S. Vecchio

Experimental validation of the filtering approach for dose monitoring in proton therapy at low energy

The higher physical selectivity of proton therapy demands higher accuracy in monitoring of the delivered dose, especially when the target volume is located next to critical organs and a fractionated therapy is applied. A method to verify a treatment plan and to ensure the high quality of the hadrontherapy is to use Positron Emission Tomography (PET), which takes advantage of the nuclear reactions between protons and nuclei in the tissue during irradiation producing beta(+)-emitting isotopes. Unfortunately, the PET image is not directly proportional to the delivered radiation dose distribution; this is the reason why, at the present time, the verification of depth dose profiles with PET techniques is limited to a comparison between the measured activity and the one predicted for the planned treatment by a Monte Carlo model. In this paper we test the feasibility of a different scheme, which permits to reconstruct the expected PET signal from the planned radiation dose distribution along beam direction in a simpler and more direct way. The considered filter model, based on the description of the PET image as a convolution of the dose distribution with a filter function, has already demonstrated its potential applicability to beam energies above 70 MeV. Our experimental investigation provides support to the possibility of extending the same approach to the lower energy range ([40, 70] MeV), in the perspective of its clinical application in eye proton therapy.

A PET Prototype for “In-Beam” Monitoring of Proton Therapy

The in-beam PET is a novel PET application to image the beta+ activity induced in biological tissues by hadronic therapeutic beams. Thanks to the correlation existing between beam-delivered dose profiles and beam-induced activity profiles, in vivo information about the effective ion paths can be extracted from the in-beam pet image. in situ measurements, immediately after patient irradiation, are recommended in order to exploit the maximum statistics, by also detecting the contribution provided by the very short lived isotopes, e.g. 15O. A compact, dedicated tomograph should then be developed for such an application, so as to be used in the treatment room. We developed a small PET prototype in order to demonstrate the feasibility of such a technique for the monitoring of proton therapy of ocular tumors at the CATANA facility (Catania, Italy). The prototype consists of two planar heads with an active area of about 5 cm times 5 cm. Each head is made up of a square position sensitive photomultiplier (Hamamatsu H8500) coupled to a matrix of the same size of LYSO scintillating crystals (2 mm times 2 mm times 18 mm pixel dimensions). Dedicated, compact electronic boards are used for the signal multiplexing, amplification and digitization. The distance between the pair can be varied from 10 cm up to a maximum of about 20 cm. The validation of the prototype was performed on plastic phantoms using 62 MeV protons at the CATANA beam line. Different dose distributions were delivered and a good correlation between the distal fall-off of the activity profiles and of the dose profiles was found, i.e., better than 2 mm along the beam direction.

A Time Efficient Optical Model for GATE Simulation of a LYSO Scintillation Matrix Used in PET Applications

A time efficient optical model is proposed for GATE simulation of a LYSO scintillation matrix coupled to a photomultiplier. The purpose is to avoid the excessively long computation time when activating the optical processes in GATE. The usefulness of the model is demonstrated by comparing the simulated and experimental energy spectra obtained with the dual planar head equipment for dosimetry with a positron emission tomograph (DoPET). The procedure to apply the model is divided in two steps. Firstly, a simplified simulation of a single crystal element of DoPET is used to fit an analytic function that models the optical attenuation inside the crystal. In a second step, the model is employed to calculate the influence of this attenuation in the energy registered by the tomograph. The use of the proposed optical model is around three orders of magnitude faster than a GATE simulation with optical processes enabled. A good agreement was found between the experimental and simulated data using the optical model. The results indicate that optical interactions inside the crystal elements play an important role on the energy resolution and induce a considerable degradation of the spectra information acquired by DoPET. Finally, the same approach employed by the proposed optical model could be useful to simulate a scintillation matrix coupled to a photomultiplier using single or dual readout scheme.

Characterization of an in-beam PET prototype for proton therapy with different target composition

Positron emission tomography is a valuable method for in situ and non invasive monitoring of the accuracy of the treatment in hadron therapy. It takes advantage of short lived β+- emitters spontaneously produced in the biological tissues during irradiation, by means of projectile and/or target nuclei fragmentations. Although β+- emitter production cross-sections and hadron stopping power exhibit a different dependence on the hadron energy, it is still possible to extract non invasively in vivo information about dose localization reconstructing the distribution of positron annihilation points.

Characterization of an In-Beam PET Prototype for Proton Therapy With Different Target Compositions

At the University of Pisa, the DoPET (Dosimetry with a Positron Emission Tomograph) project has focused on the development and characterization of an ad hoc, scalable, dual-head PET prototype for in-beam treatment planning verification of the proton therapy. In this paper we report the first results obtained with our current prototype, consisting of two opposing lutetium yttrium orthosilicate (LYSO) detectors, each one covering an area of 4.5 × 4.5 cm2. We measured the β+-activation induced by 62 MeV proton beams at Catana facility (LNS, Catania, Italy) in several plastic phantoms. Experiments were performed to evaluate the possibility to extract accurate phantom geometrical information from the reconstructed PET images. The PET prototype proved its capability of locating small air cavities in homogeneous PMMA phantoms with a submillimetric accuracy and of distinguishing materials with different 16O and 12C content by back mapping phantom geometry through the separation of the isotope contributions. This could be very useful in the clinical practice as a tool to highlight anatomical or physiological organ variations among different treatment sessions and to discriminate different tissue types, thus providing feedbacks for the accuracy of dose deposition.

A time efficient optical model for GATE simulation of a LYSO scintillation matrix used in PET applications

The dual planar head DoPET (Dosimetry with a Positron Emission Tomograph) tomograph was simulated using GATE (Geant4 Application for Emission Tomography) to evaluate the DoPET performance and its agreement with simulation results including all the relevant physical aspects. An optical model was proposed to predict with accuracy the asymmetrical deterioration of the energy resolution of the detector and at the same time to avoid a too long computation time when activating the optical processes in GATE. The proposed optical model was shown to be around three orders of magnitude faster than a DoPET simulation with GATE optical processes enabled. A good agreement was found between experimental and simulated data using the optical model. This points out that optical interactions inside the crystal elements induce a predictable degradation of the spectra information as acquired by DoPET. Finally, our proposed optical model could be useful to simulate a scintillation matrix and its read out by a position sensitive photomultiplier commonly employed in PET detectors.

A flexible acquisition system for modular dual head Positron Emission Mammography

We have developed a dedicated scanner for Positron Emission Mammography, equipped with a new detection architecture that enhances its flexibility and reduces dead time. The scanner is going to use Luthetium based scintillators, which offer good detection efficiency, and a novel modular acquisition system, capable of sustaining the high scintillation rate and being less sensitive to background radiation. The final goal is the construction of an instrument able to provide an early diagnosis and to improve the effectiveness of follow-up studies for smaller tumours with respect to those studied with present clinical equipment (e.g. PET, SPECT o scintigraphy) so as to be able to visualize and characterize breast lesions with diameters < 5 mm.

Tomographic approach to single-photon breast cancer imaging with a dedicated dual-head camera with VAOR (SPEMT): Detector characterization

We have developed a compact single photon emission mammotomography (SPEMT) scanner capable of imaging the breast for the detection of small size (T1b) tumors. The scanner has a vertical-axis-of-rotation (VAOR) geometry, in which two gamma cameras orbit around a pendulous breast of a prone patient. The SPECT system is rotating around the vicinity of the breast in order to achieve high spatial resolution. The system field-of-view is 147 mm diameter and 41.6 mm height. Each head is made up of one pixilated Nal(Tl) crystal matrix with 2.2 mm pitch and 6 mm thickness coupled to three Hamamatsu H8500 64-anodes PMTs. The measured performance confirm that the system could overcome the present clinical sensitivity limit (about 1 cm diameter) for the detection of small size tumors.

SPEMT imaging with a dedicated VAoR dual-head camera: preliminary results

We have developed a SPEMT (Single Photon Emission MammoTomography) scanner that is made up of two cameras rotating around the pendulous breast of the prone patient, in Vertical Axis of Rotation (VAoR) geometry. Monte Carlo simulations indicate that the device should be able to detect tumours of 8 mm diameter with a tumour/background uptake ratio of 5:1. The scanner field of view is 41.6 mm height and 147 mm in diameter. Each head is composed of one pixilated NaI(Tl) crystal matrix coupled to three Hamamatsu H8500 64-anodes PMTs read out via resistive networks. A dedicated software has been developed to combine data from different PMTs, thus recovering the dead areas between adjacent tubes. A single head has been fully characterized in stationary configuration both in active and dead areas using a point-like source in order to verify the effectiveness of the readout method in recovering the dead regions. The scanner has been installed at the Nuclear Medicine Division of the University of Pisa for its validation using breast phantoms. The very first tomographic images of a breast phantom show a good agreement with Monte Carlo simulation results.

A silicon pixel detector system as an imaging tool for proton beam characterization

High energy protons represent a very promising alternative in the tumor irradiation, as respect the photon and electron beams. In Italy, the first and at present the only proton-therapy facility, CATANA (Centro di AdroTerapia e Applicazioni Nucleari Avanzate), was built in Catania, at the Istituto Nazionale di Fisica Nucleare-Laboratori Nazionali del Sud (INFN-LNS). Here a 62 MeV proton beam, produced by a Superconducting Cyclotron (SC), is used for the treatment of shallow tumors like those of the ocular region. A beam monitoring system, based on ion chambers for dose monitoring and on silicon diodes scanning the beam cross section for the beam quality control. Moreover, gaf-chromic films are used to measure the geometric features of the beams. Even though these systems are stable and reliable, nevertheless they are time consuming and, in the gas-chromic film case, they require an off-line analysis. In this paper we present a feasibility study for using a silicon pixel detector as device for proton beams imaging and characterization. We present the performance of such a device exposed to the CATANA proton beam in terms of dose, dose rate and exposure time response.