Data Acquisition Systems design for Quality Assurance in advanced radiation dosimetry

During the past years, radiotherapy has become a widely used technique for treatment of tumors on different areas of the human body. The evolution of the technology has led to the necessity to assure the safety of the patient at any time during any kind of treatment. A wide range of instrumentation has been developed to perform a state of art dosimetry for the Quality Assurance (QA) of the irradiating beam. Tumors can be treated using different methodologies, depending on their position and dimension. The radiation techniques have been classified in three major groups: Microbeam Radiation Therapy (MRT), Brachytherapy and External Beam Radiation Therapy (EBRT). Each of them presents different characteristics in terms of intensity of delivered dose, timing and speed of the irradiation. The aim of this research project is to study and design a unified Data Acquisition system (DAQ) able to monitor and measure the irradiating beam of any of the three clinical scenarios. A DAQ is a very complex system comprising multiple aspects. It can be divided in four main sections: detectors, the sensitive part; electronic and analogue front end; digital core and Graphical User Interface. The goal to create a unified dosimetry platform has been achieved by the design of a modular read out electronic section and a modular digital core, which can be easily adapted to the different detectors and analogue front ends. The project started with the development of a single channel DAQ to read out a single microstrip diode for MRT application. The goal of the DAQ was to have high spatial resolution, to resolve a 50 μm wide microbeam, and a large sensitivity range. In fact, for this application, the beam instead of being flat has very narrow peaks where high radiation dose is delivered, spaced by areas where radiation is very low or none (named valley). Then the system has been upgraded into a multi-channel DAQ able to acquire a 128 diodes array for Brachytherapy treatments. In this scenario, the source moves during the treatment and can be placed far away from the detector. By using an array of diodes, the system has been able to reconstruct the movement of the source at any time during the treatment. A new high sensitivity front end has been introduced to perform dosimetry up to 12 cm distance from the irradiating source. The same DAQ has been used in multi-channel configuration for EBRT treatments and its modular design allowed the introduction of larger detectors, up to 512 channels, to best suit each different application. Moreover, the flexible timing constraints of the front-end used to read out the detector, allowed the setup of a triggered acquisition which can be synchronized with the LINAC beam to have a highly efficient dosimetry measure. Lately, a new standalone section has been introduced: it comprises a rotatable phantom and its movement control. It features an inclinometer, which detects the position of the LINAC gantry, and a control system made by an encoder and a stepper motor to align the detector to the gantry. This setup is particularly useful for application like VMAT and IMRT, where the treatment is delivered by rotating the LINAC around the patient and it is also compatible with all the previously developed DAQ. Overall this research project has given a relevant contribution to the development of a unified platform for advanced radiation dosimetry. The outcome is a modular DAQ whose setup can be chosen to best suit each particular application. Even if the DAQ can be improved and upgrades are already undergoing feasibility studies, this research study has contributed to the publication of many peer reviewed papers internationally recognized

A comparative analysis of multichannel data acquisition systems for quality assurance in external beam radiation therapy

The paper presents a comparative study performed by the Centre of Medical Radiation Physics (CMRP) on three multichannel Data Acquisition Systems (DAQ) based on different analogue front-ends to suit a wide range of radiotherapy applications. The three front-ends are: a charge-to-frequency converter developed by INFN Torino, an electrometer and a charge-to-digital converter (both commercial devices from Texas Instruments). For the first two (named DAQ A and B), the CMRP has designed the read-out systems whilst the third one (DAQ C) comes with its own evaluation board. For the purpose of the characterization DAQ A and DAQ B have been equipped with 128 channels while DAQ C has 256 channels. In terms of performances, the DAQs show good linearity over all the dynamic range. Each one has a different range of sensitivity ranging from less than 1 pC up to 13 nC, which makes the three front-ends complementary and suitable for use with different radiation detectors for different radiotherapy applications, or in a mixed solution which can house different front-ends

The evaluation of a 2D diode array in magic phantom for use in high dose rate brachytherapy pretreatment quality assurance

Purpose: High dose rate (HDR) brachytherapy is a treatment method that is used increasingly worldwide. The development of a sound quality assurance program for the verification of treatment deliveries can be challenging due to the high source activity utilized and the need for precise measurements of dwell positions and times. This paper describes the application of a novel phantom, based on a 2D 11 x 11 diode array detection system, named magic phantom (MPh), to accurately measure plan dwell positions and times, compare them directly to the treatment plan, determine errors in treatment delivery, and calculate absorbed dose. Methods: The magic phantom system was CT scanned and a 20 catheter plan was generated to simulate a nonspecific treatment scenario. This plan was delivered to the MPh and, using a custom developed software suite, the dwell positions and times were measured and compared to the plan. The original plan was also modified, with changes not disclosed to the primary authors, and measured again using the device and software to determine the modifications. A new metric, the position-time gamma index, was developed to quantify the quality of a treatment delivery when compared to the treatment plan. The MPh was evaluated to determine the minimum measurable dwell time and step size. The incorporation of the TG-43U1 formalism directly into the software allows for dose calculations to be made based on the measured plan. The estimated dose distributions calculated by the software were compared to the treatment plan and to calibrated EBT3 film, using the 2D gamma analysis method. Results: For the original plan, the magic phantom system was capable of measuring all dwell points and dwell times and the majority were found to be within 0.93 mm and 0.25 s, respectively, from the plan. By measuring the altered plan and comparing it to the unmodified treatment plan, the use of the position-time gamma index showed that all modifications made could be readily detected. The MPh was able to measure dwell times down to 0.067 ± 0.001 s and planned dwell positions separated by 1 mm. The dose calculation carried out by the MPh software was found to be in agreement with values calculated by the treatment planning system within 0.75%. Using the 2D gamma index, the dose map of the MPh plane and measured EBT3 were found to have a pass rate of over 95% when compared to the original plan. Conclusions: The application of this magic phantom quality assurance system to HDR brachytherapy has demonstrated promising ability to perform the verification of treatment plans, based upon the measured dwell positions and times. The introduction of the quantitative position-time gamma index allows for direct comparison of measured parameters against the plan and could be used prior to patient treatment to ensure accurate delivery

Quality assurance of VMAT on flattened and flattening filter-free accelerators using a high spatial resolution detector

© 2020 The Authors. Journal of Applied Clinical Medical Physics published by Wiley Periodicals, Inc. on behalf of American Association of Physicists in Medicine Purpose: This study investigated the use of high spatial resolution solid-state detectors (DUO and Octa) combined with an inclinometer for machine-based quality assurance (QA) of Volumetric Modulated Arc Therapy (VMAT) with flattened and flattening filter-free beams. Method: The proposed system was inserted in the accessory tray of the gantry head of a Varian 21iX Clinac and a Truebeam linear accelerator. Mutual dependence of the dose rate (DR) and gantry speed (GS) was assessed using the standard Varian customer acceptance plan (CAP). The multi-leaf collimator (MLC) leaf speed was evaluated under static gantry conditions in directions parallel and orthogonal to gravity as well as under dynamic gantry conditions. Measurements were compared to machine log files. Results: DR and GS as a function of gantry angle were reconstructed using the DUO/inclinometer and in agreement to within 1% with the machine log files in the sectors of constant DR and GS. The MLC leaf speeds agreed with the nominal speeds and those extracted from the machine log files to within 0.03 cm s−1. The effect of gravity on the leaf motion was only observed when the leaves traveled faster than the nominal maximum velocity stated by the vendor. Under dynamic gantry conditions, MLC leaf speeds ranging between 0.33 and 1.42 cm s−1 were evaluated. Comparing the average MLC leaf speeds with the machine log files found differences between 0.9% and 5.7%, with the largest discrepancy occurring under conditions of fastest leaf velocity, lowest DR and lowest detector signal. Conclusions: The investigation on the use of solid-state detectors in combination with an inclinometer has demonstrated the capability to provide efficient and independent verification of DR, GS, and MLC leaf speed during dynamic VMAT delivery. Good agreement with machine log files suggests the detector/inclinometer system is a useful tool for machine-specific VMAT QA

Innovative detectors for quality assurance dosimetry in SBRT of stationary and movable targets

The high spatial and temporal resolution 2D monolithic silicon detector arrays M512 and DUO for quality assurance (QA) in real time motion adaptive radiotherapy (ART) have been developed. The DUO array possesses a spatial resolution of 0.2 mm and has demonstrated agreement within 5% with EBT3 film measurements of 6MV linac beam profiles for field sizes 1 × 1 cm2 and SRS cone diameter 0.5 cm. Dynamic characterisation of the M512 for QA in real time ART evaluated the performance of M512 for small fields while the detector experiences periodic motion. It was demonstrated with M512 that MLC tracking with Calypso electromagnetic array compensates for the periodic motion and improves the agreement between static and dynamic beam profiles for field size 1 × 1 cm2 from within 75% in the penumbra to within 11% agreement. The dynamic profile is returned to a similar distribution as the static case

Characterisation of silicon diode arrays for dosimetry in external beam radiation therapy

Modern stereotactic radiation therapy modalities utilize small beams and large dose gradients to deliver radiation in few fractions, reducing the possibility to correct for mistakes during the treatment process. Therefore, in order to ensure best possible treatment for the patient, quality assurance for such treatments necessitates a stable, linear, and sensitive radiation detector with high spatial resolution and radiation hardness. In this work, two silicon detector arrays with high spatial resolution have been characterized by 6 MV and 18 MV medical LINAC irradiation, and 5.5 MeV He2+ heavy ion microprobe. A maximum discrepancy of 0.6 mm in field size has been found when comparing to two-dimensional radiochromic film dose profile, and charge collection efficiency obtained by means of ion beam induced charge collection (IBICC) is 66% when operating the array in photovoltaic mode. Radiation damage study by photons and photoneutrons is presented

Preliminary epi-diode characterization for HDR brachytherapy quality assurance

High Dose Rate vaginal brachytherapy for endometrial cancer has evolved from simple single-channel (i.e. cylindrical applicator) deliveries to treatments involving several channels (i.e. multichannel applicator) for the radiotherapy source to dwell, increasing the complexity of the dose distribution, and allowing more space for potential errors. For this reason real-time treatment verification has gained a greater importance than ever before, and more methods need to be developed in order to provide assurance that the dose delivery has been carried out as intended by the hospital staff. P-type silicon epi diodes have been designed at the Centre for Medical Radiation Physics (CMRP) in Australia to suit the specific needs of HDR BT, and characterized in the clinical BT facility of the Fondazione IRCCS (INT) in Italy. They have shown great potential for BT treatment verification in real time due to their radiation hardness, dose rate independence, flexibility in physical design, and ability to monitor the treatment at a 1-kHz readout frequency. Their dynamic range has been determined as ± 17 to ± 20 mm and dwell time calculation accuracy of \u3e 0.1 s has been shown. If placed on the same longitudinal plane of a treatment accessory, these detectors would enable coverage of about 40 mm for source position and dwell time tracking. Respective detector positioning at (0, +3, -3 mm) would extend this range to 45-50 mm, depending on the catheter location, proving to be sufficient for the majority of treatment cases. Further studies are encouraged to develop diodes with a wider dynamic range

2D mapping of the MV photon fluence and 3D dose reconstruction in real time for quality assurance during radiotherapy treatment

the photon irradiation response of a 2D solid state transmission detector array mounted in a linac block tray is used to reconstruct the projected 2D dose map in a homogenous phantom along rays that diverge from the X-ray source and pass through each of the 121 detector elements. A unique diode response-to-dose scaling factor, applied to all detectors, is utilised in the reconstruction to demonstrate that real time QA during radiotherapy treatment is feasible. Purpose: to quantitatively demonstrate reconstruction of the real time radiation dose from the irradiation response of the 11x11 silicon Magic Plate (MP) detector array operated in Transmission Mode (MPTM). Methods and Materials: in transmission mode the MP is positioned in the block tray of a linac so that the central detector of the array lies on the central axis of the radiation beam. This central detector is used to determine the conversion factor from measured irradiation response to reconstructed dose at any point on the central axis within a homogenous solid water phantom. The same unique conversion factor is used for all MP detector elements lying within the irradiation field. Using the two sets of data, the 2D or 3D dose map is able to be reconstructed in the homogenous phantom. The technique we have developed is illustrated here for different depths and irradiation field sizes, (5 x 5 cm2 to 40 x 40 cm2) as well as a highly non uniform irradiation field. Results: we find that the MPTM response is proportional to the projected 2D dose map measured at a specific phantom depth, the sweet depth . A single factor, for several irradiation field sizes and depths, is derived to reconstruct the dose in the phantom along rays projected from the photon source through each MPTM detector element. We demonstrate that for all field sizes using the above method, the 2D reconstructed and measured doses agree to within ± 2.48% (2 standard deviation) for all in-field MP detector elements. Conclusions: a 2D detector system and method to reconstruct the dose in a homogeneous phantom and in real time has been demonstrated. The success of this work is an exciting development toward real time QA during radiotherapy treatment

An innovative gynecological HDR brachytherapy applicator system for treatment delivery and real-time verification

The multichannel vaginal cylinder (MVC) applicator employed for gynecological high dose rate (HDR) brachytherapy increases dose delivery complexity, and thus makes the treatment more prone to errors. A quality assurance (QA) procedure tracking the source throughout dose delivery can detect dwell position and time errors in the multiple channels of the applicator. A new MVC system with integrated real time in vivo treatment delivery QA has been developed based on diodes embedded on the outer surface of the MVC. It has been pre-calibrated and verified using a non-clinical treatment plan with consecutive test positions and dwell times within each catheter, followed by the delivery of ten clinical plans of adjuvant vaginal cuff brachytherapy following hysterectomy for endometrial cancer. The non-clinical verification showed overall mean dwell position and time discrepancies between the nominal and measured treatment of −0.2 ± 0.5 mm and −0.1 ± 0.1 s (k = 1), respectively. The clinical plans showed mean positional discrepancies of 0.2 ± 0.4 and 0.0 ± 0.8 mm, for the central and peripheral catheters, respectively, and mean dwell time discrepancies of −0.1 ± 0.2 and −0.0 ± 0.1 s for central and peripheral catheters, respectively. The innovative prototype of the MVC system has shown the ability to track the source with sub-mm and sub-second accuracy, and demonstrated potential for its incorporation into the clinical routine