Ultra-High Dose Rate Radiotherapy (UHDR-RT) is an emerging cancer treatment that delivers radiation at dose rates exceeding 40 Gy/s. While UHDR-RT offers potential therapeutic benefits, accurate dosimetry presents significant challenges. Ionisation chambers experience ion recombination at high dose rates, reducing accuracy and increasing uncertainties when corrections are applied. In contrast, calorimeters are dose rate independent and more reliable for UHDR-RT dosimetry. However, their slow thermal response limits their use in dynamic beam monitoring. This study investigates whether a hybrid dosimeter, combining an ionisation chamber and two transmission calorimeters, can effectively provide dose-rate independent beam monitoring for both UHDR-RT and conventional radiotherapy. The goal is to leverage the fast response of ionisation chambers while maintaining the dose rate independence of calorimetry. The hybrid dosimeter was designed and evaluated using finite element modelling, to study the electrical and thermal interactions between the ionisation and calorimeter components. The device was then tested in a 6 MeV UHDR-RT beam under various beam conditions to evaluate performance. The hybrid dosimeter demonstrated a linear response in both calorimeter cores and the ionisation chamber. The ion collection efficiency was measured to be approximately 3% during the highest dose delivery of 6 Gy/pulse, while the calorimeters confirmed dose-rate independence across all tested conditions. This study successfully demonstrates the feasibility of combining ionisation and calorimetry into a single hybrid dosimeter. The device offers the potential for online beam monitoring with dose rate independence, making it a promising candidate for UHDR-RT dosimetry and further development for clinical use.

PURPOSE: Evaluate the response of electret-style organic thin film transistors (OTFTs) as ionizing radiation dosimeters in various radiation environments.

 

MATERIALS & METHODS: Pentacene OTFTs were fabricated with the addition of a polystyrene electret layer. This allows OTFTs to function wirelessly (no bias during irradiation) as well as in real-time. Devices were first tested in clinical settings with photon beams from a linear accelerator and orthovoltage unit. Both readout modes were used to assess linearity, sensitivity, energy dependence, and comparisons with OTFTs made on flexible substrate. Devices then were tested at the Australian Synchrotron with dose rates of 30-500 Gy/s.

 

RESULTS: OTFTs showed a linear relationship between current and cumulative dose with their wireless response. The sensitivities were 60 ± 5 nA/Gy at 6 MV and 80 ± 10 nA/Gy at 100 kVp. For real-time readout, devices made on silicon substrates had a sensitivity of 80 nA/Gy and 33 nA/Gy for PET substrate. OTFTs also exhibited a linear response to radiation with ultra-high dose rates at the Australian Synchrotron. Saturation of the response started past 70 Gy, but devices were reset through reprogramming and showed recovery of the sensitivity. PDDs were obtained with synchrotron radiation and in good agreement with other detectors.

 

CONCLUSIONS: Polymer electret OTFTs have demonstrated potential in radiation dosimetry applications, including ultra-high dose rate settings. They exhibited excellent linearity at megavoltage and kilovoltage energies, can monitor dose in real-time or immediately after irradiation, and can be reset to be reused through programming.

Purpose: TRS-483 does not cover small field dosimetry in the presence of magnetic field, nor does it provide recommendations for equipment. This work assesses a commercially available scintillator detector for small field dosimetry in a 1.5 T MRgRT system.

 

Materials and Methods: Measurements were performed in a 1.5 T MR-Linac (Elekta, Unity system) using small volume ion chambers (Semiflex 3D MR, Semiflex, PinPoint 3D MR, microDiamond) and a commercially available scintillator detector (MedScint HS-RP200). Output factors (OF) were measured in water at a SAD=143.5 cm, depth=10cm, FS=10x10 cm². Results were compared with Monte-Carlo TPS. To assess MRI acquisition effects, data was collected with MRI on and off.

 

Results: For FS > 2x2 cm², all detectors were within 2% tolerance. For 1x1 cm²: 18.3% error (Semiflex), 12.5% (Semiflex 3D MR), <3% (PinPoint). Scintillator: within 1.5% agreement except 0.7x0.7 cm² (5.7%). Magnetic field had negligible impact on small field output factors.

 

Conclusions: The scintillator shows strong potential for small field dosimetry in MR-Linac systems and may be integrated into end-to-end QA. More testing is needed for temporal performance.

Purpose: To characterize a water calorimeter system in a 68 MeV passively-scattered proton beam, advancing a primary standard for absorbed dose to water in clinical proton beams.

 

Materials and Methods: A water calorimeter was operated in a proton beamline previously used for ocular therapy. Measurements were made in a passively-scattered beam. Investigations studied system reproducibility and sensitivity to various influencing quantities.

 

Results: For a dose rate of 12 Gy/min, standard deviation was ~0.7%. Independent setups agreed within 0.8%. Geometry and irradiation parameter dependencies matched predictions. Raw dose varied by ~2% between vessels, consistent with convective heat flow and vessel perturbations.

 

Conclusions: The protocol is robust. An uncertainty of 1% in absorbed dose is achievable. Ongoing work includes correction factor determination using heat transport calculations and Monte Carlo simulations.

Impact/Innovation:Extrapolation chambers enable accurate relative surface dose assessment without relying on traditional extrapolation, improving radiotherapy accuracy. Introduction: Surface dose assessment in radiotherapy is challenging due to steep dose gradients. This study investigates extrapolation chambers’ accuracy in the buildup region using EGSnrc simulations.

 

Methods: Dose to the cavity of two extrapolation chambers was simulated and compared with literature data. Photon beams (60Co, 6, 10, 25 MV) were modeled using BEAMnrc.

 

Results: Simulated chamber responses closely matched measured values. Gaps ≤1 mm provided accurate surface dose values. Differences ranged from 0.1% to 3.8% of Dmax across beam energies.

 

Conclusions: Extrapolation chambers can provide accurate surface dose values when used with a defined small gap, challenging traditional extrapolation assumptions.

Purpose: Real-time dose monitoring is difficult in UHDR electron beams. This study validates BCT calibration and long-term stability using alanine dosimetry and PSDs.

 

Methods: UHDR beam output from 6 and 9 MeV electrons was monitored with a Bergoz BCT. Calibration was performed using alanine dosimetry and tracked over time. PSD was used for independent validation.

 

Results: Alanine calibration showed high accuracy (±0.05%). Dose predictions within 0.42–0.52% post-calibration. PSD stability was within 0.82%.

 

Conclusion: Alanine-calibrated BCTs are accurate and stable for UHDR monitoring. PSDs offer effective independent validation.