This work presents the tests of a multi-gap detector (MGD), composed of three parallel-plate ionization chambers (ICs) with different gap widths, assembled to prove the capability of correcting for ...charge volume recombination which is expected to occur when high fluence rates are delivered. Such beam conditions occur with a compact accelerator for charged particle therapy developed to reduce the costs, to accomplish faster treatments and to exploit different beam delivery techniques and dose rates as needed, for example, for range modulation and FLASH irradiations, respectively. The MGD was tested with carbon ions at the Centro Nazionale di Adroterapia Oncologica (CNAO Pavia, Italy), and with protons in two different beam lines: at Bern University Hospital with continuous beams and at the Laboratori Nazionale del Sud (Catania, Italy) of the Italian National Center of Nuclear Physics (INFN) with pulsed beams. For each accelerator, we took measurements with different beam intensities (up to the maximum rate of ionization achievable) and changed the detector bias voltage (V) in order to study the charge collection efficiency. Charge recombination models were used to evaluate the expected collected charge and to measure the linearity of the rate of ionization with the beam fluence rate. A phenomenological approach was used to determine the collection efficiency (f1) of the chamber with thinnest gap from the relative efficiencies, f1/f2 and f1/f3, exploiting the condition that, for each measurement, the three chambers were exposed to the same rate of ionization. Results prove that two calibration curves can be determined and used to correct the online measurements for the charge losses in the ICs for recombination.
The emergent FLASH RadioTherapy (RT) uses ultrahigh dose-rate irradiation (up to 107 Gy/s instantaneous dose-rate in each μs pulse) to deliver a single high dose of irradiation in a very short time ...(<200 ms). Pre-clinical studies at ultrahigh dose-rates recently showed an increased ratio between tumoricidal effect and normal tissue toxicity (therapeutic index), compared to conventional RT at standard Gy/min dose-rates. If confirmed by biological in vivo validations, this could represent a breakthrough in cancer treatment. However, the reliability and the accuracy of experimental studies are nowadays limited by the lack of detectors able to measure online the beam fluence at FLASH dose-rates. The behavior of standard beam monitors (gas-filled ionization chambers) is compromised by the volume recombination caused by the amount of charges created per unit volume and unit time, due to the large dose-rate. Moreover, due to the lack of proper monitoring devices and to the uncertainties of its future applications, very few facilities are able to deliver at present FLASH irradiations. In this contribution, we report about the physical and technological challenges of monitoring high and ultra-high dose-rates with electrons and photon beams, starting from the pre-clinical and clinical constraints for new devices. Based on the extensive experience in silicon detectors for monitoring applications in RT with external beams, the work then investigates silicon sensors as a possible option to tackle such extreme requirements and a rugged thin and large (e.g., 10 × 10 cm2) flat detector (silicon-based sensor + readout electronics) is therefore outlined. This study aims at presenting the FLASH-RT dosimetry problem and analyzing the possibilities for a silicon sensor to be employed as sensing device for several FLASH scenarios, including some ideas on the readout part. However, more detailed simulations and studies are demanded to delineate more precisely the technical choices to be undertaken in order to tackle the clinical accuracy required on the beam fluence, typically a few %, during photon and electron high and ultra-high irradiations, the required minimal perturbation of the beam and the high level of radiation resistance.
Introduction. The fractures that occurred around trochanteric nails (perinail fractures, PNFs) are becoming a huge challenge for the orthopaedic surgeon. Although presenting some specific critical ...issues (i.e., patients’ outcomes and treatment strategies), these fractures are commonly described within peri-implant ones and their treatment was based on periprosthetic fracture recommendations. The knowledge gap about PNFs leads us to convene a research group with the aim to propose a specific classification system to guide the orthopaedic surgeon in the management of these fractures. Materials and Methods. A steering committee, identified by two Italian associations of orthopaedic surgeons, conducted a comprehensive literature review on PNFs to identify the unmet needs about this topic. Subsequently, a panel of experts was involved in a consensus meeting proposing a specific classification system and formulated treatment statements for PNFs. Results and Discussion. The research group considered four PNF main characteristics for the classification proposal: (1) fracture localization, (2) fracture morphology, (3) fracture fragmentation, and (3) healing status of the previous fracture. An alphanumeric code was included to identify each characteristic, allowing to describe up to 54 categories of PNFs, using a 3- to 4-digit code. The proposal of the consensus-based classification reporting the most relevant aspects for PNF treatment might be a useful tool to guide the orthopaedic surgeon in the appropriate management of these fractures.
Purpose
Advanced ion beam therapeutic techniques, such as hypofractionation, respiratory gating, or laser‐based pulsed beams, have dose rate time structures which are substantially different from ...those found in conventional approaches. The biological impact of the time structure is mediated through the β parameter in the linear quadratic (LQ) model. The aim of this study was to assess the impact of changes in the value of the β parameter on the treatment outcomes, also accounting for noninstantaneous intrafraction dose delivery or fractionation and comparing the effects of using different primary ions.
Methods
An original formulation of the microdosimetric kinetic model (MKM) is used (named MCt‐MKM), in which a Monte Carlo (MC) approach was introduced to account for the stochastic spatio‐temporal correlations characteristic of the irradiations and the cellular repair kinetics. A modified version of the kinetic equations, validated on experimental cell survival in vitro data, was also introduced. The model, trained on the HSG cells, was used to evaluate the relative biological effectiveness (RBE) for treatments with acute and protracted fractions. Exemplary cases of prostate cancer irradiated with different ion beams were evaluated to assess the impact of the temporal effects.
Results
The LQ parameters for a range of cell lines (V79, HSG, and T1) and ion species (H, He, C, and Ne) were evaluated and compared with the experimental data available in the literature, with good results. Notably, in contrast to the original MKM formulation, the MCt‐MKM explicitly predicts an ion and LET‐dependent β compatible with observations. The data from a split‐dose experiment were used to experimentally determine the value of the parameter related to the cellular repair kinetics. Concerning the clinical case considered, an RBE decrease was observed, depending on the dose, ion, and LET, exceeding up to 3% of the acute value in the case of a protraction in the delivery of 10 min. The intercomparison between different ions shows that the clinical optimality is strongly dependent on a complex interplay between the different physical and biological quantities considered.
Conclusions
The present study provides a framework for exploiting the temporal effects of dose delivery. The results show the possibility of optimizing the treatment outcomes accounting for the correlation between the specific dose rate time structure and the spatial characteristic of the LET distribution, depending on the ion type used.
. In this study we introduce spatiotemporal emission reconstruction prompt gamma timing (SER-PGT), a new method to directly reconstruct the prompt photon emission in the space and time domains inside ...the patient in proton therapy.
. SER-PGT is based on the numerical optimisation of a multidimensional likelihood function, followed by a post-processing of the results. The current approach relies on a specific implementation of the maximum-likelihood expectation maximisation algorithm. The robustness of the method is guaranteed by the complete absence of any information about the target composition in the algorithm.
. Accurate Monte Carlo simulations indicate a range resolution of about 0.5 cm (standard deviation) when considering 10
primary protons impinging on an homogeneous phantom. Preliminary results on an anthropomorphic phantom are also reported.
. By showing the feasibility for the reconstruction of the primary particle range using PET detectors, this study provides significant basis for the development of an hybrid in-beam PET and prompt photon device.
Purpose
A retrospective analysis of the dose delivery system (DDS) performances of the initial clinical operation at CNAO (Centro Nazionale di Adroterapia Oncologica) is reported, and compared with ...the dose delivery accuracy following the implementation of a position feedback control.
Methods
Log files and raw data of the DDS were analyzed for every field of patients treated with protons and carbon ions between January 2012 and April 2013 (~3800 fields). To investigate the DDS accuracy, the spot positions and the number of particles per spot measured by the DDS and prescribed by the treatment planning system were compared for each field. The impact of deviations on dose distributions was studied by comparing, through the gamma‐index method, 2 three‐dimensional (3D) physical dose maps (one for prescribed, one for measured data), generated by a validated dose computation software. The maximum gamma and the percentage of points with gamma ≤ 1 (passing volume) were studied as a function of the treatment day, and correlated with the deviations from the prescription in the measured number of particles and spot positions. Finally, delivered dose distributions of same treatment plans were compared before and after the implementation of a feedback algorithm for the correction of small position deviations, to study the effect on the delivery quality. A double comparison of prescribed and measured 3D maps, before and after feedback implementation, is reported and studied for a representative treatment delivered in 2012, redelivered on a polymethyl methacrylate (PMMA) block in 2018.
Results
Systematic deviations of spot positions, mainly due to beam lateral offsets, were always found within 1.5 mm, with the exception of the initial clinical period. The number of particles was very stable, as possible deviations are exclusively related to the quantization error in the conversion from monitor counts to particles. For the chosen representative patient treatment, the gamma‐index evaluation of prescribed and measured dose maps, before and after feedback implementation, showed a higher variability of maximum gamma for the 2012 irradiation, with respect to the reirradiation of 2018. However, the 2012 passing volume is >99.8% for the sum of all fields, which is comparable to the value of 2018, with the exception of one day with 98.2% passing volume, probably related to an instability of the accelerating system.
Conclusions
A detailed retrospective analysis of the DDS performances in the initial period of CNAO clinical activity is reported. The spot position deviations are referable to beam lateral offset fluctuations, while almost no deviation was found in the number of particles. The impact of deviations on dose distributions showed that the position feedback implementation and the increased beam control capability acquired after the first years of clinical experience led to an evident improvement in the DDS stability, evaluated in terms of gamma‐index as a measure of the impact on dose distributions. However, the clinical effect of the maximum gamma variability found in the 2012 representative irradiation is mitigated by averaging along the number of fractions, and the high percentage of passing volumes confirmed the accuracy of the delivery even before the feedback implementation.
The beam energy is one of the most significant parameters in particle therapy since it is directly correlated to the particles' penetration depth inside the patient. Nowadays, the range accuracy is ...guaranteed by offline routine quality control checks mainly performed with water phantoms, 2D detectors with PMMA wedges, or multi-layer ionization chambers. The latter feature low sensitivity, slow collection time, and response dependent on external parameters, which represent limiting factors for the quality controls of beams delivered with fast energy switching modalities, as foreseen in future treatments. In this context, a device based on solid-state detectors technology, able to perform a direct and absolute beam energy measurement, is proposed as a viable alternative for quality assurance measurements and beam commissioning, paving the way for online range monitoring and treatment verification.
This work follows the proof of concept of an energy monitoring system for clinical proton beams, based on Ultra Fast Silicon Detectors (featuring tenths of ps time resolution in 50 μm active thickness, and single particle detection capability) and time-of-flight techniques. An upgrade of such a system is presented here, together with the description of a dedicated self-calibration method, proving that this second prototype is able to assess the mean particles energy of a monoenergetic beam without any constraint on the beam temporal structure, neither any a priori knowledge of the beam energy for the calibration of the system.
A new detector geometry, consisting of sensors segmented in strips, has been designed and implemented in order to enhance the statistics of coincident protons, thus improving the accuracy of the measured time differences. The prototype was tested on the cyclotron proton beam of the Trento Protontherapy Center (TPC). In addition, a dedicated self-calibration method, exploiting the measurement of monoenergetic beams crossing the two telescope sensors for different flight distances, was introduced to remove the systematic uncertainties independently from any external reference.
The novel calibration strategy was applied to the experimental data collected at TPC (Trento) and CNAO (Pavia). Deviations between measured and reference beam energies in the order of a few hundreds of keV with a maximum uncertainty of 0.5 MeV were found, in compliance with the clinically required water range accuracy of 1 mm.
The presented version of the telescope system, minimally perturbative of the beam, relies on a few seconds of acquisition time to achieve the required clinical accuracy and therefore represents a feasible solution for beam commission, quality assurance checks, and online beam energy monitoring.
Pile-up effects due to the overlap of signals within the system dead-time (τ) influence the counting capability of radiation detection devices, necessitating the use of correction algorithms to ...compensate for the count-losses at high radiation rates. Count-rate linearity is especially critical for clinical applications like X-ray imaging or beam monitoring in particle therapy. In particular, in proton therapy the number of delivered particles must be measured online during the treatment session with a maximum error of 1 % up to an average input beam flux of about 1010cm−2s−1. For a segmented detector used to identify and count the single beam particles, assuming a channel area of 1 mm2 and a dead-time τ=2ns, a maximum counting inefficiency of 1 % is required up to τ.fin=0.2 for each detector channel, where fin represents the input rate. Moreover, the beam is often delivered in bunches with higher instantaneous particle rates, and the saturation model of the detector and electronic chain could not be easily determined. Similar considerations are applicable for pixelated detectors used for photon counting.
Two methods are proposed to mitigate counting inefficiencies with radiation sources of variable time-structures. Both methods are based on the collection of logic signals provided by two independent detector channels exposed to the same radiation field after discriminating the detector analog outputs with a fixed threshold, assuming that the duration of the discriminator output signal corresponds to the system dead-time. The correction algorithms employ the measurements of the time durations, the number of signals from the two channels and of their AND/OR combinations. The methods provide count-loss corrections without the need to know the dead-time model.
The performances of the proposed algorithms are evaluated by using simulations of ideal boxcar signals of fixed duration τ, distributed randomly in time to emulate the dead-time behavior of the system. Both methods provide an effective count-loss correction with a maximum deviation of 1% for different input rates up to τ.fin=1, assuming a uniform random time distribution of input events for both paralyzable and non-paralyzable systems. The simulations of pulsed radiation fluxes provide the same results as a function of the instantaneous input rates.
These results are similar to those obtainable by the standard live-time correction algorithm. However, the latter algorithm can only be applied to continuous particle fluxes, while the proposed algorithms work also for pulsed beams, without any hypothesis on the bunch duration or frequency. The robustness of the algorithms with respect to the resolution of the time measurement is studied and the potential limitations in more realistic systems are discussed.
The algorithms can be easily implemented in standard logical circuits with multiple input signals provided by segmented detectors. Even if the methods are intended for real-time correction in beam particle counting, they could be applied in a wider range of applications of radiation measurements.
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