Journal of Applied Clinical Medical Physics最新文献

Background: Computed tomography (CT) examinations of the head, paranasal sinus (PNS), and cervical spine (C-spine) are frequently performed in emergency settings, raising concerns about radiation exposure to the radiosensitive eye lens. Overexposure can cause radiation-induced ocular damage. To address this concern, organ dose modulation (ODM) has emerged as a promising technique for reducing eye lens dose in CT examinations.

Purpose: This study aimed to evaluate radiation exposure to the eye lens and objective image quality metrics for head, PNS, and C-spine CT examinations using fixed tube current, automatic tube current modulation (ATCM), and ODM techniques.

Methods: Eye lens doses were measured using nanoDot optically stimulated luminescence dosimeters (OSLDs) placed bilaterally to the eye lens of a whole-body anthropomorphic phantom (PBU-60). CT scans were performed using a Revolution EX CT scanner with three scanning techniques: fixed tube current, ATCM, and ODM. The phantom was scanned twice for each examination type (head, PNS, and C-spine) with all three techniques. Eye lens dose reductions with the ODM technique were quantified relative to fixed tube current and ATCM techniques. Image quality was quantitatively evaluated in terms of image noise, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR).

Results: Mean eye lens doses ± standard deviation (SD) using the ODM technique were 38.44 ± 1.37, 17.92 ± 1.01, and 9.77 ± 0.38 mGy for head, PNS, and C-spine, respectively. These eye lens doses were reduced by 4.28%, 21.33%, and 47.97% compared to the fixed tube current techniques and by 19.40%, 24.70%, and 13.69% compared to the ATCM techniques, for head, PNS, and C-spine, respectively. These dose reductions were achieved while maintaining image quality metrics (image noise, SNR, and CNR) with no statistically significant differences (p > 0.05) compared to fixed tube current and ATCM techniques.

Conclusion: Implementation of the ODM technique resulted in significant eye lens dose reduction (4.28%-47.97%) across head, PNS, and C-spine CT examinations with no significant differences in image noise, SNR, and CNR compared to both fixed tube current and ATCM techniques. ODM demonstrates potential as a practical dose optimization strategy for routine emergency head and neck CT imaging. Further studies with subjective image quality assessment are recommended to evaluate clinical diagnostic acceptability in hospital settings.

Purpose: This study describes and evaluates the acceptance procedure for a ViewRay (VR) MRIdian 0.35T MR-Linac, emphasizing key challenges, limitations, and recommendations to enhance clinical performance and accuracy.

Methods: A comprehensive acceptance test was conducted at a single institution, following the manufacturer's protocols and aligned with established acceptance guidelines. Specific tools and phantoms were used to assess three primary components: mechanical, dosimetric, and Magnetic Resonance Imaging (MRI).

Results: Overall, the test outcomes satisfied the manufacturer's specifications. However, certain issues were identified: high couch attenuation at specific gantry angles (leading to their exclusion from treatment), variations in magnetic field homogeneity at different gantry angles, and discrepancies between TPS calculations and measurements for field output factors smaller than 0.83 cm × 0.83 cm.

Conclusion: This work provides a detailed account of the acceptance testing procedure and establishes a QA baseline for 0.35T MR-Linac systems. In doing so, it also identifies key system limitations, such as couch attenuation, magnetic field inhomogeneity, and small-field output discrepancies, underscoring the need for careful gantry angle selection, field homogeneity optimization, and meticulous validation of very small fields.

Background: In the IAEA TRS-483 Code of Practice (CoP), rectangular small field sizes are approximated to equivalent square small fields using the definition proposed by Cranmer-Sargison et al. However, the CoP estimates the uncertainties associated with this formula only for rectangular fields with dimensions within the range .

Purpose: The objective of the present study was to compare the accuracy of the Cranmer-Sargison definition with other formulas for equivalent square small fields in the context of measuring field output factors (FOFs) for rectangular small fields, both within and outside the range covered by the CoP.

Methods: Measurements were conducted using Gafchromic EBT4 radiochromic films. The models compared included Cranmer-Sargison, Sterling, Superellipse, Sterling-Partial Superellipse, Sterling-Superellipse, Vadash and Bjärngard, and Fogliata. The most accurate definition of equivalent square field size was identified as the one yielding the lowest discrepancy between measured and analytical values, with the log-likelihood of the measurements selected as the metric. Analytical values were derived using the function introduced by Sauer and Wilbert, which relates FOFs to equivalent square field sizes.

Results: The best results were achieved with the Fogliata model, followed in terms of accuracy by the Sterling-Partial Superellipse model. The Sterling-Superellipse and Vadash and Bjärngard models came next. It should be noted that the Sterling-Partial Superellipse and Sterling-Superellipse models rely solely on the geometric shape of the irradiation field size, whereas the Fogliata and Vadash and Bjärngard models incorporate a fitting parameter. The Sterling definition, while less accurate than these models, improved upon the Cranmer-Sargison definition and retained computational simplicity. Finally, the Cranmer-Sargison and Superellipse models exhibited the largest discrepancies.

Conclusions: This study identified several definitions of equivalent square small field size that could refine the IAEA TRS-483 CoP by improving the accuracy of field output correction factors for rectangular small fields. Among these definitions, the Fogliata model obtained the best results.

Background: Conventional cone-beam computed tomography (CBCT) systems are limited by suboptimal image quality, inaccurate Hounsfield unit (HU) calibration, and reduced reliability for dose calculation. HyperSight CBCT on the Halcyon platform offers improved HU accuracy, expanded field-of-view (FOV), and enhanced image quality.

Purpose: This study aimed to assess the dosimetric accuracy of treatment planning using HyperSight CBCT through phantom-based dose verification.

Methods: This study included three institutions equipped with the HyperSight imaging system on the Halcyon platform, with all procedures performed after acceptance testing and calibration. Each institution generated HU-to-density calibration curves using computed tomography (CT) scanners and standardized phantoms, and corresponding CBCT for planning (CBCTp) scans were also acquired. Additional CBCTp scans were acquired using a consistent phantom model (062 M) across the three institutions. Reference treatment plans were created on CT images and transferred to CBCTp and CBCT datasets for dose recalculation using identical parameters. Dosimetric assessment included gamma analysis and comparisons of DVH-based dosimetric metrics for relevant regions of interest (ROIs). End-to-end testing with an anthropomorphic phantom was performed using ion chamber measurements and film dosimetry at brain, bone, and thorax locations.

Results: HU-to-density curves showed consistent behavior across institutions, with larger variability only at higher densities. CBCTp calibrations agreed well with vendor references. DVH-based dosimetric metrics showed differences generally within 1% for both CBCTp and CBCT when compared with CT. Across institutions, gamma analysis of both CBCTp and CBCT yielded high passing rates (≥ 98.5% at 3%/2 mm). End-to-end testing with film dosimetry showed that CBCT-based plans agreed with measured doses within ± 4%, while CT-based plans were within ± 3%. Ion chamber measurements showed all dose differences within ± 2.3%, with both CBCTp and CBCT within ± 1.0% of CT.

Conclusions: HyperSight CBCT provides accurate dose calculations when properly calibrated. Phantom-based validation demonstrated sub-2% deviations and strong agreement with CT, supporting its clinical use in adaptive radiotherapy.