Characterization and optimization of a β detector for 18F radio-guided surgery

Radio-Guided Surgery (RGS) is a nuclear medicine technique to support the surgeon during surgery towards a complete tumor resection. It is based on intraoperative detection of radiation emitted by a radio-pharmaceutical that bounds selectively to tumoral cells. In the past years, an approach that exploits β^- emitting radiotracers has been pursued to overtake some limitations of the traditional RGS based on γ emission. A particle detector dedicated to this application, demonstrating very high efficiency to β^- particles and remarkable transparency to photons, has been thus developed. As a by-product, its characteristics suggested the possibility to utilize it with β⁺ emitting sources, more commonly in use in nuclear medicine. In this paper, performances of such detector on ¹⁸F liquid sources are estimated by means of Monte Carlo simulations (MC) and laboratory measurements. The experimental setup with a ¹⁸F saline solution comprised a "positron signal" spot (a 7 × 10 mm cylinder representing the tumor residual), and a surrounding "far background" volume, that represented for the detector an almost isotropic source of annihilation photons. Experimental results show good agreement with MC predictions, thus confirming the expected performances of the detector with ¹⁸F, and the validity of the developed MC simulation as a tool to predict the gamma background determined by a diffuse source of annihilation photons.

Evolution of Portable Sensors for In-Vivo Dose and Time-Activity Curve Monitoring as Tools for Personalized Dosimetry in Molecular Radiotherapy

Treatment personalization in Molecular Radiotherapy (MRT) relies on pre- and post-treatment SPECT/ PET-based images and measurements to obtain a patient-specific absorbed dose-rate distribution map and its evolution over time. Unfortunately, the number of time points that are available per patient to investigate individual pharmacokinetics is often reduced by limited patient compliance or SPECT or PET/CT scanner availability for dosimetry in busy departments. The adoption of portable sensors for in-vivo dose monitoring during the entire treatment could improve the assessment of individual biokinetics in MRT and, thus, the treatment personalization. The evolution of portable devices, non-SPECT/PET-based options, already used for monitoring radionuclide activity transit and accumulation during therapy with radionuclides (i.e., MRT or brachytherapy), is presented to identify valuable ones, which combined with conventional nuclear medicine imaging systems could be effective in MRT. External probes, integration dosimeters and active detecting systems were included in the study. The devices and their technology, the range of applications, the features and limitations are discussed. Our overview of the available technologies encourages research and development of portable devices and dedicated algorithms for MRT patient-specific biokinetics study. This would represent a crucial advancement towards personalized treatment in MRT.

A wearable radiation measurement system for collection of patient-specific time-activity data in radiopharmaceutical therapy: system design and Monte Carlo simulation results

Purpose: A high level of personalization in Molecular Radiotherapy (MRT) could bring advantages in terms of treatment effectiveness and toxicity reduction. Individual organ‐level dosimetry is crucial to describe the radiopharmaceutical biodistribution expressed by the patient, to estimate absorbed doses to normal organs and target tissue(s). This paper presents a proof‐of‐concept Monte Carlo simulation study of “WIDMApp” (Wearable Individual Dose Monitoring Apparatus), a multi‐channel radiation detector and data processing system for in vivo patient measurement and collection of radiopharmaceutical biokinetic data (i.e., time‐activity data). Potentially, such a system can increase the amount of such data that can be collected while reducing the need to derive it via nuclear medicine imaging.

Methods: a male anthropomorphic MIRD phantom was used to simulate photons (i.e., gamma‐rays) propagation in a patient undergoing a 131I thyroid treatment. The administered activity was set to the amount usually administered for the treatment of differentiated carcinoma while its initial distribution in different organs was assigned following the ICRP indications for the 131I biokinetics. Using this information, the simulation computes the Time‐dependent Counts Curves (TCCs) that would have been measured by seven WIDMApp‐like sensors placed and oriented to face each one of five emitting organs plus two thyroid lobes. A deconvolution algorithm was then applied on this simulated data set to reconstruct the Time‐Activity Curve (TAC) of each organ. Deviations of the reconstructed TACs parameters from values used to generate them were studied as a function of the deconvolution algorithm initialization parameters and assuming non‐Poisson fluctuation of the TCCs data points.

Results: This study demonstrates that it is possible, at least in the simple simulated scenario, to reconstruct the organ cumulated activity by measuring the time dependence of counts recorded by several detectors placed at selected positions on the patient's body. The ability to perform in vivo sampling more frequently than conventional biokinetic studies increases the number of time points and therefore the accuracy in TAC estimates. In this study, an accuracy on cumulated activity of 5% is obtained even with a 20% error on the TCC data points and a 50% error on the initial guess on the parameters of the deconvolution algorithm.

Conclusions: the WIDMApp approach could provide an effective tool to characterize more accurately the radiopharmaceutical biokinetics in MRT patients, reducing the need of resources of nuclear medicine departments, such as technologist and scanner time, to perform individualized biokinetics studies. The relatively simple hardware for the approach proposed would allow its application to large numbers of patients. The results obtained justify development of an actual prototype system to characterize this technique under realistic conditions.

Monitoring Carbon Ion Beams Transverse Position Detecting Charged Secondary Fragments: Results From Patient Treatment Performed at CNAO

Particle therapy in which deep seated tumours are treated using ¹²C ions (Carbon Ions RadioTherapy or CIRT) exploits the high conformity in the dose release, the high relative biological effectiveness and low oxygen enhancement ratio of such projectiles. The advantages of CIRT are driving a rapid increase in the number of centres that are trying to implement such technique. To fully profit from the ballistic precision achievable in delivering the dose to the target volume an online range verification system would be needed, but currently missing. The ¹²C ions beams range could only be monitored by looking at the secondary radiation emitted by the primary beam interaction with the patient tissues and no technical solution capable of the needed precision has been adopted in the clinical centres yet. The detection of charged secondary fragments, mainly protons, emitted by the patient is a promising approach, and is currently being explored in clinical trials at CNAO. Charged particles are easy to detect and can be back-tracked to the emission point with high efficiency in an almost background-free environment. These fragments are the product of projectiles fragmentation, and are hence mainly produced along the beam path inside the patient. This experimental signature can be used to monitor the beam position in the plane orthogonal to its flight direction, providing an online feedback to the beam transverse position monitor chambers used in the clinical centres. This information could be used to cross-check, validate and calibrate, whenever needed, the information provided by the ion chambers already implemented in most clinical centres as beam control detectors. In this paper we study the feasibility of such strategy in the clinical routine, analysing the data collected during the clinical trial performed at the CNAO facility on patients treated using ¹²C ions and monitored using the Dose Profiler (DP) detector developed within the INSIDE project. On the basis of the data collected monitoring three patients, the technique potential and limitations will be discussed.

First Ex Vivo Results of β--Radioguided Surgery in Small Intestine Neuroendocrine Tumors with 90Y-DOTATOC

Background: In neuroendocrine tumor (NET), complete surgery could better the prognosis. Radioguided surgery (RGS) with β^--radioisotopes is a novel approach focused on developing a new probe that, detecting electrons and operating with low background, provides a clearer delineation of the lesions with low radiation exposition for surgeons. As a first step to validate this procedure, ex vivo specimens of tumors expressing somatostatin receptors, as small intestine neuroendocrine tumor (SI-NET), were tested. Materials and Methods: SI-NET presents a high uptake of a beta-emitting radiotracer, ⁹⁰Y-DOTATOC. Five SI-NET patients were enrolled after performing a ⁶⁸Ga-DOTATOC positron emission tomography/computed tomography (CT) and a CT enterography; 24 h before surgery, they received 5 mCi of ⁹⁰Y-DOTATOC. Results: Surgery was performed as routine. Tumors and surrounding tissue were sectioned in different samples and examined ex vivo with the beta-detecting probe. All the tumor samples showed high counts of radioactivity that was up to a factor of 18 times higher than the corresponding cutoff value, with a sensitivity of 96% and a specificity of 100%. Conclusions: These first ex vivo RGS tests showed that this probe can discriminate very effectively between tumor and healthy tissues by the administration of low activities of ⁹⁰Y-DOTATOC, allowing more precise surgery.

Tumor-non-tumor discrimination by a β- detector for Radio Guided Surgery on ex-vivo neuroendocrine tumors samples

This paper provides a first insight of the potential of the β^- Radio Guided Surgery (β^--RGS) in a complex surgical environment like the abdomen, where multiple sources of background concur to the signal at the tumor site. This case is well reproduced by ex-vivo samples of 90^Y-marked Gastro-Entero-Pancreatic Neuroendocrine Tumors (GEP NET) in the bowel. These specimens indeed include at least three wide independent sources of background associated to three anatomical districts (mesentery, intestine, mucose). The study is based on the analysis of 37 lesions found on 5 samples belonging to 5 different patients. We show that the use of electrons, a short range particle, instead of γ particles, allows to limit counts read on a lesion to the sum of the tumor signal plus the background generated by the sole hosting district.The background on adjacent districts in the same specimen/patient is found to differ up to a factor 4, showing how the specificity and sensitivity of the β^--RGS technique can be fully exploited only upon a correct measurement of the contributing background. This locality has been used to set a site-specific cut-off algorithm to discriminate tumor and healthy tissue with a specificity of 100% and a sensitivity, on this test data sample, close to 100%. Factors influencing the sensitivity are also discussed. One of the specimens set allowed us evaluate the volume of the lesions, thus concluding that the probe was able to detect lesions as small as 0.04 mL in that particular case.

Characterisation of a β detector on positron emitters for medical applications

PURPOSE

Radio Guided Surgery (RGS) is a technique that helps the surgeon to achieve an as complete as possible tumor resection, thanks to the intraoperative detection of particles emitted by a radio tracer that bounds to tumoral cells. In the last years, a novel approach to this technique has been proposed that, exploiting β^- emitting radio tracers, overtakes some limitations of established γ-RGS. In this context, a first prototype of an intraoperative β particle detector, based on a high light yield and low density organic scintillator, has been developed and characterised on pure β^- emitters, like ⁹⁰Y. The demonstrated very high efficiency to β^- particles, together with the remarkable transparency to photons, suggested the possibility to use this detector also with β⁺ emitting sources, that have plenty of applications in nuclear medicine. In this paper, we present upgrades and optimisations performed to the detector to reveal such particles.

METHODS

Laboratory measurement have been performed on liquid Ga68 source, and were used to validate and tune a Monte Carlo simulation.

RESULTS

The upgraded detector has an ~80% efficiency to electrons above ~110keV, reaching a plateau value of ~95%. At the same time, the probe is substantially transparent to photons below ~200keV, reaching a plateau value of ~3%.

CONCLUSIONS

The new prototype seems to have promising characteristics to perform RGS also with β⁺ emitting isotopes.

Design of a new tracking device for on-line beam range monitor in carbon therapy

Charged particle therapy is a technique for cancer treatment that exploits hadron beams, mostly protons and carbon ions. A critical issue is the monitoring of the beam range so to check the correct dose deposition to the tumor and surrounding tissues. The design of a new tracking device for beam range real-time monitoring in pencil beam carbon ion therapy is presented. The proposed device tracks secondary charged particles produced by beam interactions in the patient tissue and exploits the correlation of the charged particle emission profile with the spatial dose deposition and the Bragg peak position. The detector, currently under construction, uses the information provided by 12 layers of scintillating fibers followed by a plastic scintillator and a pixelated Lutetium Fine Silicate (LFS) crystal calorimeter. An algorithm to account and correct for emission profile distortion due to charged secondaries absorption inside the patient tissue is also proposed. Finally detector reconstruction efficiency for charged particle emission profile is evaluated using a Monte Carlo simulation considering a quasi-realistic case of a non-homogenous phantom.

Monitoring of Hadrontherapy Treatments by Means of Charged Particle Detection

The interaction of the incoming beam radiation with the patient body in hadrontherapy treatments produces secondary charged and neutral particles, whose detection can be used for monitoring purposes and to perform an on-line check of beam particle range. In the context of ion-therapy with active scanning, charged particles are potentially attractive since they can be easily tracked with a high efficiency, in presence of a relatively low background contamination. In order to verify the possibility of exploiting this approach for in-beam monitoring in ion-therapy, and to guide the design of specific detectors, both simulations and experimental tests are being performed with ion beams impinging on simple homogeneous tissue-like targets (PMMA). From these studies, a resolution of the order of few millimeters on the single track has been proven to be sufficient to exploit charged particle tracking for monitoring purposes, preserving the precision achievable on longitudinal shape. The results obtained so far show that the measurement of charged particles can be successfully implemented in a technology capable of monitoring both the dose profile and the position of the Bragg peak inside the target and finally lead to the design of a novel profile detector. Crucial aspects to be considered are the detector positioning, to be optimized in order to maximize the available statistics, and the capability of accounting for the multiple scattering interactions undergone by the charged fragments along their exit path from the patient body. The experimental results collected up to now are also valuable for the validation of Monte Carlo simulation software tools and their implementation in Treatment Planning Software packages.