Technical Note: Vendor miscalibration of preclinical orthovoltage irradiator identified through independent output check

Abstract

Purpose: Accurate radiation dosimetry in radiobiological experiments is crucial for preclinical research in advancement of cancer treatment. Vendors of cell irradiators often perform calibration for end-users. However, calibration accuracy remains unclear due to missing detailed information on calibration equipment and procedures. In this study, we report our findings of a vender miscalibration of the radiation output and our investigation on the root cause of the discrepancy. Methods: Independent calibration verification for a commercial preclinical orthovoltage irradiator was conducted. Initially, in the absence of ionization chambers calibrated at kV energy, radiochromic films (EBT3) was first calibrated at MV energy. Energy correction factors from literature were used to create an in-house kV dosimetry system. The miscalibration identified with the in-house kV EBT3 dosimetry was later confirmed by ADCL calibrated ionization chambers (Exradin A1SL and PTW 30013) at kV energy. Ionization chambers were suspended in-air following TG-61 recommendation for output calibration. To investigate the root cause of the miscalibration, additional measurements were performed with ionization chambers placed on the shelf. A validated Monte Carlo simulation code was also used to investigate the impact of placing the ionization chamber on the shelf instead of suspending it in air during the vendor-performed calibration process. Results: Up to a 6% dosimetry error was observed when comparing the vendor calibrated output of the preclinical irradiator with our independent calibration check. Further investigation showed incorrect setups in the vendor's calibration procedure which may result in dose errors up to 11% from the backscatter of the shelf board during calibration, and up to 5% from omitting temperature and pressure corrections to ionization chamber readings. Conclusion: Our study revealed large dose calibration errors caused by incorrect setup and the omission of temperature/pressure correction in the vendor's calibration procedure. The findings also highlighted the importance of performing an independent check of the dose calibration for preclinical kV irradiators. More absolute dosimetry training is needed for both vendors and end users for establishing accurate absolute dosimetry.

A review of small animal dosimetry techniques: image-guided and spatially fractionated therapy

“…An absorbed dose variation of only ±5% can affect tumour tissue control probability by 10-20% [1,157,158]. This is of course valid for both clinical and preclinical radiotherapy and accurate dosimetry is of highly importance during the commissioning of clinical linear accelerators as well as for small animal irradiators [159].…”

A preclinical radiotherapy dosimetry audit using a realistic 3D printed murine phantom

“…Some centres rely on the manufacturer to complete the dosimetry checks, which has the advantage that the checks are done in a consistent fashion by staff who are expertly trained on the system. However, this is not a guarantee that the irradiator has been properly calibrated 37 , and further checks should be performed by appropriate personnel for validation. The responses to the questionnaire also indicate that there may be a lack of support at the institutional level, and the lack of independence in the QA process increases the risk of a systemic problem going unnoticed 37 .…”

“…However, this is not a guarantee that the irradiator has been properly calibrated 37 , and further checks should be performed by appropriate personnel for validation. The responses to the questionnaire also indicate that there may be a lack of support at the institutional level, and the lack of independence in the QA process increases the risk of a systemic problem going unnoticed 37 . Also of importance is the wide variation in the frequency of output checks (bi-monthly to annually) and the traceability of the chambers used for those checks.…”

Extracellular vesicles released after cranial radiation: An insight into an early mechanism of brain injury

“…Irradiated mice received one dose of 10 Gray cranial radiation using the X-ray irradiator, X-RAD 225XL, Precision X-ray (PXi), Inc. (225 kVp peak voltage, 0.3 mm Cu filtration, and 0.92 mm Cu beam quality). The irradiator was calibrated with in-air method following American Association of Physicist in Medicine TG-61 protocol ( Ma et al, 2001 ) with an ADCL calibrated A1SL ion chamber (Standard Imaging Inc.) ( Chen et al, 2021 ). Radiation field was set at 1.41 by 1.2 cm with field light guidance to cover the mice brain.…”