Plastic Scintillation Dosimeters Research Articles

Plastic scintillation dosimeter with a conical mirror for measuring 3D dose distribution.

To test the measurement technique of the three-dimensional (3D) dose distribution measured image by capturing the scintillation light generated using a plastic scintillator and a scintillating screen. Our imaging system constituted a column shaped plastic scintillator covered by a Gd2 O2 S:Tb scintillating screen, a conical mirror and a cooled CCD camera. The scintillator was irradiated with 6 MV photon beams. Meanwhile, the irradiated plan was prepared for the static field plans, two-field plan (2F plan) and the conformal arc plan (CA plan). The 2F plan contained 16 mm2 and 10 mm2 fields irradiated from gantry angles of 0° and 25°, respectively. The gantry was rotated counterclockwise from 45° to 315° for the CA plan. The field size was then obtained as 10 mm2 . A Monte Carlo simulation was performed in the experimental geometry to obtain the calculated 3D dose distribution as the reference data. Dose response was acquired by comparing between the reference and the measurement. The dose rate dependence was verified by irradiating the same MU value at different dose rates ranging from 100 to 600 MU/min. Deconvolution processing was applied to the measured images for the correction of light blurring. The measured 3D dose distribution was reconstructed from each measured image. Gamma analysis was performed to these 3D dose distributions. The gamma criteria were 3% for the dose difference, 2mm for the distance-to-agreement and 10% for the threshold. Dose response for the scintillation light was linear. The variation in the light intensity for the dose rate ranging from 100 to 600 MU/min was less than 0.5%, while our system presents dose rate independence. For the 3D dose measurement, blurring of light through deconvolution processing worked well. The 3D gamma passing rate (3D GPR) for the 10 × 10 mm2 , 16 × 16 mm2 , and 20 × 20 mm2 fields were observed to be 99.3%, 98.8%, and 97.8%, respectively. Reproducibility of measurement was verified. The 3D GPR results for the 2F plan and the CA plan were 99.7% and 100%, respectively. We developed a plastic scintillation dosimeter and demonstrated that our system concept can act as a suitable technique for measuring the 3D dose distribution from the gamma results. In the future, we will attempt to measure the 4D dose distribution for clinical volumetric modulated arc radiation therapy (VMAT)-SBRTplans.

In-line MRI-LINAC depth dose measurements using an in-house plastic scintillation dosimeter

Plastic scintillation dosimeters (PSDs) have many properties that make them desirable for relative dosimetry with MRI-LINACs. An in-house PSD, Farmer ionisation chamber and Gafchromic EBT3 film were used to measure central axis percentage depth dose distributions (PDDs) at the Australian MRI-LINAC Mean errors were calculated between each detector’s responses, where the in-house PSD was on average within 0.7% of the Farmer chamber and 1.4% of film, while the Farmer chamber and film were on average within 1.1% of each other. However, the PSD systematically over-estimated the dose as depth increased, approaching a maximum overestimation of the order of 3.5% for the smallest field size measured. This trend was statistically insignificant for all other field sizes measured; further investigation is required to determine the source of this effect. The calculated values of mean absolute error are comparable to the those of trusted dosimeters reported in the literature. These mean absolute errors, and the ubiquity of desirable dosimetric qualities inherent to PSDs suggest that PSDs in general are accurate for relative dosimetry with the MRI-LINAC. Further investigation is required into the source of the reported systematic trends dependent on field-size and depth of measurement.

PO-1335: Characterization of novel 3D printed plastic scintillation dosimeters

PO-1335: Characterization of novel 3D printed plastic scintillation dosimeters

Characterization of novel 3D printed plastic scintillation dosimeters

We propose a new methodology for the fabrication and evaluation of scintillating detector elements using a consumer grade fusion deposition modeling (FDM) 3D printer. In this study we performed a comprehensive investigation into both the effects of the 3D printing process on the scintillation light output of 3D printed plastic scintillation dosimeters (PSDs) and their associated dosimetric properties. Fabrication properties including print variability, layer thickness, anisotropy and extrusion temperature were assessed for 1 cm3 printed samples. We then examined the stability, dose linearity, dose rate proportionality, energy dependence and reproducibility of the 3D printed PSDs compared to benchmarks set by commercially available products. Experimental results indicate that the shape of the emission spectrum of the 3D printed PSDs do not show significant spectral differences when compared to the emission spectrum of the commercial sample. However, the magnitude of scintillation light output was found to be strongly dependent on the parameters of the fabrication process. Dosimetric testing indicates that the 3D printed PSDs share many desirable properties with current commercially available PSDs such as dose linearity, dose rate independence, energy independence in the MV range, repeatability, and stability. These results demonstrate that not only does 3D printing offer a new avenue for the production and manufacturing of PSDs but also allows for further investigation into the application of 3D printing in dosimetry. Such investigations could include options for 3D printed, patient-specific scintillating dosimeters that may be used as standalone dosimeters or incorporated into existing 3D printed patient devices (e.g. bolus or immobilization) used during the delivery of radiation therapy.

MRI-LINAC beam profile measurements using a plastic scintillation dosimeter.

Plastic scintillation dosimeters (PSDs) possess many desirable qualities for dosimetry with LINACs. These qualities are expected to make PSDs effective for MRI-LINAC dosimetry, however little research has been conducted investigating their dosimetric performance with MRI-LINACs. In this work, an in-house PSD was used to measure 8 beam profiles with an in-line MRI-LINAC, compared with film measurements. One dimensional global gamma indices (γ) and corresponding γ pass rates were calculated to compare PSD and film profiles for the 1%/1 mm, 2%/2 mm and 3%/3 mm criterion. The mean global pass rates were 85.8%, 97.5% and 99.4% for the 1%/1 mm, 2%/2 mm and 3%/3 mm criteria, respectively. The majority of the γ failures occurred in the penumbral regions. Penumbra widths were measured to be slightly narrower with the PSD compared to film, however, the uncertainties in the measured penumbra widths brought the PSD and film penumbra widths into agreement. Differences in dose were calculated between the PSD and film, and remained within 2.2% global agreement for the central regions and 1.5% global agreement for out of field regions. These values for range of agreement were similar to the those reported in the literature for other dosimeters which are trusted for relative MRI-LINAC dosimetry.

Dose rate measurement of Leksell Gamma Knife Perfexion using a 3D printed plastic scintillation dosimeter

In recent years, 3D printing technology has received significant research attention. Additionally, 3D printing technology is being applied to study radiation dosimeters of various materials. In this study, a plastic scintillator for 3D printing was developed in a laboratory and used to manufacture a plastic scintillation dosimeter (PSD) with a shape identical to that of the ionization chamber PTW31010. The 16-mm beam of Gamma Knife® Perfexion™ was irradiated to derive the absorbed dose rates of the PSD and PTW31010; they were subsequently compared with the dose rates of the treatment plan. The differences in the dose rates of the Gamma Knife treatment plan and the absorbed dose rates of PTW31010 were within 0.87%. The difference between the dose rates of the Gamma Knife treatment plan and the absorbed dose rates of the PSD were within 4.1%. A linear fit of the absorbed dose rates of four shots involving different dose rates and irradiation angles yielded an adjusted R-square value exceeding 0.9999. A total of 10 repeated measurements were conducted for the same shot to confirm its reproducibility, with a relative error of 0.56%.

First measurements with a plastic scintillation dosimeter at the Australian MRI-LINAC

MRI-LINACs combine MRI and LINAC technologies with the potential for image guided radiation therapy with optimal soft-tissue contrast. In this work, we present the advantages and limitations of plastic scintillation dosimeters (PSDs) for relative dosimetry with MRI-LINACs. PSDs possess many desirable qualities, including magnetic field insensitivity and irradiation angle independence, which are expected to make them suitable for dosimetry with MRI-LINACs. An in-house PSD was used to measure field size output factors as well as a percent depth dose distribution and the beam quality index TPR20/10 at a cm2 field size. Measurements were repeated with a Scanditronix/Wellhofer FC65-G ionisation chamber and PTW 60019 microDiamond detector for comparison. Relative differences were calculated between the three detectors, where the mean difference in dose was 1.2% between the PSD and ionisation chamber, 1.9% between the PSD and microDiamond detector and 1.3% between the microDiamond detector and the ionisation chamber. The closeness between the three mean differences in doses suggests that PSDs are feasible for relative dosimetry with MRI-LINACs.

Technical Note: Identification of an optimal electromagnetic sensor for invivo electromagnetic-tracked scintillation dosimeter for HDR brachytherapy.

Brachytherapy is a treatment modality which delivers large doses of radiation in a reduced number of visits. Since a small number of large dose-per-fraction is administered in high dose rate brachytherapy, ensuring the right dose is delivered is highly critical. In this work, a scintillation detector is coupled to an electromagnetic (EM) sensor (NDI, Waterloo, ON, Canada) having submillimeter positional accuracy for real-time tracking of the dosimeter position. However, adding an EM sensor adds materials in the path to the scintillator and thus could potentially perturb the dose measurements. This study assesses four different sensors for a plastic scintillation detector-EM sensor coupled dosimeter. To confirm the perturbation presence, different sensors were placed in front of the scintillator so the radiation does not arrive to it directly. Variation of the distance between the sensor and the scintillator was used to quantify the effect on the signal at and . To test the signal's angular dependence for each sensor, the signal measurement was taken from to with increment. The Aurora 5DOF-610090 sensor showed an increased signal of almost 20% with increasing beam angle. Sensors Aurora 5DOF-610099, Aurora 5DOF-610157, and Aurora Micro 6DOF-610059 showed no significant angle dependance. The Aurora Micro 6DOF-610059 and Aurora 5DOF-610157 sensors' cable signal revealed no extra signal attenuation. The latter gives a smaller overall attenuation. Therefore, the Aurora 5DOF-610157 sensor is chosen to be part of the novel dosimeter construction. It has a jitter error (average standard deviation of each individual measurement) of ±0.06mm and a reproducibility of ±0.008mm. In the optimal operating range, the average positional uncertainty is less than 0.2 mm. Average angle errors are not higher than . It is feasible to integrate an EM tracking sensor to a plastic scintillation dosimeter with minimal impact to the collected signal as well as sufficient positional accuracy to keep dose uncertainty below 5%.

ELPHA: Dynamically deformable liver phantom for real-time motion-adaptive radiotherapy treatments.

Real-time motion-adaptive radiotherapy of intrahepatic tumors needs to account for motion and deformations of the liver and the target location within. Phantoms representative of anatomical deformations are required to investigate and improve dynamic treatments. A deformable phantom capable of testing motion detection and motion mitigation techniques is presented here. The dynamically dEformable Liver PHAntom (ELPHA) was designed to fulfill three main constraints: First, a reproducibly deformable anatomy is required. Second, the phantom should provide multimodality imaging contrast for motion detection. Third, a time-resolved dosimetry system to measure temporal effects should be provided. An artificial liver with vasculature was casted from soft silicone mixtures. The silicones allow for deformation and radiographic image contrast, while added cellulose provides ultrasonic contrast. An actuator was used for compressing the liver in the inferior direction according to a prescribed respiratory motion trace. Electromagnetic (EM) transponders integrated in ELPHA help provide ground truth motion traces. They were used to quantify the motion reproducibility of the phantom and to validate motion detection based on ultrasound imaging. A two-dimensional ultrasound probe was used to follow the position of the vessels with a template-matching algorithm. This detected vessel motion was compared to the EM transponder signal by calculating the root-mean-square error (RMSE). ELPHA was then used to investigate the dose deposition of dynamic treatment deliveries. Two dosimetry systems, radio-chromic film and plastic scintillation dosimeters (PSD), were integrated in ELPHA. The PSD allow for time-resolved measurement of the delivered dose, which was compared to a time-resolved dose of the treatment planning system. Film and PSD were used to investigate dose delivery to the deforming phantom without motion compensation and with treatment-couch tracking for motion compensation. ELPHA showed densities of 66 and 45HU in the liver and the surrounding tissues. A high motion reproducibility with a submillimeter RMSE (<0.32mm) was measured. The motion of the vasculature detected with ultrasound agreed well with the EM transponder position (RMSE<1mm). A time-resolved dosimetry system with a 1Hz time resolution was achieved with the PSD. The agreement of the planned and measured dose to the PSD decreased with increasing motion amplitude: A dosimetric RMSE of 1.2, 2.1, and 2.7cGy/s was measured for motion amplitudes of 8, 16, and 24mm, respectively. With couch tracking as motion compensation, these values decreased to 1.1, 1.4, and 1.4cGy/s. This is closer to the static situation with 0.7cGy/s. Film measurements showed that couch tracking was able to compensate for motion with a mean target dose within 5% of the static situation (-5% to +1%), which was higher than in the uncompensated cases (-41% to -1%). ELPHA is a deformable liver phantom with high motion reproducibility. It was demonstrated to be suitable for the verification of motion detection and motion mitigation modalities. Based on the multimodality image contrast, a high accuracy of ultrasound based motion detection was shown. With the time-resolved dosimetry system, ELPHA is suitable for performance assessment of real-time motion-adaptive radiotherapy, as was shown exemplary with couch tracking.

Experimental investigation on the accuracy of plastic scintillators and of the spectrum discrimination method in small photon fields.

To investigate the accuracy of output factor measurements using a commercial (Exradin W1, SI) and a prototype, "in-house" developed, plastic scintillation dosimeter (PSD) in small photon fields. Repetitive detector-specific output factor OFdet measurements were performed in water (parallel to the CAX) using two W1 PSDs (SI), a PTW microLion, a PTW microDiamond and an unshielded diode D1V (SI) to which Monte Carlo calculated corrections factors were applied. Four sets of repetitive measurements were performed with the W1 PSD positioned parallel and perpendicular to the CAX, each set on a different day, and with analytically calculated volume averaging corrections applied. The W1 OFdet measurements were compared to measurements using an "in-house" developed PSD in water (CHUQ) and both were validated against a previously commissioned Monte Carlo beam model in small photon fields. The performance of the spectrum discrimination calibration procedure was evaluated under different fiber orientations and wavelength threshold choices and the impact on the respective OFdet was reported. For all detectors in the study an excellent agreement was observed down to a field size of 1×1 cm2 . For the smallest field size of 0.5×0.5 cm2 , the W1 PSDs presented OFdet readings higher by 3.8 to 5.0% relative to the mean corrected OFdet of the rest of the detectors and by 5.8 to 6.1% relative to the CHUQ PSD. The repetitive W1 OFdet measurements in water (parallel CAX) were higher by 3.9% relative to the OFdet measurements in Solid WaterTM (perpendicular CAX) even after volume averaging corrections were applied, indicating a potential fiber orientation dependency in small fields. Uncertainties in jaw and detector repositioning as well as source variations with time were estimated to be less than 0.9% (1 σ) for the W1 under both orientations. The CHUQ PSD agreed with the MC dose calculations in water, for the smallest field size, within 1.1-1.7% before any corrections and within 0.3-0.8% after volume averaging corrections. The spectrum discrimination method provided reproducible Cherenkov spectra under the different calibration set-ups with noisier spectra extracted if the calibration is performed in water and parallel to the CAX. The impact of fiber orientation and wavelength threshold during calibration on OFdet was in general minimal. Clinically relevant differences were observed between similar scintillator dosimeters in photon fields smaller than 1× 1 cm2 . Further research on PSDs is needed that can explain the origin of these differences especially related to the Cherenkov spectrum dependencies on the optical fiber technical characteristics.

TH-CD-304-09: Simple and Automatic Calibration Technique for Plastic Scintillation Dosimeters

Purpose: To validate a new faster, simpler and potentially more accurate calibration technique for plastic scintillation dosimeters (PSD). The proposed technique calibrates the dosimeter automatically using either a specific irradiation sequence or simply by using it during treatments or QA. Methods: PSDesigner, a multi-purpose PSD simulation platform written in Python, is used to simulate PSDs under a 6 MV, 600MU/min calibrating sequence at dmax. The signal is generated as a linear combination of experimentally measured emission spectra. The user specifies both the beam sequence parameters (jaws positions and treatment table displacement) and the optical probe geometry (e.g. positions of the scintillating elements). As noisy measurement data are collected, a computational technique, Independent Components Analysis (ICA), evaluates the system's eigenvectors that compose the calibration matrix. Known dose profiles are used to convert the eigenvectors units from current (from collected light by a PMT) to dose. Error on doses is evaluated as a function of total irradiation time to quantify the system's performance and ability to calibrate itself automatically as it is being used. Results: The decreasing error on doses measurements as the probe is being irradiated validates the use of ICA to calibrate the system. A stable calibration is achieved in less than 30s of real-time irradiation, showing promising results in favor of future auto-calibrating PSDs. Comparison between noisy and noise-less simulations also indicates that such system is insensitive to random errors. Introduction of a system defect shows how the system can correct itself by shifting its original set of eigenvectors. The system thus exhibit auto-correction and auto-diagnostic capabilities. Conclusion: This work validates a new auto-calibration technique for PSDs that relies on ICA. Unlike traditional calibration techniques, this simple and straightforward method does not get more complex as the number of scintillating elements and measurement points increases.

SU‐C‐304‐03: Experimental Investigation On the Accuracy of Plastic Scintillation Dosimeters in Small Fields

Purpose:To investigate the accuracy of the Exradin W1 (SI) and of an “in‐house” plastic scintillation dosimeter (CHUQ PSD) in small radiation fields.Methods:Output factor (OF) measurements with the W1 and CHUQ PSD were performed for field sizes of 0.5 × 0.5, 1 × 1 and 2 × 2 cm2. Both detectors were placed parallel to the central axis (CAX) in water. The spectrum discrimination calibration method was performed in each set‐up to account for the Cerenkov (CRV) signal created in the fiber. The OFs were compared to the expected field factors in water derived using i) Monte Carlo (MC) simulations of an accurate accelerator model and ii) microLion (PTW) and D1V diode (SI) OFs. MC‐derived correction factors were applied to both the microLion and D1V OFs. For the CHUQ PSD the calibration was repeated in water (// CAX), solid water (perpendicular to CAX) and under a shielded configuration. The signal was collected using a spectrometer (wavelength range = 185–1100 nm). Spectral analysis was performed to evaluate potential changes of the spectral distributions under the various calibration set‐up configurations.Results:The W1 OFs presented an over‐response for the 0.5 × 0.5 cm2 in the range of 3 – 4.1% relative to the expected field factor. The CHUQ PSD presented an under‐response in the range of 1.5 – 2.7%, without accounting for volume averaging. The CRV spectra under the various calibration procedures appeared similar to each other and only minor changes were observed to the respective OFs.Conclusion:The W1 and CHUQ PSD can be used in small fields down to a 1 × 1 cm2 field size. Discrepancies were encountered between the two detectors for the smallest field size of 0.5 × 0.5 cm2 with the CHUQ PSD exhibiting a closer agreement to the expected field factor.Funding sources: 1) Alexander S. Onassis Public Benefit Foundation in Greece and 2) CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council, Grant number: 432290, RGPIN‐2014‐06475

Sci-Sat AM: Stereo - 07: Suitability of a plastic scintillator dosimeter for composite clinical fields delivered using the Cyberknife robotic radiosurgery system

Plastic scintillation dosimeters (PSDs) have favourable characteristics for small and composite field dosimetry in radiosurgery, however, imperfect corrections for the Cerenkov radiation contamination could limit their accuracy for complex deliveries. In this work, we characterize the dose and dose-rate linearity, directional dependence, and compare output factors with other stereotactic detectors for a new commercially available PSD (Exradin W1). We provide some preliminary comparisons of planned and measured dose for composite fields delivered clinically by a Cyberknife radiosurgery system. The W1 detector shows good linearity with dose (<0.5%) and dose rate (<0.8%) relative to the signal obtained using an ion chamber under the same conditions. A maximum difference of 2% was observed depending on the detector's angular orientation. Output factors for all detectors agree within a range of ±3.2% and ±1.5% for the 5 and 7.5 mm collimators, respectively, provided Monte-Carlo corrections for detector effects are applied to diode and ion chambers (without corrections the range is ±5.5% and ±3.1% for these two collimators). For clinical beam deliveries using 5 and 7.5 mm collimators, four of the six patients showed better agreement with planned dose for the PSD detector compared to a micro ion chamber. Two of the six patients investigated,more » however, showed 5% differences between PSD and planned dose, film measurements and the ratio of PSD and micro ion chamber signal suggest that further investigation is warranted for these plans. The W1 detector is a promising tool for stereotactic plan verification under the challenging dosimetric conditions of stereotactic radiosurgery.« less

Current status of scintillation dosimetry for megavoltage beams

Plastic scintillation dosimeters (PSD) have been a topic of a keen research interest for the past 20 years. Numerous PSD systems have been proposed, most times differentiating from the previous by a slight change in one or more components, such as the photodetector. However, a few major technological and engineering innovations have also been made. Over the past few years PSDs have been evaluated for small field dosimetry, in vivo dose measurements, building of arrays and much more. The present manuscript is intended to present the basic physics and properties of PSDs and its application over the past two decades.

TU‐A‐116‐09: Real‐Time Plastic Scintillation Dosimeter for Fluoroscopically‐Guided Interventional Procedures

Purpose: To monitor and measure, in real‐time, patient skin dose during fluoroscopically‐guided interventional (FGI) procedures using a novel plastic scintillation dosimeter (PSD). Methods: A clear optical fiber of 1 mm diameter, 10 m long, is optically coupled to a plastic scintillation fiber (BCF‐60) of 1 mm diameter, 10 mm long. On its other end, the clear fiber is connected to an avalanche photodiode, APD (C4777‐01, Hamamatsu). An electrometer collects the APD charge and displays live graph and dose values. Initial calibration is achieved with a NRC calibrated Farmer‐type ionization chamber (Exradin A12, Standard Imaging). The PSD and the chamber are set on an Allura Xper Fd10/10 (Philips) intervention table at the Air Kerma reference point, 150 mm below isocentre. In compliance with TG‐125 data collection methodology, slabs of solid water are gradually added over the instruments. In order to preserve image quality, the automatic controls increase tube current and kVp accordingly. Acquisitions are made in both cine‐fluorography and fluoroscopy operation modes, at 15 frames/s. Measurements were repeated by replacing the solid water by an Alderson RANDO phantom, with image intensifier as close as possible of the surface to replicate clinical protocols. Results: The calibration curves indicate that PSD response is linear below 80 kVp, but exhibits a quadratic behavior above, with a rate of 20–40 pA/mGy over Air Kerma Rate range of 0.10–16 mGy/s. The PSD can read dose rate as low as 1.7 mGy/s ± 5 %. However, uncertainties rise above 10 % for dose rate less than 1 mGy/s. Phantom measurements indicate that the dosimeter is accurate for clinical applications. Conclusion: A PSD has been calibrated and tested in order to monitor and measure low dose rates involved in FGI procedures in real‐time. The next steps will include Monte Carlo calculations to assess accuracy before starting clinical procedures.

TU‐C‐108‐08: Characterization of a Fiber‐Taper CCD Photo‐Counting System for Plastic Scintillation Dosimetry and Comparison to the Traditional Lens System

Purpose: To build a lens‐less CCD photo‐counting system for plastic scintillation dosimeters (PSDs), characterize its signal‐to‐noise ratio (SNR), its dose and dose‐rate sensitivity, and compare it to traditional CCD+lens optical systems. Methods: The detector used in this study is made of a 1 mm diameter by 2 mm long BCF60 scintillating‐fiber coupled to a 2.6 meters clear plastic fiber. This PSD is coupled to two different photon‐counting systems. The first is composed of a fiber‐taper system (FTS) of a 2048 × 2048 pixels attached to an uncooled Alta (Apogee) 4020 polychromatic CCD‐camera and a second one consisting of a 1600 × 1200 pixels, Alta (Apogee) 2020 polychromatic CCD‐camera (cooled to ‐ 18°C) using a 50 mm lens of F#=1. The same identical dose measurements were made under the same irradiation conditions. For both systems, the chromatic Cerenkov removal method is used to account for the stem effects. Results: The FTS increases the light collection by a factor of 4. This gain is possible because we are no longer restricted by the geometric limitation or the optical aberration of a lens system. Despite a difference of 45°C in operating temperature, we observed a SNR 1.8 times higher. This produces a good stability of measure vs. integration time with at most 1% difference on low‐dose measurements. This also reduces by at least a factor of 2 the uncertainties, allowing us to realize low‐dose measurements of 1.0 and 0.5 cGy with accuracy of 3.4% and 5.6% vs. 5.8% and 15.9% for the lens‐based system. Conclusion: This study demonstrates the superiority of the simpler FTS over a lens‐based system. It opens the possibility of using PSDs of smaller radii (125 micron) to obtain greater spatial resolution. Reducing the size of PSDs could further simplify their use for in‐vivo monitoring and small field dosimetry. Development and funding supported by Standard Imaging.

A prototype scintillation dosimeter customized for small and dynamic megavoltage radiation fields

A prototype plastic scintillation dosimeter has been developed with a small sensitive volume, rapid response and good dosimetric performance. The novelty of this design is the use of an air core light guide to transport the scintillation signal out of the primary radiation field. The significance of this innovation is that it eliminates the Cerenkov background signal that is generated in conventional optical fibres. The dosimeter performance was compared to existing commercial dosimeters in 6 MV and 18 MV photon beams and 6 MeV and 20 MeV electron beams, in both static and dynamic fields. The dosimeter was tested in small static fields and in dynamically delivered fields where the detector volume is shielded, while the stem is irradiated. The depth dose measurements for the photon beams agreed with ionization chamber measurements to within 1.6%, except in the build-up region due to positional uncertainty. For the 6 MeV and 20 MeV electron beams, the percentage depth dose measurements agreed with the ionization chamber measurements to within 3.6% and 4.5%, respectively. For field sizes of 1 cm × 1 cm and greater, the air core dosimeter readings agreed with diamond detector readings to within 1.2%. The air core dosimeter was accurate in dynamically delivered fields and had no measurable stem effect. The air core dosimeter was accurate over a range of field sizes, energies and dose rates, confirming that it is a sensitive and accurate dosimeter with high spatial resolution suitable for use in megavoltage photon and electron beams.

Cerenkov-free scintillation dosimetry in external beam radiotherapy with an air core light guide

Plastic scintillators have many advantages for dosimetry in external beam radiotherapy. The current method of transmitting the scintillation light to a remote detector is through a solid core optical fibre. When exposed in a high energy therapeutic radiotherapy beam this fibre is subject to an unwanted background signal from Cerenkov light which can exceed the scintillation signal at characteristic angles. We have constructed a plastic scintillation dosimeter that uses an air core light guide to transport the light from the scintillator to the light detector. We show that there is sufficient signal propagation in the air core light guide to allow the scintillator signal to be carried outside the primary beam of a radiotherapy linear accelerator and for a dosimeter to be constructed using a scintillator inserted into the end of the light guide. Studies of the background light generated in the air core light guide, as a function of the angle between the beam and the fibre axis, show that there is no characteristic Cerenkov peak generated in the air core. Depth dose measurements using the air core scintillation dosimeter with no correction for Cerenkov are compared to ionization chamber measurements for a 6 MV photon beam and a 9 MeV electron beam.

Recent developments of optically stimulated luminescence materials and techniques for radiation dosimetry and clinical applications

During the last 10 years, optically stimulated luminescence (OSL) has emerged as a formidable competitor not only to thermoluminescence dosimetry (TLD) but also to several other dosimetry systems. Though a large number of materials have been synthesized and studied for OSL, Al2O3:C continues to dominate the dosimetric applications. Re-investigations of OSL in BeOindicate that this material might provide an alternative to Al2O3:C. Study of OSL of electronic components of mobile phones and ID cards appears to have opened up a feasibility of dosimetry and dose reconstruction using the electronic components of gadgets of everyday use in the events of unforeseen situations of radiological accidents, including the event of a dirty bomb by terrorist groups. Among the newly reported materials, a very recent development of NaMgF3:Eu2+ appears fascinating because of its high OSL sensitivity and tolerable tissue equivalence. In clinical dosimetry, an OSL as a passive dosimeter could do all that TLD can do, much faster with a better or at least the same efficiency; and in addition, it provides a possibility of repeated readout unlike TLD, in which all the dose information is lost in a single readout. Of late, OSL has also emerged as a practical real-time dosimeter for in vivo measurements in radiation therapy (for both external beams and brachytherapy) and in various diagnostic radiological examinations including mammography and CT dosimetry. For in vivo measurements, a probe of Al2O3:C of size of a fraction of a millimeter provides the information on both the dose rate and the total dose from the readout of radioluminescence and OSL signals respectively, from the same probe. The availability of OSL dosimeters in various sizes and shapes and their performance characteristics as compared to established dosimeters such as plastic scintillation dosimeters, diode detectors, MOSFET detectors, radiochromic films, etc., shows that OSL may soon become the first choice for point dose measurements in clinical applications. A brief review of the recent developments is presented.

Dosimetric calibration of solid state detectors with low energy β sources

A PTW Optidos plastic scintillation and a PTW natural diamond detectors were calibrated in terms of absorbed dose to water with β fields produced by 90Sr+90Y and 85Kr reference sources. Each source was characterized at the Italian National Metrological Institute – the Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti of ENEA (ENEA-INMRI) – for two different series, 1 and 2, of ISO reference β-particle radiation fields. Beam flattening filters were used for the series 1 β fields to give uniform absorbed dose rates over a large area at a source-to-reference plane distance of 30cm. The series 2 β fields were produced at source-to-reference plane distance of 10cm, without the beam flattening filters, in order to obtain higher absorbed dose rates.The reference absorbed dose rate values were directly determined by the Italian national standard for β-particle dosimetry (a PTW extrapolation ionization chamber) for the series 1 β fields and by a calibrated transfer standard chamber, (a Capintec thin fixed-volume parallel plate ionization chamber) for the series 2 β fields. Finally the two solid state detectors were calibrated in terms of absorbed dose to water with the series 2 β field.The expanded uncertainties of the calibration coefficients obtained for the plastic scintillation dosimeter were 10% and 12% (2SD) for the 90Sr+90Y and the 85Kr sources, respectively. The expanded uncertainties obtained for the diamond dosimeter were 10% (2SD) and 16% (2SD) for the 90Sr+90Y and the 85Kr sources, respectively.The good results obtained with the 90Sr+90Y and the 85Kr β sources encourage to implement this procedure to calibrate this type of detectors at shorter distances and with other β sources of interest in brachytherapy, for example the 106Ru source.