Real-time Treatment Verification Research Articles

Time-resolved clinical dose volume metrics, calculations and predictions based on source tracking measurements and uncertainties to aid treatment verification and error detection for HDR brachytherapy—a proof-of-principle study

Objective. High-dose-rate (HDR) brachytherapy lacks routinely available treatment verification methods. Real-time tracking of the radiation source during HDR brachytherapy can enhance treatment verification capabilities. Recent developments in source tracking allow for measurement of dwell times and source positions with high accuracy. However, more clinically relevant information, such as dose discrepancies, is still needed. To address this, a real-time dose calculation implementation was developed to provide more relevant information from source tracking data. A proof-of-principle of the developed tool was shown using source tracking data obtained from a 3D-printed anthropomorphic phantom. Approach. Software was developed to calculate dose-volume-histograms (DVH) and clinical dose metrics from experimental HDR prostate treatment source tracking data, measured in a realistic pelvic phantom. Uncertainty estimation was performed using repeat measurements to assess the inherent dose measuring uncertainty of the in vivo dosimetry (IVD) system. Using a novel approach, the measurement uncertainty can be incorporated in the dose calculation, and used for evaluation of cumulative dose and clinical dose-volume metrics after every dwell position, enabling real-time treatment verification. Main results. The dose calculated from source tracking measurements aligned with the generated uncertainty bands, validating the approach. Simulated shifts of 3 mm in 5/17 needles in a single plan caused DVH deviations beyond the uncertainty bands, indicating errors occurred during treatment. Clinical dose-volume metrics could be monitored in a time-resolved approach, enabling early detection of treatment plan deviations and prediction of their impact on the final dose that will be delivered in real-time. Significance. Integrating dose calculation with source tracking enhances the clinical relevance of IVD methods. Phantom measurements show that the developed tool aids in tracking treatment progress, detecting errors in real-time and post-treatment evaluation. In addition, it could be used to define patient-specific action limits and error thresholds, while taking the uncertainty of the measurement system into consideration.

Radiotherapy delivery error detection with electronic portal imaging device (EPID)-based in vivo dosimetry.

Radiotherapy delivery error detection with electronic portal imaging device (EPID)-based in vivo dosimetry.

Towards 2D dosimetry using monolithic active pixel sensors and a copper grating

Higher energy and intensity X-ray radiotherapy treatments are coming into wider use, having the benefit of requiring fewer treatment fractions and fewer hospital visits per patient. However, small percentage errors in multileaf collimator positioning and dose become bigger problems with higher doses per fraction. Hence, real-time treatment verification becomes essential. Where devices downstream from the patient suffer from scattering in the patient, upstream devices can disturb the therapeutic beam. Here, a method is proposed for performing dosimetry upstream using monolithic active pixel sensors, which can be made thin enough to disturb the beam by <1%. In order to calculate the dose to the tumour, a verification device needs to make a measurement of the photon field. Some photons will Compton scatter an electron in the silicon and generate a signal. However, this signal is obscured by energy deposits from contamination electrons, originating from Compton scattering in the accelerator head and air. Often extensive build-up material is added to verification devices to reduce the electron contamination and enhance the photon signal. However, this leads to degradation of the beam intensity to the patient. Instead we propose using thin strips of 50 µm thick copper in a grating pattern and measuring the difference in the signal with and without it. The contamination electrons are relatively undisturbed by the presence of the thin copper strips and the photon signal generated via Compton scattering is enhanced under the strips. Hence the difference in the two signals mostly consists of energy deposits originating from the therapeutic photons. From this the dose to the patient can be derived. The 50 μm of copper is thin enough to keep the beam attenuation below 1%, but in itself would give a total signal response which consists of 38% contamination electrons. Using the grating technique, we show that the electron contamination can be reduced to 2.6% of the total signal. This allows the photon signal only to be extracted from the data and thus the dose to patient with a very thin upstream detector can be calculated using a Monte Carlo model to extrapolate the photon flux into the tumour.

Complex field verification using a large area CMOS MAPS upstream in radiotherapy

A multileaf collimator (MLC) is an integral component in modern radiotherapy machines as it dynamically shapes the photon field used for patient treatment. Currently, the MLC leaves which collimate the treatment field are mechanically calibrated to ±1 mm every 3 months and during pre-treatment calibration are calibrated to the mechanically set leaf positions. Leaf drift can occur between calibration dates and hence exceed the ±1 mm tolerance. Pre-treatment verification, increases LINAC usage time so is seldom performed for each individual patient treatment, but instead for an acceptable sample of patients and/or treatment fractions. Independent real-time treatment verification is therefore desirable. We are developing a large area CMOS MAPS upstream of the patient to monitor MLC leaf positions for real-time treatment verification. CMOS MAPS are radiation hard for photon and electron irradiation, have high readout speeds and low attenuation which makes them an ideal upstream radiation detector for radiotherapy. Previously, we reported on leaf position reconstruction for single leaves using the Lassena, a 12 × 14 cm2, three side buttable MAPS suitable for clinical deployment. Sobel operator based methods were used for edge reconstruction. It was shown that the correspondence between reconstructed and set leaf position was excellent and resolutions ranged between 60.6 ± 8 and 109 ± 12 μm for a single central leaf with leaf extensions ranging from 1 to 35 mm using 0.3 sec of treatment beam time at 400 MU/min. Here, we report on leaf edge reconstruction using updated methods for complex leaf configurations, as occur in clinical use. Results show that leaf positions can be reconstructed with resolutions of 62 ± 6 μm for single leaves and 86 ± 16 μm for adjacent leaves at the isocenter using 0.15 sec at 400 MU/min of treatment beam. These resolutions are significantly better than current calibration standards.

OC-0076 Real time treatment verification in HDR brachytherapy: an in-phantom proof of principle

OC-0076 Real time treatment verification in HDR brachytherapy: an in-phantom proof of principle

PO-1050 A gynecological multichannel applicator including a real-time treatment verification system

PO-1050 A gynecological multichannel applicator including a real-time treatment verification system

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 > 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.

OA190] Novel inorganic scintillation detector system for real-time treatment verification during brachytherapy

Purpose Afterloaded brachytherapy administers a sealed radioisotope that is guided inside pre-inserted catheters in the tumor volume. The goal with the treatment modality is to deliver high doses to the tumor with steep dose gradients to spare adjacent organs and normal tissue. Real-time treatment verification could have an important role during brachytherapy treatments to detect errors that can lead to harmful consequences for the patient. However, real-time treatment verification is presently not performed during brachytherapy, partially because commercial technology does not exhibit adequate signal intensities over the entire range of absorbed dose rates. The limited use of treatment verification is problematic because errors can occur unnoticed. The purpose of this study was to develop scintillation-based point detectors that could have an important role for real-time verification during brachytherapy to detect and prevent treatment errors. Methods A real-time treatment verification system based on miniature inorganic scintillation detectors (ISDs) was developed. The ISDs consisted of a 1 mm-size scintillator that was optically coupled to a 1 mm-diameter and 15 m-long fiber-optic cable. The ISDs were based on the scintillation materials Al2O3:Cr, Y2O3:Eu, YVO4:Eu, ZnSe:O or CsI:Tl. The fiber-optic cable transmitted the scintillation to the photodetector system which consisted of a charge-coupled device camera or a spectrometer spectrograph. We tested the ISDs under brachytherapy treatment irradiation conditions using an Ir-192 source, and compared their performance with organic scintillators BCF-12 and BCF-60 which are the current standard for scintillation detectors in radiotherapy. Results The ISDs exhibited scintillation intensities that were up to 3 orders of magnitude greater than the detectors based on the organic scintillators. The large intensities of the ZnSe:O and CsI:Tl materials made it possible to develop an in vivo dosimetry system based on low-cost photodetector and data acquisition components. The prototype in vivo dosimetry system measured dose rates with Conclusions High-intensity ISD materials make it possible to develop low-cost in vivo dosimetry systems that precisely can monitor brachytherapy treatments. Miniature ISDs can therefore facilitate dissemination of real-time treatment verification technology for brachytherapy.

MR-based source localization for MR-guided HDR brachytherapy

For the purpose of MR-guided high-dose-rate (HDR) brachytherapy, a method for real-time localization of an HDR brachytherapy source was developed, which requires high spatial and temporal resolutions. MR-based localization of an HDR source serves two main aims. First, it enables real-time treatment verification by determination of the HDR source positions during treatment. Second, when using a dummy source, MR-based source localization provides an automatic detection of the source dwell positions after catheter insertion, allowing elimination of the catheter reconstruction procedure. Localization of the HDR source was conducted by simulation of the MR artifacts, followed by a phase correlation localization algorithm applied to the MR images and the simulated images, to determine the position of the HDR source in the MR images. To increase the temporal resolution of the MR acquisition, the spatial resolution was decreased, and a subpixel localization operation was introduced. Furthermore, parallel imaging (sensitivity encoding) was applied to further decrease the MR scan time. The localization method was validated by a comparison with CT, and the accuracy and precision were investigated. The results demonstrated that the described method could be used to determine the HDR source position with a high accuracy (0.4–0.6 mm) and a high precision (⩽0.1 mm), at high temporal resolutions (0.15–1.2 s per slice). This would enable real-time treatment verification as well as an automatic detection of the source dwell positions.

Inorganic scintillation detectors based on Eu-activated phosphors for 192Ir brachytherapy

The availability of real-time treatment verification during high-dose-rate (HDR) brachytherapy is currently limited. Therefore, we studied the luminescence properties of the widely commercially available scintillators using the inorganic materials Eu-activated phosphors Y2O3:Eu, YVO4:Eu, Y2O2S:Eu, and Gd2O2S:Eu to determine whether they could be used to accurately and precisely verify HDR brachytherapy doses in real time. The suitability for HDR brachytherapy of inorganic scintillation detectors (ISDs) based on the 4 Eu-activated phosphors in powder form was determined based on experiments with a 192Ir HDR brachytherapy source. The scintillation intensities of the phosphors were 16–134 times greater than that of the commonly used organic plastic scintillator BCF-12. High signal intensities were achieved with an optimized packing density of the phosphor mixture and with a shortened fiber-optic cable. The influence of contaminating Cerenkov and fluorescence light induced in the fiber-optic cable (stem signal) was adequately suppressed by inserting between the fiber-optic cable and the photodetector a 25 nm band-pass filter centered at the emission peak. The spurious photoluminescence signal induced by the stem signal was suppressed by placing a long-pass filter between the scintillation detector volume and the fiber-optic cable. The time-dependent luminescence properties of the phosphors were quantified by measuring the non-constant scintillation during irradiation and the afterglow after the brachytherapy source had retracted. We demonstrated that a mixture of Y2O3:Eu and YVO4:Eu suppressed the time-dependence of the ISDs and that the time-dependence of Y2O2S:Eu and Gd2O2S:Eu introduced large measurement inaccuracies. We conclude that ISDs based on a mixture of Y2O3:Eu and YVO4:Eu are promising candidates for accurate and precise real-time verification technology for HDR BT that is cost effective and straightforward to manufacture. Widespread dissemination of this technology could lead to an improved understanding of error types and frequencies during BT and to improved patient safety during treatment.

O4. Monte Carlo modelling of a prototype beam delivery check system for intensity modulated radiation therapy plan

Introduction Monte Carlo (MC) simulation techniques can accurately solve complex radiation transport challenges. This study applied the BEAMnrc Monte Carlo code technique to model an online dose monitoring system, the Integral Quality Monitoring (IQM) device (iRT Systems GmbH, Koblenz, Germany) and study the signal response of an IMRT treatment plan using the IQM model. IMRT treatments have generated complexities in their pretreatment quality assurance (QA) and there is huge curiosity to verify the treatment plan data during the real-time treatment. The Integral Quality Monitor (IQM) is an independent real-time treatment verification system which verifies the integrity and validates the accuracy of the treatment plan data. The IQM also functions as a pretreatment quality assurance tool for radiotherapy. Materials and methods The EGSnrc/BEAMnrc MC codes were utilized to model the Elekta Synergy linac equipped with Agility 160-leaf MLC as well as a MC model of the IQM. Full clinical IMRT plans were extracted from a DICOM plan file to create input files for the MC simulations. The simulated output signals from the IQM model were generated. Results IQM model was successfully developed and the Monte Carlo calculated IMRT signals show a good response from one segment to another of maximum IMRT plan signal. Conclusion Application of MC techniques to online dose monitoring is authentic. It demonstrates that MC radiation transport method is virtually unlimited when it comes to solving radiation transport and dose calculation challenges.

TU‐F‐BRE‐01: A High Resolution Micro Fiber Scintillator Detector Optimized for SRS and SBRT in Vivo Real Time Treatment Verification

Purpose:We have built a high resolution real time scintillating fiber detector prototype to determine in real time the accuracy of stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT) treatments when only a fraction of the planned dose was delivered. The motivation of this work is to enhance dose delivery accuracy and to achieve error free radiosurgery.Methods:A high density array of scintillating fibers and a high speed photo detectors array were integrated to implement a high resolution real time dosimeter that can sample with high resolution pulsed SRS and SBRT beams cross sections. The high efficiency of the developed system allows to read each linac pulse in real time and to compute the accumulated dose and dose errors when only a fraction of the beam was delivered. The fibers are highly packed in a substrate that is directly coupled to two 128 pixel arrays with a pitch matching the fiber spacing to achieve accurate spatial localization. The small cross section of the fiber array allows stacking multiple fiber arrays to measure independent angular profiles that are digitally processed in parallel for real time dosimetry.Results:We implemented a high density array detector prototype with a pitch of 0.5 mm, readout speed of 1.2 msec, and a response time of 0.5 usec. The fast reading speed has the capability to determining the dose in flattening free filter beams. The detector can be installed in transmission mode at the output port of a micro‐MLC. Treatment deviations smaller than 3% are detected when less than 1/100 of the planned dose was delivered.Conclusions:We built a prototype of a high resolution fiber scintillator array detector for SRS and SBRT in vivo dosimetry. Results show that the developed detector has the potential to assure error free SRS and SBRT treatments.

SU‐E‐T‐443: Measuring Prompt Gamma Emission During Passive Beam Proton Radiotherapy for Real Time Treatment Verification

Purpose: Recent studies demonstrate the feasibility of prompt‐gamma imaging for pencil beam proton therapy. In contrast to passive‐scatter proton radiotherapy, the existence of high background signal from secondary radiation produced by protons in the beam line components makes the detection of prompt‐gammas challenging. Our primary goal is to produce a more complete measurement of the prompt‐gammas produced in tissue for clinical treatment conditions. Methods: The proton treatment facility at iThemba Labs was studied in detail and all the nozzle components that interact with the proton beam were built and positioned at the locations specified by the manufacturer using the Geant4 Monte Carlo toolkit. The treatment nozzle was validated against depth dose and lateral profiles in a water phantom at therapeutic energies. The model included NaI detectors of different dimensions to detect the prompt‐gamma spectra, and standard gamma emitting sources in the energy range 0.661 to 4.438 MeV were used to determine the Gaussian broadening parameters of the detectors. Finally, prompt‐gamma spectra from the tissue phantom were simulated over a range of energies in order to make comparisons with the experimental spectra currently being measured. Results: The prompt‐gamma energy spectra show prominent peaks caused by collisions of proton with nuclei in the tissue phantom. Single and double escape peaks of 4.44 MeV were clearly seen, while the 6.129 MeV peak was unresolved due to the Doppler broadening effect, peaks at 6.916 and 7.115 MeV were not observed, but the single and double escape peaks of 6.129 MeV were visible. Conclusion: Our study confirms the feasibility of prompt‐gamma imaging during passive proton radiotherapy to assist for on‐line treatment verification. The energy range from 3.0 MeV to 5.0 MeV is a suitable candidate for further investigation and the presence of background radiation can be minimized by introducing proper shielding and collimation.

Sci-Wed PM: Delivery-02: Characterization and Clinical Evaluation of RADPOS 4D in vivo Dosimetry System

A novel 4D in vivo dosimetry system, RADPOS, has recently been developed. The system combines a MOSFET dosimeter with an electromagnetic positioning sensor and is currently used in a clinical trial to determine its potential for application in external beam treatments. The probe is small (1.3 mm diameter) and can be used either on patient skin or inside natural body cavities as a real time treatment verification tool providing immediate feedback about patient motion and the accuracy of the delivered dose. Preliminary tests have shown that the RADPOS has dosimetric characteristics similar to microMOSFET detectors, when the last one is used alone. The system can measure displacements with an accuracy ranging from (1.1–1.3) mm, within a volume defined by the coordinates: , , and , related to the transmitter. The stability of the RADPOS position readout is within 0.21 ± 0.07 mm and is not affected by the linac beam. The clinical trial involves lung cancer patients treated with external beam radiotherapy. Four RADPOS detectors are positioned at marked points on the patients' abdomen and chest during an initial 4DCT and subsequent treatment fractions. Position coordinates of the sensors are read in real time and compared to determine deviation from initial breathing patterns measured during the 4DCT. Total dose is read at the end of each fraction. Initial clinical results show that the RADPOS system can measure the amplitude and period of the patients breathing pattern and can provide a means to identify corresponding changes in dose.

121P CLINICAL EVALUATION OF A NOVEL 4D IN-VIVO DOSIMETRY SYSTEM FOR LUNG CANCER RADIOTHERAPY

Background: A novel 4D dosimetry system called RADPOS has recently been developed and is in use at The Ottawa Hospital Cancer Centre as part of a clinical trial to determine its potential for use in external beam treatment for lung cancer patients. The RADPOS combines a MOSFET dosimeter with an electromagnetic positioning sensor to allow for simultaneous measurement of dose and spatial coordinates at a specific location. The probe is small (1.3mm diameter) and it can be used either on patient skin or inside natural body cavities. The RADPOS system can be used on patients as a real time treatment verification tool providing immediate feedback about patient motion and the accuracy of the delivered dose. Preliminary tests have shown that the RADPOS has dosimetric characteristics similar to microMOSFET detectors, when used alone. The system can measure displacement with an accuracy of (0.75±0.07)mm for total displacements up to 200mm. The stability of the RADPOS position readout is within 0.21±0.07mm and is not affected by the linac beam. Aluminum, brass, steel and lead were the only materials of those tested that caused significant interference with this accuracy. Methods: The clinical trial involves lung cancer patients treated with external beam radiotherapy. Four RADPOS detectors are positioned at marked points on the patients’ abdomen during an initial 4DCT and subsequent treatment fractions. Position coordinates of the sensors are read in real time at a rate of 20 25Hz and total dose is read at the end of each fraction. Amplitude and period of the breathing motion are analyzed and compared to determine deviation from initial breathing patterns measured during the 4DCT. Results: Results show that the RADPOS system can be set-up quickly, requiring minimal additional time for each scheduled treatment fraction. The RADPOS measured breathing pattern agrees very well with the simultaneous measurement done with the Philips Bellows System and Varian RPM systems. When used to measure simulated breathing motion on a 4D Quasar phantom, the RADPOS-measured amplitude was within 0.2mm of measurements made by the Philips Bellows System. Conclusions: In conclusion, the RADPOS provides sufficient information to identify changes in the patients breathing pattern and other patient motion.

122P ENDOBRONCHIAL RADIOTHERAPY IN THE TREATMENT OF TRACHEA MALIGNANT TUMORS

Background: A novel 4D dosimetry system called RADPOS has recently been developed and is in use at The Ottawa Hospital Cancer Centre as part of a clinical trial to determine its potential for use in external beam treatment for lung cancer patients. The RADPOS combines a MOSFET dosimeter with an electromagnetic positioning sensor to allow for simultaneous measurement of dose and spatial coordinates at a specific location. The probe is small (1.3mm diameter) and it can be used either on patient skin or inside natural body cavities. The RADPOS system can be used on patients as a real time treatment verification tool providing immediate feedback about patient motion and the accuracy of the delivered dose. Preliminary tests have shown that the RADPOS has dosimetric characteristics similar to microMOSFET detectors, when used alone. The system can measure displacement with an accuracy of (0.75±0.07)mm for total displacements up to 200mm. The stability of the RADPOS position readout is within 0.21±0.07mm and is not affected by the linac beam. Aluminum, brass, steel and lead were the only materials of those tested that caused significant interference with this accuracy. Methods: The clinical trial involves lung cancer patients treated with external beam radiotherapy. Four RADPOS detectors are positioned at marked points on the patients’ abdomen during an initial 4DCT and subsequent treatment fractions. Position coordinates of the sensors are read in real time at a rate of 20 25Hz and total dose is read at the end of each fraction. Amplitude and period of the breathing motion are analyzed and compared to determine deviation from initial breathing patterns measured during the 4DCT. Results: Results show that the RADPOS system can be set-up quickly, requiring minimal additional time for each scheduled treatment fraction. The RADPOS measured breathing pattern agrees very well with the simultaneous measurement done with the Philips Bellows System and Varian RPM systems. When used to measure simulated breathing motion on a 4D Quasar phantom, the RADPOS-measured amplitude was within 0.2mm of measurements made by the Philips Bellows System. Conclusions: In conclusion, the RADPOS provides sufficient information to identify changes in the patients breathing pattern and other patient motion.

Three-dimensional, Real-time Treatment Verification with Simultaneous, Orthogonal kV-MV Imaging

Three-dimensional, Real-time Treatment Verification with Simultaneous, Orthogonal kV-MV Imaging