Wesley Culberson | Searcharxiv

arXiv2026

Evaluation of the Exradin A30 Parallel Plate Ion Chamber as a Reference Dosimeter in Ultra-High Dose Rate (UHDR) Electron Beams

Reliable reference dosimetry for ultra-high dose-rate (UHDR) beams (>40 Gy/s) is challenging because conventional ionization chambers (ICs) exhibit saturation from ion recombination. The Exradin A30 IC uses an ultra-thin 0.3-mm electrode spacing to improve charge-collection efficiency (CCE). This study evaluated the commercial A30 as a reference dosimeter for UHDR electron beams by characterizing leakage current, CCE, polarity correction (Ppol), and beam-quality correction factors (kQ). Measurements were performed with a 9-MeV IntraOp Mobetron from the accelerator head, achieving up to 9 Gy per pulse (DPP) and an instantaneous dose rate of 2.25 MGy/s. Data were acquired in grounded water-equivalent plastic, distilled water, and saline water. DPP was varied by changing SSD at a fixed 4-{\mu}s pulse width, while pulse repetition frequency (PRF) ranged from 5 to 90 Hz. CCE was determined using EBT-XD film under matched UHDR and conventional dose and energy conditions. CCE and Ppol were also evaluated as functions of DPP and PRF in distilled and saline water. Values of kQ were calculated using Monte Carlo simulations and measured in TrueBeam electron beams. Leakage current was <2 fA. Both CCE and Ppol decreased with increasing DPP; however, CCE remained 90-99% across all three phantoms, while Ppol decreased from 0.990 to 0.981 in liquid and solid water. Neither CCE nor Ppol depended on PRF over 5-90 Hz. Measured and calculated kQ values agreed within 0.8% at all energies except 9 MeV, where they differed by 2%. The A30 exhibited 5% recombination at DPP up to 5 Gy in distilled and saline water. Its response in solid phantoms was affected by charge buildup, which was mitigated by grounding. With appropriate CCE corrections and grounded solid phantoms, the commercial A30 is suitable for reference dosimetry in UHDR electron beams.

physics.med-ph↗

arXiv2025

Correlative Symmetric Index: An alternative mathematical evaluation for beam profile symmetry

Background: Current mathematical quantification methods for beam symmetry are highly sensitive to noise, especially in beam profiles with significant variation. Purpose: This study evaluates the accuracy of standard radiotherapy beam symmetry metrics and compares them to a proposed cross-correlation-based metric called the Correlative Symmetric Index (CSI), as well as the Structural Similarity Index (SSIM). We aim to demonstrate that CSI is less susceptible to noise than traditional methods. Methods: Simulated non-symmetric beam profiles with similar left and right areas were analyzed using both standard and proposed symmetry metrics. To test the robustness of each method to noise, a noisy non-symmetric beam profile was also generated. Measured beam profiles at various depths in a water tank were used to compare the performance of each symmetry metric under realistic clinical conditions. Results: In the noisy, non-symmetric case, CSI and SSIM values were 0.387 and 0.5401, respectively. Traditional metrics such as the Point Difference Quotient (PDQ) and area-based symmetry yielded values of 0.312 and 0.400. The percentage change between the non-symmetric (no noise) and non-symmetric (with noise) cases for PDQ, area-based symmetry, SSIM, and CSI were 11.1%, 5.6%, 16.25%, and 1.43%, respectively. For clinically measured symmetric profiles, all metrics produced values above 0.9. Conclusion: The Correlative Symmetric Index (CSI) demonstrates greater robustness to noise than both pointwise and area-based symmetry methods. CSI provides a reliable measure for quantifying beam symmetry, particularly under noisy conditions.

physics.med-ph↗