Implementation of alanine/EPR as transfer dosimetry system in a radiotherapy audit programme in Belgium

Towards liquid EPR dosimetry using nitroxides in aqueous solution

Objective. Water-equivalent dosimeters are desirable for dosimetry in radiotherapy. The present work investigates basic characteristics of novel aqueous detector materials and presents a signal loss approach for electron paramagnetic resonance (EPR) dosimetry.Approach. The proposed principle is based on the radiation dose dependent annihilation of EPR active nitroxides (NO·) in aqueous solutions. Stable nitroxide radicals (3-Maleimido-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (MmP), 3-Carbamoyl-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (CmP)) in aqueous solutions containing dimethyl sulfoxide (DMSO) as an additive were filled in glass capillaries for irradiation and EPR readout. Radiation doses ranging from 1 to 64 Gy were applied with a clinical 6 MV flattening filter free photon beam. EPR readout was then performed with a X-band benchtop spectrometer. The dose response, temporal stability and reproducibility of the samples' EPR signal amplitudes as well as the influence of the nitroxide concentration between 10 and 160μM on the absolute signal loss were investigated using MmP. CmP was used to examine the dependence of the dose response on DMSO concentration between 0 and 10 vol%. An indirect effect model was fitted to the experimental data assuming irradiation induced radical reactions as the underlying mechanism.Main results. For an initial MmP concentration of 20μM, absolute EPR signal loss is linear up to a dose of 16 Gy with a yield G(-NO·) of approximately 0.4μmol J-1. Within five weeks upon sample irradiation to doses between 0 and 32 Gy relative EPR signal fluctuations were on average (126 readouts) below 1% (1σ). For c(MmP) ≥ 20μM, absolute signal loss is only weakly dependent on c(MmP), whereas it increases strongly with increasing c(DMSO) in the range 0-5 vol%. An indirect effect model is applicable to describe the reaction mechanism resulting in the observed dose response curve.Significance. Liquids consisting of nitroxides in aqueous solution and small amounts of DMSO (2 vol%) show promising basic characteristics for application as water-equivalent EPR dosimeter materials in radiotherapy. The EPR signal loss is based on an indirect effect mediated by diffusing radicals originating from the radiolysis of the water/DMSO mixture.

Early results of a remote dosimetry audit program for lung stereotactic body radiation therapy

Background and purpose

A dosimetry audit program based on alanine electron paramagnetic resonance (EPR) and radiochromic film dosimetry, may be a valuable tool for monitoring and improving the quality of lung stereotactic body radiotherapy (SBRT). The aim of this study was to report the initial, independent assessment of the dosimetric accuracy for lung SBRT practice using these dosimeters in combination with a novel phantom design.

Materials and Methods

The audit service was a remote audit program performed on a commercial lung phantom preloaded with film and alanine detectors. An alanine pellet was placed in the centre of the target simulated using silicone in a 3D-printed mould. Large film detectors were placed coronally through the target and the lung/tissue interface and analysed using gamma analysis. The beam output was always checked on the same day with alanine dosimetry in water. We audited 29 plans from 14 centres up to now.

Results

For the alanine results 28/29 plans were within 5 % with 19/29 plans being within 3 %. The passing rates were > 95 % for the film through the target for 27/29 plans and 17/29 plans for the film at the lung/tissue interface. For three plans the passing rate was < 90 % for the film on top of the lungs.

Conclusions

The preliminary results were very satisfactory for both detectors. The high passing rates for the film in the interface region indicate good performance of the treatment planning systems. The phantom design was robust and performed well on several treatment systems.

Calibration of the comet assay using ionising radiation

Several trials have attempted to identify sources of inter-laboratory variability in comet assay results, aiming at achieving more equal responses. Ionising radiation induces a defined level of DNA single-strand breaks (per dose/base pairs) and is used as a reference when comparing comet results but relies on accurately determined radiation doses. In this ring test we studied the significance of dose calibrations and comet assay protocol differences, with the object of identifying causes of variability and how to deal with them. Eight participating laboratories, using either x-ray or gamma radiation units, measured dose rates using alanine pellet dosimeters that were subsequently sent to a specialised laboratory for analysis. We found substantial deviations between calibrated and nominal (uncalibrated) dose rates, with up to 46% difference comparing highest and lowest values. Three additional dosimetry systems were employed in some laboratories: thermoluminescence detectors and two aqueous chemical dosimeters. Fricke's and Benzoic Acid dosimetry solutions gave reliable quantitative dose estimations using local equipment. Mononuclear cells from fresh human blood or mammalian cell lines were irradiated locally with calibrated (alanine) radiation doses and analysed for DNA damage using a standardised comet assay protocol and a lab-specific protocol. The dose response of eight laboratories, calculated against calibrated radiation doses, was linear with slope variance CV= 29% with the lab-specific protocol, reduced to CV= 16% with the standard protocol. Variation between laboratories indicate post-irradiation repair differences. Intra-laboratory variation was very low judging from the dose response of 8 donors (CV=4%). Electrophoresis conditions were different in the lab-specific protocols explaining some dose response variations which were reduced by systematic corrections for electrophoresis conditions. The study shows that comet assay data obtained in different laboratories can be compared quantitatively using calibrated radiation doses and that systematic corrections for electrophoresis conditions are useful.

Radiotherapy-specific quality indicators at national level: How to make it happen

PURPOSE /OBJECTIVE

To promote best practice and quality of care, the Belgian College of Physicians for Radiotherapy Centers established a set of radiotherapy specific quality indicators for benchmarking on a national level. This paper describes the development, the collected QIs, the observed trends and the departments' evaluation of this initiative.

MATERIAL AND METHODS

The Donabedian approach was used, focussing on structural, process and outcome QIs. The criteria for QI selection were availability, required for low-threshold regular collection, and applicability to guidelines and good practice. The QIs were collected yearly and individualized reports were sent out to all RT departments. In 2021, a national survey was held to evaluate the ease of data collection and submission, and the perceived importance and validity of the collected QIs.

RESULTS

18 structural QI and 37 process and outcome parameters (n = 25 patients/pathology/department) were collected. The participation rate amounted to 95 % overall. The analysis gave a national overview of RT activity, resources, clinical practice and reported acute toxicities. The individualized reports allowed departments to benchmark their performance. The 2021 survey indicated that the QIs were overall easy to collect, relevant and reliable. The collection of acute recorded toxicities was deemed a weak point due to inter-observer variabilities and lack of follow-up time.

CONCLUSION

QI collection on a national level is a valuable process in steering quality improvement initiatives. The feasibility and relevance was demonstrated with a high level of participation. The national initiative will continue to evolve as a quality monitoring and improvement tool.

Dosimetry audit for megavoltage photon beams applied in non-reference conditions

PURPOSE

We have conducted for the first time a Malaysian postal dosimetry audit of external beam under non-reference conditions by evaluating the output performance while screening for systematic errors within the dosimetry chain. The potential use from the choice of detector were investigated along with the search for other sources of discrepancies.

METHODS

Ten radiotherapy centres were audited, encompassing 16 megavoltage photon beam arrangements, adopting the IAEA postal dosimetry protocol for non-reference conditions, with a holder modified to accommodate three TLD types: Ge-doped cylindrical silica fibres (CF), Ge-doped flat silica fibres (FF), and TLD-100 powder.

RESULTS

Eight of the centres operated within ± 5% of stated dose, one other exceeding tolerance for all measured points, and one did not return any dosimeters for analysis after failing the initial irradiations. Post remedial measures, the mean relative response for CF, FF, and TLD-100 was 1.00, 0.99, and 0.98 respectively, with associated coefficients of variation 6.87%, 6.45%, and 5.06%.

CONCLUSION

High quality radiotherapy clinical practice postal dosimetry audits that are based on sensitive TLDs are seen to be particularly effective in identifying and resolving dose delivery discrepancies.

Dose Variations Using an X-Ray Cabinet to Establish in vitro Dose-Response Curves for Biological Dosimetry Assays

In biological dosimetry, dose-response curves are essential for reliable retrospective dose estimation of individual exposure in case of a radiation accident. Therefore, blood samples are irradiated in vitro and evaluated based on the applied assay. Accurate physical dosimetry of the irradiation performance is a critical part of the experimental procedure and is influenced by the experimental setup, especially when X-ray cabinets are used. The aim of this study was to investigate variations and pitfalls associated with the experimental setups used to establish calibration curves in biological dosimetry with X-ray cabinets. In this study, irradiation was performed with an X-ray source (195 kV, 10 mA, 0.5 mm Cu filter, dose rate 0.52 Gy/min, 1st and 2nd half-value layer = 1.01 and 1.76 mm Cu, respectively, average energy 86.9 keV). Blood collection tubes were irradiated with a dose of 1 Gy in vertical or horizontal orientation in the center of the beam area with or without usage of an additional fan heater. To evaluate the influence of the setups, physical dose measurements using thermoluminescence dosimeters, electron paramagnetic resonance dosimetry and ionization chamber as well as biological effects, quantified by dicentric chromosomes and micronuclei, were compared. This study revealed that the orientation of the sample tubes (vertical vs. horizontal) had a significant effect on the radiation dose with a variation of -41% up to +49% and contributed to a dose gradient of up to 870 mGy inside the vertical tubes due to the size of the sample tubes and the associated differences in the distance to the focal point of the tube. The number of dicentric chromosomes and micronuclei differed by ~30% between both orientations. An additional fan heater had no consistent impact. Therefore, dosimetric monitoring of experimental irradiation setups is mandatory prior to the establishment of calibration curves in biological dosimetry. Careful consideration of the experimental setup in collaboration with physicists is required to ensure traceability and reproducibility of irradiation conditions, to correlate the radiation dose and the number of aberrations correctly and to avoid systematical bias influencing the dose estimation in the frame of biological dosimetry.

SABR pre-treatment checks using alanine and nanoDot dosimeters

Stereotactic Ablative Radiotherapy (SABR) remains one of the preferred treatment techniques for early-stage cancer. It can be extended to more treatment locales involving the sternum, scapula and spine. This work investigates SABR checks using Alanine and nanoDot dosimeter for three treatment sites, including sternum, spine and scapula. Alanine and nanoDot dosimeters' performances were verified using a 6 MV photon beam before SABR pretreatment verifications. Each dosimeter was placed inside customized designed inserts into a Rod Phantom (in-house phantom) made of Perspex that mimics the human body for a SABR check. Electron Paramagnetic Resonance (EPR) spectrometer, Bruker EleXsys E500 (9.5 GHz) and Microstar (Landauer Inc.) Reader was employed to acquire the irradiated alanine and nanoDot dosimeters' signal, respectively. Both dosimeters treatment sites are expressed as mean ± standard deviation (SD) of the measured and Eclipse calculated dose Alanine (19.59 ± 0.24, 17.98 ± 0.15, 17.95 ± 0.18) and nanoDot (19.70 ± 0.43, 17.05 ± 0.08, 17.95 ± 0.98) for spine, scapula and sternum, respectively. The percentage difference between alanine and nanoDot dosimeters was within 2% for sternum and scapula but 2.4% for spine cases. These results demonstrate Alanine and nanoDot dosimeters' potential usefulness for SABR pretreatment quality assurance (QA).

Characterization of passive dosimeters in proton pencil beam scanning - A EURADOS intercomparison for mailed dosimetry audits in proton therapy centres

The lack of mailed dosimetry audits of proton therapy centres in Europe has encouraged researchers of EURADOS Working Group 9 (WG9) to compare response of several existing passive detector systems in therapeutic pencil beam scanning. Alanine Electron Paramagnetic Resonance dosimetry systems from 3 different institutes (ISS, Italy; UH, Belgium and IFJ PAN, Poland), ^natLiF:Mg, Ti (MTS-N) and ^natLiF:Mg, Cu, P (MCP-N) thermoluminescent dosimeters (TLDs), GD-352M radiophotoluminescent glass dosimeters (RPLGDs) and Al₂O₃:C optically stimulated dosimeters (OSLDs) were evaluate. Dosimeter repeatability, batch reproducibility and response in therapeutic Pencil Beam Scanning were verified for implementation as mail auditing system. Alanine detectors demonstrated the lowest linear energy transfer (LET) dependence with an agreement between measured and treatment planning system (TPS) dose below 1%. The OSLDs measured on average a 6.3% lower dose compared to TPS calculation, with no significant difference between varying modulations and ranges. Both GD-352M and MCP-N measured a lower dose than the TPS and luminescent response was dependent on the LET of the therapeutic proton beam. Thermoluminescent response of MTS-N was also found to be dependent on the LET and a higher dose than TPS was measured with the most pronounced increase of 11%. As alanine detectors are characterized by the lowest energy dependence for different parameters of therapeutic pencil beam scanning they are suitable candidates for mail auditing in proton therapy. The response of luminescence detector systems have shown promises even though more careful calibration and corrections are needed for its implementation as part of a mailed dosimetry audit system.

A practical EPR dosimetry system for routine use in radiotherapy: uncertainty analysis of lithium formate dosimeters at the therapeutic dose level

In electron paramagnetic resonance (EPR) dosimetry, solid dosimeter materials such as alanine (AL) or, more recently, lithium formate monohydrate (LFM) are typically used. These materials offer high potential for applications in radiotherapy based on their favorable dosimetric properties. Nevertheless, EPR dosimetry is not widespread in the clinics. This work presents an uncertainty analysis of EPR dosimetry in the dose range from 1 to 70 Gy using a compact spectrometer and applying a practical procedure being suitable for routine use in radiotherapy. The performances of self-pressed LFM pellets and commercial AL pellets are compared side by side. All pellets had a diameter of 4 mm and a height of 2 mm (AL) or 4 mm (LFM). The mean pellet mass was 35.81 mg and 73.81 mg for AL and LFM, respectively. Before irradiation, the pellets were stored for at least 8 weeks at 34 ± 2% relative humidity. For irradiation, the pellets were put inside an airtight capsule. In total, 25 pellets per material were examined. The pellets were irradiated at a temperature of 25 ± 2.5 (2σ) °C to doses of either 1, 5, 20, 50 or 70 Gy (five pellets per dose value and material) by a clinical 6 MV photon beam. Measurement uncertainties were obtained from five independent readouts per pellet within five weeks following irradiation using a benchtop EPR spectrometer. The measurement time of a single readout was restricted to 10 min per pellet. Dose values were derived from EPR signal amplitudes using a specifically developed spectral fitting procedure. Signal fading characteristics were analyzed and taken into account during evaluation. The relative dose uncertainties (1σ) for a single readout at doses ≥ 5 Gy are below 2.8% (AL) and 1.1% (LFM) but increase to 12.3% (AL) and 2.6% (LFM) at 1 Gy. By averaging five independent readouts, the uncertainties at 1 Gy decrease to 2.6% (AL) and 0.8% (LFM). In terms of dose uncertainty, the LFM pellets are superior to the commercial AL pellets owing to their narrower EPR spectrum and approximately doubled mass resulting in higher EPR signal intensities. In case of the LFM pellets, the EPR dosimetry system shows a high level of precision (< 3%) down to 1 Gy being preferable for applications in radiotherapy. The uncertainties can be further decreased by averaging multiple dose values from independent readouts.

Feasibility and impact of national peer reviewed clinical audits in radiotherapy departments

PURPOSE/OBJECTIVE

A national incentive brought about the instauration of systematic clinical audits of all Belgian radiotherapy departments (n = 25) from 2011 to 2015 using the International Atomic Energy Agency QUATRO (Quality Improvement Quality Assurance Team for Radiation Oncology) methodology. The impact of these audits was evaluated and the emitted recommendations originating from the audit reports were analysed to identify areas of weakness on a national basis.

METHOD

The QUATRO audits performed in each radiotherapy department gave rise to reports in which each department received a list of recommendations that it is free to implement. These audit reports were analyzed to identify common areas for which improvements were recommended. Moreover, questionnaires were sent to all departments in order to evaluate the overall usefulness of the recommendations as well as the relevancy and the actual impact of each individual recommendation.

RESULTS

Of the 381 emitted recommendations, 34% concerned process optimization of which a quarter involved process improvement and protocol development. Twenty-seven percent of the recommendations concerned infrastructure of which one-third was related to the quality of the equipment or facility. Nineteen and 20% of recommendations addressed department organisational and staff issues respectively. When analysing the departments' feedback questionnaires, 54% of the departments evaluated the audits' recommendations as being very useful. Furthermore, 42.7% of the recommendations were found to be very relevant and 23.5% were deemed to have an important impact.

CONCLUSION

This first round of audits in Belgium allowed for the identification of common areas for improvements of practice in radiation oncology departments, with a focus on process optimization and infrastructure elements. Similarly, the audits' emitted recommendations were globally deemed very relevant. Encouraged, by this analysis, a second cycle of audits has started in Belgium with a modified version of the QUATRO document (B-QUATRO).

Clinical radiotherapy audits in Belgium, 2011-2014

Systematic clinical radiotherapy audits have been introduced in Belgium in 2011, as part of the Federal Cancer Plan. This is in compliance with article 11 of the 97/43 Council directive of Euratom states, translated into the Belgian legislation by royal decree in 2002. The principle of clinical audits has thus been part of the federal legal requirements for more than 10 years. However, its application had to wait for the development of a practical approach: what authority will audit, who will be the auditors, along which methodology, at what frequency, etc. Since 2002, the Federal College of Radiotherapy has the mission to monitor quality of radiotherapy at large. It was therefore decided after discussions with the relevant administration at the Ministry of Health and the Federal Agency for Nuclear Control that the College would practically organise the audits. Early in the 2000s, the IAEA developed a manual for comprehensive audits, as a tool for quality improvement. Auditors were professionals of the domain and the audit visit took the form of a peer review. Great care was taken to assemble an audit party able to cover all aspects of clinical radiotherapy with a radiation oncologist, a medical physicist, a radiation therapist and, on demand, a quality officer. The IAEA manual contains a series of questionnaires to be prepared by the audited centre in advance (pre-audit and self-assessment), indicating what specific areas the auditors would assess. It is also a template for the auditors, ensuring that no area is left aside or forgotten during the site visit. The report, at the end of the visit, is drafted according to a specific report template, also developed by IAEA. Several members of the Belgian radiotherapy community have developed their auditor's skills by participating to the IAEA audit program; they are the core of the auditor Belgian team.

Performance characteristics of the EPR dosimetry system with table sugar in radiotherapy applications

Performance characteristics of the megavoltage photon dose measurements with EPR and table sugar were analyzed. An advantage of sugar as a dosimetric material is its tissue equivalency. The minimal detectable dose was found to be 1.5Gy for both the 6 and 18MV photons. The dose response curves are linear up to at least 20Gy. The energy dependence of the dose response in the megavoltage energy range is very weak and probably statistically insignificant. Reproducibility of measurements of various doses in this range performed with the peak-to-peak and double-integral methods is reported. The method can be used in real-time dosimetry in radiation therapy.

A high precision method for quantitative measurements of reactive oxygen species in frozen biopsies

OBJECTIVE

An electron paramagnetic resonance (EPR) technique using the spin probe cyclic hydroxylamine 1-hydroxy-3-methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine (CMH) was introduced as a versatile method for high precision quantification of reactive oxygen species, including the superoxide radical in frozen biological samples such as cell suspensions, blood or biopsies.

MATERIALS AND METHODS

Loss of measurement precision and accuracy due to variations in sample size and shape were minimized by assembling the sample in a well-defined volume. Measurement was carried out at low temperature (150 K) using a nitrogen flow Dewar. The signal intensity was measured from the EPR 1st derivative amplitude, and related to a sample, 3-carboxy-proxyl (CP•) with known spin concentration.

RESULTS

The absolute spin concentration could be quantified with a precision and accuracy better than ±10 µM (k = 1). The spin concentration of samples stored at -80°C could be reproduced after 6 months of storage well within the same error estimate.

CONCLUSION

The absolute spin concentration in wet biological samples such as biopsies, water solutions and cell cultures could be quantified with higher precision and accuracy than normally achievable using common techniques such as flat cells, tissue cells and various capillary tubes. In addition; biological samples could be collected and stored for future incubation with spin probe, and also further stored up to at least six months before EPR analysis, without loss of signal intensity. This opens for the possibility to store and transport incubated biological samples with known accuracy of the spin concentration over time.