Dose Calibration Research Articles

Split Bolus Method in Computerized Tomography.

The split-bolus method in computed tomography (CT) is the method used in the evaluation of renal dynamic enhancement stages and in the detection of pathologies. When designing the CT urography technique, there are several important options such as single-bolus and split bolus techniques. The single-bolus method consists of three separate post-contrast phases: arterial, nephrographic, and excretory (pyelogram), as a result raising the total radiation dose imparted to patients. On the other hand, in the split-bolus technique, the contrast dose is divided into several separate administrations to obtain the nephrographic and excretory phases simultaneously. With the split-bolus technique, by reducing the radiation dose and the number of phases that the patient will be exposed to, urinary system evaluation and the whole abdomen pathological evaluations can be performed. The device to be used in imaging must be a tomography device with at least 16 Multidetector CT sections. The bolus tracking method is one of the most accurate contrast delivery methods for renal dynamics and the split-bolus technique. Automatic dose calibration is used.

Dosimetric evaluation of irradiation geometry and potential air gaps in an acrylic miniphantom used for external audit of absolute dose calibration for a hybrid 1.5T MR-linac system.

IntroductionTo investigate the impact of partial lateral scatter (LS), backscatter (BS) and presence of air gaps on optically stimulated luminescence dosimeter (OSLD) measurements in an acrylic miniphantom used for dosimetry audit on the 1.5 T magnetic resonance‐linear accelerator (MR‐linac) system.MethodsThe following irradiation geometries were investigated using OSLDs, A26 MR/A12 MR ion chamber (IC), and Monaco Monte Carlo system: (a) IC/OSLD in an acrylic miniphantom (partial LS, partial BS), (b) IC/OSLD in a miniphantom placed on a solid water (SW) stack at a depth of 1.5 cm (partial LS, full BS), (c) IC/OSLD placed at a depth of 1.5 cm inside a 3 cm slab of SW/buildup (full LS, partial BS), and (d) IC/OSLD centered inside a 3 cm slab of SW/buildup at a depth of 1.5 cm placed on top of a SW stack (full LS, full BS). Average of two irradiated OSLDs with and without water was used at each setup. An air gap of 1 and 2 mm, mimicking presence of potential air gap around the OSLDs in the miniphantom geometry was also simulated. The calibration condition of the machine was 1 cGy/MU at SAD = 143.5 cm, d = 5 cm, G90, and 10 × 10 cm2.ResultsThe Monaco calculation (0.5% uncertainty and 1.0 mm voxel size) for the four setups at the measurement point were 108.2, 108.1, 109.4, and 110.0 cGy. The corresponding IC measurements were 109.0 ± 0.03, 109.5 ± 0.06, 110.2 ± 0.02, and 109.8 ± 0.03 cGy. Without water, OSLDs measurements were ∼10% higher than the expected. With added water to minimize air gaps, the measurements were significantly improved to within 2.2%. The dosimetric impacts of 1 and 2 mm air gaps were also verified with Monaco to be 13.3% and 27.9% higher, respectively, due to the electron return effect.ConclusionsA minimal amount of air around or within the OSLDs can cause measurement discrepancies of 10% or higher when placed in a high b‐field MR‐linac system. Care must be taken to eliminate the air from within and around the OSLD.

Thyroid Uptake Exceeding 100%: Causes and Prevention.

Background: Radionuclide thyroid uptake measurements reflect the metabolic activity of the thyroid gland. Thyroid uptake is measured as a percentage of radioactivity retained by the gland at a specified time versus the activity administered to the patient; thus, uptake measurements must fall between 0 and 100%. In this study we review sources of errors that can lead to uptake >100% through a case study and describe a novel quality control (QC) indicator to improve the accuracy of uptake measurements in the clinic. Methods: Probe efficiency is determined as the ratio between dose counts of the probe relative to the independent dose calibrator activity readings. The nominal probe efficiency value (M) was calculated as the mean of readings (n≥20) and variance was characterized using the standard-deviation (SD). Warning levels were set at M±1.96×SD and error levels were set to M±2.58×SD. In subsequent routine clinical use, prior to administrating a capsule, the probe efficiency is calculated and compared with the warning and error limits. We derived M for three pairs of probe and dose-calibrator devices using several doses and measured independently by several nuclear medicine technologists. Results: The recorded data indicated that nominal efficiency was statistically different between our old device and the one that replaced it (P = 0.01) but coefficient-of-variation(CV% =SD/M×100%) was not (P = 0.42) when technologists were made aware of the expected efficiency value. Using efficiency measurements of the first 20 first patients acquired on the replacement device new QC values were derived (M= 910, SD =36). In 22 patients measured at our sister site, with the same device models but without technologists being aware of QC initiative, derived QC values were (M =1025, SD =116), demonstrating a significant difference between nominal values of individual devices (p<0.001). Furthermore, variability was significantly lower (p<0.001) when QC was followed compared to when it was not applied. Conclusion: Adding the probe efficiency as a quality control indicator during thyroid uptake measurement is simple, can produce more precise clinical measurement and help mitigate operator and instrumentation errors.

Characterization of an under-development capacitor dosimeter equipped with a silicon x-ray diode.

Herein, we evaluated a capacitor dosimeter under development by a manufacturer, which is designed to monitor the entrance dose in x-ray diagnosis and comprises a silicon x-ray diode (Si-XD), a 0.1 µF capacitor, and a dosimeter dock. The Si-XD is a high-sensitivity photodiode optimized for x-ray detection. The dosimeter was charged to 3.30V using the dock before x-ray irradiation. The charging voltage was reduced by photocurrents flowing through the Si-XD during irradiation, and the discharging voltage was measured. For the fundamental characterization of this capacitor dosimeter, we investigated the x-ray tube-current and tube-voltage dependences of the measured dose using an industrial x-ray tube; the angular dependence was also investigated. A commercially available semiconductor dosimeter (RaySafe ThinX) was used for dose calibration. The doses were proportional to the tube current at a constant tube voltage of 100kV and increased with increasing tube voltage at a constant tube current of 1.0 mA. The dose difference with respect to the commercially available semiconductor dosimeter was within 1.0% when the tube current was varied and it was within 3.0% when the tube voltage was varied. In the angular dependence measurement, a difference of up to 6.0% was observed as the angle varied from 0° to 355° in steps of 5°. The dose-calibration results indicated that the determination of the initial charging voltage was important for dose conversion using the capacitor dosimeter.

P08 - Study of ion chamber dose calibration in kV-CBCT-based IGRT using different protocols

P08 - Study of ion chamber dose calibration in kV-CBCT-based IGRT using different protocols

Implementation of Beam Matching Concept for the New Installed Elekta Precise Treatment System Medical LINACs in Indonesia

A concept of radiation beam matching of some medical linear accelerators (LINACs) that have identical characteristics of the models, radiation quality, and multileaf collimator features may be implemented as long as the manufacturer provides complete specifications so that a Treatment Planning System (TPS) can be used for many beam-matched LINACs. This paper describes a preliminary study on the implementation of the beam matching concept for five units Elekta Precise Treatment System LINACs that have recently been installed in Indonesia. The beam matching criteria were based on the percentage depth dose (PDD) and beam profile for photon and electron beams. Dosimetry measurements were carried out by using an SNC 125 ionization chamber of 0.125 cm 3 in volume, PTW Pinpoint 3D of 0.016 cm 3 in volume, and PTW Farmer Chamber of 0.6 cm 3 in volume. The results indicated that the PDD 10 of 6 and 10 MV photon beams among installed five units LINACs have excellent compatibility each others with a maximum deviation of less than 0.4 %, while the maximum deviation for dose depth of 80 % (R 80 ) for the electron beams with nominal energies of 4, 6, 8, 10, 15 and 18 MeV is 1 mm. The measurement results for the flatness profile were less than 6 %, and symmetry profiles were less than 3 %. It also outlines the determination of the absorbed dose to water under reference conditions. The results of the calibration of output doses show that the absorbed dose in the water was 1 cGy ≈ 1 MU. The data obtained from measurements for each LINAC conform with the requirements of the beam matching process set by the manufacturer.

Organ Dose Reconstruction Applicable for a Japanese Nuclear Worker Cohort: J-EPISODE.

An evaluation of cancer risk based on organ-absorbed dose is underway for the Japanese Epidemiological Study on Low-Dose Radiation Effects (J-EPISODE), which has analyzed health effects in association with radiation exposure evaluated with the personal dose equivalent Hp(10). Although the concept of effective dose and its operational definition of Hp(10) are widely used for radiological protection purposes, effective dose is not recommended for epidemiological evaluation. Organ-absorbed dose was instead adopted for the IARC 15-Country Collaborative study (15-Country study), the International Nuclear Workers Study (INWORKS), the Mayak worker study, and the Life Span Study (LSS) of atomic bomb survivors. The reconstruction method in J-EPISODE followed in principle the approach adopted in the 15-Country Study. As part of the approach of J-EPISODE, a conversion factor from photon dosimeter reading to air kerma was developed using dosimeter response data, which were measured by the experiment using an anthropomorphic phantom, and it was confirmed that the 15-Country study’s assumption of photon energy and geometry distribution in a work environment applied to Japanese nuclear workers. This article focuses on a method for reconstructing the conversion factor from photon dosimeter reading to organ-absorbed photon dose for a Japanese nuclear worker cohort. The model for estimating the conversion factor was defined under the assumption of a lognormal distribution from three concerned bias factors: (1) a dosimeter reading per air kerma, i.e., dosimeter response; (2) an organ-absorbed dose per air kerma; and (3) a factor relating to the differences in dose concepts and calibration practices between the roentgen dosimeter era and the present. Dosimeter response data were cited from the companion paper. Data on organ-absorbed photon dose per air kerma were estimated using a voxel phantom with the average Japanese adult male height and weight. The bias factor for the recorded dose in the roentgen era was defined, considering the backscatter radiation from the human body. The estimated values of organ-absorbed photon dose per air kerma were almost the same as those in ICRP Publication 116, revealing that the effect of differences in body size was almost negligible. The conversion factors from dosimeter reading to organ-absorbed dose were estimated by period (the roentgen era or from then), nuclear facility type (nuclear power plant or other), dosimeter type, and tissue or organ. The estimated conversion factors ranged from 0.7 to 0.9 (Gy Sv−1). The estimated cumulative organ-absorbed photon dose for the participants of J-EPISODE demonstrated that organ-absorbed dose values were approximately 0.8 times the recorded doses if neglecting dose-unit differences. J-EPISODE reconstructed an organ-absorbed dose conversion factor and will evaluate the risk of cancer mortality and morbidity using the organ-absorbed dose in the future.

Traceable reference dosimetry in MRI guided radiotherapy using alanine: calibration and magnetic field correction factors of ionisation chambers

Magnetic resonance imaging (MRI)-guided radiotherapy (RT) (MRIgRT) falls outside the scope of existing high energy photon therapy dosimetry protocols, because those protocols do not consider the effects of the magnetic field on detector response and on absorbed dose to water. The aim of this study is to evaluate and demonstrate the traceable measurement of absorbed dose in MRIgRT systems using alanine, made possible by the characterisation of alanine sensitivity to magnetic fields reported previously by Billas et al (2020 Phys. Med. Biol. 65 115001), in a way which is compatible with existing standards and calibrations available for conventional RT. In this study, alanine is used to transfer absorbed dose to water to MRIgRT systems from a conventional linac. This offers an alternative route for the traceable measurement of absorbed dose to water, one which is independent of the transfer using ionisation chambers. The alanine dosimetry is analysed in combination with measurements with several Farmer-type chambers, PTW 30013 and IBA FC65-G, at six different centres and two different MRIgRT systems (Elekta Unity™ and ViewRay MRIdian™). The results are analysed in terms of the magnetic field correction factors, and in terms of the absorbed dose calibration coefficients for the chambers, determined at each centre. This approach to reference dosimetry in MRIgRT produces good consistency in the results, across the centres visited, at the level of 0.4% (standard deviation). Farmer-type ionisation chamber magnetic field correction factors were determined directly, by comparing calibrations in some MRIgRT systems with and without the magnetic field ramped up, and indirectly, by comparing calibrations in all the MRIgRT systems with calibrations in a conventional linac. Calibration coefficients in the MRIgRT systems were obtained with a standard uncertainty of 1.1% (Elekta Unity™) and 0.9% (ViewRay MRIdian™), for three different chamber orientations with respect to the magnetic field. The values obtained for the magnetic field correction factor in this investigation are consistent with those presented in the summary by de Pooter et al (2021 Phys. Med. Biol. 66 05TR02), and would tend to support the adoption of a magnetic field correction factor which depends on the chamber type, PTW 30013 or IBA FC65-G.

A disposable OSL dosimeter for in vivo measurement of rectum dose during brachytherapy.

We aimed to develop a disposable rectum dosimeter and to demonstrate its ability to measure exposure dose to the rectum during brachytherapy for cervical cancer treatment using high-dose rate 192 Ir. Our rectum dosimeter measures the dose with an optically stimulated luminescence (OSL) sheet which was furled to a catheter. The catheter we used is 6mm in diameter; therefore, it is much less invasive than other rectum dosimeters. The rectum dosimeter developed in this study has the characteristics of being inexpensive and disposable. It is also an easy-to-use detector that can be individually sterilized, making it suitable for clinical use. To obtain a dose calibration curve, phantom experiments were performed. Irradiation was performed using a cubical acrylic phantom, and the response of the OSL dosimeter was calibrated with the calculation value predicted by the treatment planning system (TPS). Additionally, the dependence of catheter angle on the dosimeter position and repeatability were evaluated. We also measured the absorbed dose to the rectum of patients who were undergoing brachytherapy for cervical cancer (n=64). The doses measured with our dosimeters were compared with the doses calculated by the TPS. In order to examine the causes of large differences between measured and planned doses, we classified the data into common and specific cases when performing this clinical study. For specific cases, the following three categories were considered: (a) patient movement, (b) gas in the vagina and/or rectum, and (c) artifacts in the X-ray image caused by applicators. A dose calibration curve was obtained in the range of 0.1Gy-10.0Gy. From the evaluation of the dependence of catheter angle on the dosimeter position and repeatability, we determined that our dosimeter can measure rectum dose with an accuracy of 3.1% (k=1). In this clinical study, we succeeded in measuring actual doses using our rectum dosimeter. We found that the deviation of the measured dose from the planned dose was derived to be 12.7% (k=1); this result shows that the clinical study included large elements of uncertainty. The discrepancies were found to be due to patient motion during treatment, applicator movement after planning images were taken, and artifacts in the planning images. We present the idea that a minimally invasive rectum dosimeter can be fabricated using an OSL sheet. Our clinical study demonstrates that a rectum dosimeter made from an OSL sheet has sufficient ability to evaluate rectum dose. Using this dosimeter, valuable information concerning organs at risk can be obtained during brachytherapy.

Thermoluminescent relative efficiency of TLD-100 glow peaks after exposure to X-rays of 20 kV–300 kV, 137Cs and 60Co gamma

Thermoluminescent dosimeter LiF:Mg, Ti (TLD-100) has been considered as the “gold standard” to measure absorbed dose from low-energy photon fields in medical applications. However, to calibrate the dosimeters, high energy beams (6 MV X-ray or 60Co gamma) are usually used since low photon energy beams are not always available. Thus, knowledge of the relative efficiency, RE, is necessary in order to convert the absorbed dose calibration from high energy beams to low-energy photon fields like those used in brachytherapy and diagnostic. In this work, RE as a function of the photon energy, besides, the influence of the phantom material used to collocate the dosimeters during irradiation has been studied. Thermoluminescent glow curves were deconvolved into 9 individual glow peaks by using an algorithm developed in Matlab based on the Podgorsak approximation using the first-order kinetics model. The influence of the phantom material on the dosimeter response is observed in every glow peak, where RE values are lower in air compared to other materials such as polymethyl methacrylate. For all the glow peaks, RE as a function of the mean photon energy shows a region with a minimum and a maximum.

High spatial resolution dosimetry with uncertainty analysis using Raman micro-spectroscopy readout of radiochromic films.

The purpose of this work is to develop a new approach for high spatial resolution dosimetry based on Raman micro-spectroscopy scanning of radiochromic film (RCF). The goal is to generate dose calibration curves over an extended dose range from 0 to 50Gy and with improved sensitivity to low (<2Gy) doses, in addition to evaluating the uncertainties in dose estimation associated with the calibration curves. Samples of RCF (EBT3) were irradiated at a broad dose range of 0.03-50Gy using an Elekta Synergy clinical linear accelerator. Raman spectra were acquired with a custom-built Raman micro-spectroscopy setup involving a 500 mW, multimode 785 nm laser focused to a lateral spot diameter of 30µm on the RCF. The depth of focus of 34µm enabled the concurrent collection of Raman spectra from the RCF active layer and the polyester laminate. The preprocessed Raman spectra were normalized to the intensity of the 1614cm-1 Raman peak from the polyester laminate that was unaltered by radiation. The mean intensities and the corresponding standard deviation of the active layer Raman peaks at 696, 1445, and 2060cm-1 were determined for the 150×100µm2 scan area per dose value. This was used to generate three calibration curves that enabled the conversion of the measured Raman intensity to dose values. The experimental, fitting, and total dose uncertainty was determined across the entire dose range for the dosimetry system of Raman micro-spectroscopy and RCF. In contrast to previous work that investigated the Raman response of RCFs using different methods, high resolution in the dose response of the RCF, even down to 0.03Gy, was obtained in this study. The dynamic range of the calibration curves based on all three Raman peaks in the RCF extended up to 50Gy with no saturation. At a spatial resolution of 30×30µm2 , the total uncertainty in estimating dose in the 0.5-50Gy dose range was [6-9]% for all three Raman calibration curves. This consisted of the experimental uncertainty of [5-8]%, and the fitting uncertainty of [2.5-4.5]%. The main contribution to the experimental uncertainty was determined to be from the scan area inhomogeneity which can be readily reduced in future experiments. The fitting uncertainty could be reduced by performing Raman measurements on RCF samples at further intermediate dose values in the high and low dose range. The high spatial resolution experimental dosimetry technique based on Raman micro-spectroscopy and RCF presented here, could become potentially useful for applications in microdosimetry to produce meaningful dose estimates in cellular targets, as well as for applications based on small field dosimetry that involve high dose gradients.

Cortical Responses to Vagus Nerve Stimulation Are Modulated by Brain State in Nonhuman Primates.

Vagus nerve stimulation (VNS) has been tested as therapy for several brain disorders and as a means to modulate cortical excitability and brain plasticity. Cortical effects of VNS, manifesting as vagal-evoked potentials (VEPs), are thought to arise from activation of ascending cholinergic and noradrenergic systems. However, it is unknown whether those effects are modulated by brain state at the time of stimulation. In 2 freely behaving macaque monkeys, we delivered short trains of 5 pulses to the left cervical vagus nerve at different frequencies (5-300 Hz) while recording local field potentials (LFPs) from sites in contralateral prefrontal, sensorimotor and parietal cortical areas. Brain states were inferred from spectral components of LFPs and the presence of overt movement: active awake, resting awake, REM sleep and NREM sleep. VNS elicited VEPs in all sampled cortical areas. VEPs comprised early (<70 ms), intermediate (70-250 ms) and late (>250 ms) components. The magnitude of the intermediate and late components was largest during NREM sleep and smallest during wakefulness, whereas that of the early component was not modulated by brain state. VEPs during NREM were larger for stimuli delivered at the depolarized phase of ongoing delta oscillations. Higher pulsing frequencies generated larger VEPs. These short VNS trains did not affect brain state transitions during wakefulness or sleep. Our findings suggest that ongoing brain state modulates the evoked effects of VNS on cortical activity. This has implications for the role of ongoing cortical activity and brain state in shaping cortical responses to peripheral stimuli, for the modulation of vagal interoceptive signaling by cortical activity, and for the dose calibration of VNS therapies.

Implant- and anesthesia-related factors affecting cardiopulmonary threshold intensities for vagus nerve stimulation

Objective. Vagus nerve stimulation (VNS) is typically delivered at increasing stimulus intensity until a neurological or physiological response is observed (‘threshold’) for dose calibration, preclinically and therapeutically. Factors affecting VNS thresholds have not been studied systematically. In a rodent model of VNS we measured neural and physiological responses to increasing VNS intensity, determined neurological and physiological thresholds and examined the effect of implant- and anesthesia-related factors on thresholds. Approach. In acute and chronic vagus implants (45 and 20 rats, respectively) VNS was delivered under isoflurane, ketamine-xylazine, or awake conditions. Evoked compound action potentials (CAPs) were recorded and activation of different fiber types was extracted. Elicited physiological responses were registered, including changes in heart rate (HR), breathing rate (BR), and blood pressure (BP). CAP and physiological thresholds were determined. Main results. The threshold for evoking discernable CAPs (>10 µV) (CAP threshold) is significantly lower than what elicits 5%–10% drop in heart rate (heart rate threshold, HRT) (25 µA ± 1.8 vs. 80 µA ± 5.1, respectively; mean ± SEM). Changes in BP and small changes in BR (bradypnea) occur at lowest intensities (70 µA ± 8.3), followed by HR changes (80 µA ± 5.1) and finally significant changes in BR (apnea) (310 μA ± 32.5). HRT and electrode impedance are correlated in chronic (Pearson correlation r= 0.47; p< 0.001) but not in acute implants (r = −0.34; p NS); HRT and impedance both increase with implant age (r= 0.44; p< 0.001 and r = 0.64; p < 0.001, respectively). HRT is lowest when animals are awake (200 µA ± 35.5), followed by ketamine-xylazine (640 µA ± 151.5), and isoflurane (1000 µA ± 139.5). The sequence of physiological responses with increasing VNS intensity is the same in anesthetized and awake animals. Pulsing frequency affects physiological responses but not CAPs. Significance. Implant age, electrode impedance, and type of anesthesia affect VNS thresholds and should be accounted for when calibrating stimulation dose.

Improving intensity-modulated radiation therapy quality assurance by adopting statistical process control

PurposeTo use statistical process control for intensity-modulated radiation therapy (IMRT) quality assurance (QA) and improve tolerance limits and action limits. MethodsAn electronic portal imaging device (EPID) was selected to verify IMRT QA. The I-chart and the exponentially weighted moving averages (EWMA) chart were used to analyze the corresponding results. ResultsTwenty samples were used to enable the sampling requirements for building the control limits to be met. The I-chart showed that isolated data points beyond the control limits were mainly derived from complex plans. The EWMA made predictions of systematic errors earlier than the I-chart. Systematic errors primarily originated from the dose calibration on the EPID, and recalibrating the EPID could eliminate such errors. ConclusionStatistical process control is an effective tool to detect controllable and can be used in IMRT QA. After calibrating the EPID, the tolerance and action limits all improved and satisfied the requirements/recommended values of the AAPM TG-218 report.

Investigation of absolute dose calibration accuracy for TomoTherapy using real water.

A systematic bias in TomoTherapy output calibration was reported by the Imaging and Radiation Oncology Core Houston (IROC‐H) after analyzing intensity‐modulated radiation therapy (IMRT) credentialing results from hundreds of TomoTherapy units. Multiple theories were developed to explain this observation. One theory was that the use of a solid water “cheese” phantom instead of real water in the calibration measurement was the culprit. A phantom filled with distilled water was built to investigate whether our TomoTherapy was miscalibrated due to the use of a solid water phantom. A miscalibration of −1.47% was detected on our TomoTherapy unit. It is found that despite following the vendor's updated recommendation on computed tomography (CT) number to density calibration, the cheese phantom was still mapped to a density of 1.028 g/cm3, rather than the 1.01 g/cm3 value reported in literature. When the density of the cheese phantom was modified to 1.01 g/cm3 in the treatment planning system, the measurement also indicated that our TomoTherapy machine was miscalibrated by −1.52%, agreeing with the real water phantom findings. Our single‐institution finding showed that the cheese phantom density assignment can introduce greater than 1% errors in the TomoTherapy absolute dose calibration. It is recommended that the absolute dose calibration for TomoTherapy be performed either in real water or in the cheese phantom with the density in TPS overridden as 1.01 g/cm3.

Comparing calculated and experimental activity and dose values obtained from image-based quantification of 90Y SPECT/CT Data

PurposeSelective internal radiation therapy (SIRT) is a treatment for various kinds of liver tumours by injecting 90Y bearing microspheres into the liver vessels. To perform meaningful post-treatment dosimetry, quantitative imaging is performed. MethodsThis work uses a Monte-Carlo based reconstruction software with scatter and attenuation correction and collimator modelling that allows the quantification of 90Y bremsstrahlung SPECT/CT data. A dataset comprising 17 patients and measurements on a Jaszczak phantom, a NEMA IEC Body phantom and an anthropomorphic liver phantom are analysed and activities and dose values are acquired. These measured values are compared with applied activities and pre-treatment calculations, allowing to assess the quality of the SPECT reconstruction. A detailed uncertainty budget is presented, including uncertainties of the dose calibrator, the count rate, non-included interactions and other factors. ResultsThe applied method is validated by finding measurements repeatable within the given uncertainty, and it is shown the influence of various parameters on the reconstruction process is negligible. Furthermore, activities and doses measured in the phantoms show good agreement with calculated values, if they are corrected for partial volume effects. ConclusionsThe strict observation of metrological requirements and the creation of an uncertainty budget increase the reliability and traceability of this novel approach to 90Y dosimetry. It gives an example of successful voxel-based dosimetry based on quantitative 90Y SPECT/CT image data.

Output dose investigation and analysis of external beam radiotherapy in Beijing

Objective To study the output dose of external beam radiotherapy in Beijing and analyze the cause of output dose deviation. Guidance is given on the selection of measurement methods, calculation formulas and parameters. Methods During 2020, the output doses of medical electron accelerators, tomotherapy, cyberknife, and γ-knife were measured based on the water absorbed dose calibration factor combined with the requirements in the IAEA TRS398 and TRS483 reports, and low-energy X-rays were measured based on the air kerma calibration factor combined with the requirements in the AAPM protocal. The output dose was measured and compared with the preset irradiation value. Results A total of 59 external beam radiation therapy equipments’s output dose were investigated. 2 of the 49 accelerators’s absolute value of relative deviation between the system indication value and the measurement exceeded 3%; the deviation of other external beam radiotherapy equipments was less than 3%. Conclusion A unified method and multi-party quality control can ensure the accuracy of the output dose. In the routine operation of the equipment, the hospital should strictly implement periodic inspections to ensure accurate and reliable radiotherapy. 摘要： 目的 调查北京地方所属外照射放射治疗设备输出剂量的情况, 分析输出剂量相对偏差原因, 明确测量方法、 计算公式及参数的选取。 方法 2020 年期间, 采用水中吸收剂量校准因子结合国际原子能机构 (IAEA) TRS398 和 TRS483 号报告中的要求对医用电子加速器、螺旋断层放射治疗装置、机械臂放射治疗装置、 γ 射线立体定向放射治疗 系统的输出剂量进行测量, 采用空气比释动能校准因子结合 AAPM 方案中的要求对低能 X 射线治疗机的输出剂量进 行测量, 并与预置照射值进行比较分析输出剂量相对偏差。 结果共调查了 59 台外照射放射治疗设备的输出剂量, 49 台医用电子加速器中 2 台输出剂量相对偏差绝对值超过 3%; 其他外照射放射治疗设备的输出剂量相对偏差绝对值 均小于 3%。 结论 统一明确的测量方法和多方的质量控制检测可确保放射治疗设备输出剂量的准确, 在设备常规运 行中医院应严格执行周期性检验保证实施精确可靠的放射治疗。

Radioembolization With Holmium-166 Polylactic Acid Microspheres: Distribution of Residual Activity in the Delivery Set and Outflow Dynamics During Planning and Treatment Procedures

Purpose: To evaluate the microsphere outflow dynamics and residual Ho-166 activity during and after transarterial radioembolization planning and treatment procedures, and to assess the distribution and predilection sites of residual activity in the proprietary delivery set and the microcatheter. Materials and Methods: Fifteen planning and 12 therapeutic radioembolization procedures were performed with poly-l-lactic acid microspheres loaded with Ho-166. The amount and distribution of residual activity was assessed by dose calibrator measurements and SPECT imaging. The activity flow profile from the microcatheter was assessed dynamically. For planning procedures, different injection methods were evaluated in order to attempt to decrease the residual activity. Results: The median residual activities for planning and treatment procedures using standard injection methods were 31.2% (range 17.3%–44.1%) and 4.3% (range 3.5%–6.9%), respectively. Planning residual activities could be decreased significantly with 2 injection methods similar to treatment procedures, to 17.5% and 10.9%, respectively (P = 0.002). Main predilection sites of residual microspheres were the 3-way stopcock and the outflow needle connector. During treatment procedures, more than 80% of the injected activity is transferred during the first 3 injection cycles. Conclusion: After treatment procedures with holmium-loaded microspheres, mean residual activity in the delivery set is reproducibly low and between reported values for glass and resin microspheres. The majority of microspheres is transferred to the patient during the second and third injection cycle. An estimated residual waste of 3% to 4% may be included in the treatment activity calculation. For planning procedures, a modified injection technique should be used to avoid high residual activities.

Feasibility study of radiophotoluminescent glass dosimeter for in vivo dosimetry in external photon beam radiotherapy

The purpose of this study was to check the applicability of radiophotoluminescent glass dosimeter (RPLGD) for in vivo dosimetry in external photon beams radiotherapy. The entrance dose measurements were performed. The influence of beam set-up parameters on the entrance dose was investigated using the RPLGDs. In this study, the RPLGD and 6-MV photon beam produced by a linear accelerator were used. The entrance dose calibration factors of the RPLGDs and beam energy and geometry-dependent correction factors were measured using a water phantom. For a correction factor related to the irradiation set-up, Monte Carlo (MC) simulation was performed. The entrance dose verification was performed on an Alderson Rando phantom during 64 single beam irradiations using different beam set-up. One RPLGD was attached on surface during a single beam irradiation for delivery of clinically planned dose. Additionally, during 50 irradiations, one RPLGD was placed directly inside the Alderson Rando phantom at different points in the beam path to verify the dose delivery. The mean ratio of the measured entrance dose using RPLGD on the surface of the phantom to the expected dose which was calculated by treatment planning system was equal to 1.001 ± 0.030. The mean ratio of the dose measured using RPLGDs inside the Alderson Rando phantom to the expected dose was equal to 1.019 ± 0.019. This study concludes that the RPLGD is suitable for in vivo dosimetry in external photon beam radiotherapy for dose verification within ± 5% at the clinically acceptable level.

Multicenter Study of Quantitative SPECT: Reproducibility of 99mTc Quantitation Using a Conjugated-Gradient Minimization Reconstruction Algorithm.

This multicenter study aimed to determine the reproducibility of quantitative SPECT images reconstructed using a commercially available method of ordered-subset conjugate-gradient minimization. Methods: A common cylindric phantom containing a 100 kBq/mL concentration of 99mTc-pertechnetate solution in a volume of 7 L was scanned under standard imaging conditions at 6 institutions using the local clinical protocol of each. Interinstitutional variation among the quantitative SPECT images was evaluated using the coefficient of variation. Dose calibrator accuracy was also investigated by measuring the same lot of commercially available 99mTc vials at each institution. Results: The respective radioactivity concentrations under standard and clinical conditions ranged from 95.71 ± 0.60 (mean ± SD) to 108.35 ± 0.36 kBq/mL and from 96.78 ± 0.64 to 108.49 ± 0.11 kBq/mL, respectively. Interinstitutional variation in radioactivity concentration was 4.20%. The bias in the radioactivity concentrations in SPECT images was associated with the accuracy of the dose calibrator at each institution. Conclusion: The reproducibility of the commercially available quantitative SPECT reconstruction method is high and comparable to that of PET, for comparatively large (∼7 L), homogeneous objects.