pavia, Italy) hadrontherapy facility and the first experimental measurements have been carried out. Homogeneous PMMA phantoms and PMMA phantoms with small air and bone inserts were irradiated with monoenergetic clinical proton beams. The activity range was evaluated at various benchmark positions within the field of view to assess the homogeneity of response of the PET system. Repeated irradiations of PMMA phantoms with clinical spread out Bragg peak proton beams were performed to evaluate the reproducibility of the PET signal. The results found in this work show that the response of the INSIDE PET scanner is independent of the position within the radiation field. Results also show the capability of the INSIDE PET scanner to distinguish variations of the activity range due to small tissue inhomogeneities. Finally, the reproducibility of the activity range measurement was within 1 mm.

The TRIMAGE PET Data Acquisition System: Initial Results

We present the first results obtained with a prototype of the PET read-out electronics of the trimodal PET/MRI/EEG TRIMAGE scanner. The read-out is based on the 64-channel TRIROC ASIC and on an acquisition board that will control up to 12 ASICs. The output of each ASIC is processed in parallel and sent to a host system that in the final version will receive data from 18 acquisition boards. Blocks of 64 SiPMs are one-to-one coupled to a dual-layer staggered LYSO crystal matrix and read by a single ASIC. The FPGA reads the sparse output from the ASICs and reconstructs for each event a full image of the light pattern coming from the LYSO matrix. This pattern can be then processed on-line or sent to the host PC for post-processing. Early tests were conducted by using a prototype board with single LYSO crystals of

A detector module composed of pixellated crystals coupled to SiPM strips

{3.3} \textrm {mm}\times {3.3} \textrm {mm} \times {8} \textrm {mm}

SiPM-based PET module with depth of interaction

and dual layer staggered LYSO matrices. Results show that the ASIC can sustain input rates above 58 kHz on all its channels, with small variations depending on the discriminating thresholds, being this limit due its digital output stage. With the single crystals setup, we obtained an energy resolution of 10.7% at 511 keV and a coincidence time resolution of 420 ps FWHM. With the staggered matrix the obtained mean energy resolution was 16% on the top layer and 18% on the bottom layer. The flood maps obtained with the LYSO matrix setup show that the pixels on both the staggered levels are clearly identifiable.

DoPET: an in-treatment monitoring system for proton therapy at 62 MeV

A detector based on a pixellated scintillator crystal coupled on two opposite sides to Silicon Photomultiplier (SiPM) strips is presented. In one direction the width of the SiPM strips matches the crystal pitch, while in the other direction the strip length is equal to the crystal pitch times the number of pixels in a row. The SiPM strips on one side are orthogonal to the strips on the other side. The crystal position can be identified using a row-column coding method. As a proof of concept, a small prototype using an array of 8 × 8 LYSO crystals, each one 1.5 mm × 1.5 mm × 10 mm in dimensions, has been built. The crystal is coupled on both sides to monolithic matrices composed of 8 SiPM strips, each one 1.5 mm wide (pitch) and 12 mm long by means of silicon grease. SiPMs strips have been obtained connecting in parallel single pixels belonging to a monolithic matrix, where each pixel has the same pitch of the scintillating crystal coupled to it. This arrangement allows a reduction from N2 to 2N of the number of analog channels needed to read-out the entire crystal array. Furthermore, this method provides the information about the Depth of Interaction of the primary particles impinging on the detector. The results of the prototype characterization in terms of energy and Depth Of Interaction resolution capabilities are presented here.

Monte Carlo simulation tool for online treatment monitoring in hadrontherapy with in-beam PET: A patient study

Silicon Photomultipliers are used in many new generation PET block detectors. High granularity pixel SiPMs allow a high precision measurement of the photon interaction coordinates along the crystal surface. In order to further improve the resolution it is necessary to measure the photon Depth of Interaction (DOl), so as to reduce the parallax error in the Line of Response reconstruction. An innovative technique for DOl determination is proposed and tested. Measurements are made with a 2 cm × 2 cm × 1 cm LYSO slab with readout on the front and back large sides by means of two 4 × 4 square SiPM pixel matrices of 5 mm pitch. The data acquisition is based on the new BASIC32 chip read out with an FPGA-based system.

TRIMAGE: A dedicated trimodality (PET/MR/EEG) imaging tool for schizophrenia

Proton beam radiotherapy is highly effective in treating cancer thanks to its conformal dose deposition. This superior capability in dose deposition has led to a massive growth of the treated patients around the world, raising the need of treatment monitoring systems. An in-treatment PET system, DoPET, was constructed and tested at CATANA beam-line, LNS-INFN in Catania, where 62 MeV protons are used to treat ocular melanoma. The PET technique profits from the beta+ emitters generated by the proton beam in the irradiated body, mainly 15-O and 11-C. The current DoPET prototype consists of two planar 15 cm × 15 cm LYSO-based detector heads. With respect to the previous versions, the system was enlarged and the DAQ up-graded during the years so now also anthropomorphic phantoms, can be fitted within the field of view of the system. To demonstrate the capability of DoPET to detect changes in the delivered treatment plan with respect to the planned one, various treatment plans were used delivering a standard 15 Gy fraction to an anthropomorphic phantom. Data were acquired during and after the treatment delivery up to 10 minutes. When the in-treatment phase was long enough (more than 1 minute), the corresponding activated volume was visible just after the treatment delivery, even if in presence of a noisy background. The after-treatment data, acquired for about 9 minutes, were segmented finding that few minutes are enough to be able to detect changes. These experiments will be presented together with the studies performed with PMMA phantoms where the DoPET response was characterized in terms of different dose rates and in presence of range shifters: the system response is linear up to 16.9 Gy/min and has the ability to see a 1 millimeter range shifter.