Yunjie Yang

Accurate in vivo dosimetry is crucial for dose monitoring of cardiac implantable electronic devices (CIED) and for dose verification for special procedures such as total body irradiation (TBI) and total skin electron therapy (TSET). A new near real‐time in vivo dosimetry system using radiochromic films (RCF) is investigated for clinical use in megavoltage external beam radiotherapy. The Pnt‐Dos™ in vivo dosimetry system comprises of a new type of RCF and a dedicated software module. Each Pnt‐Dos device is a small piece of RCF individually packed with a unique QR code for identification and record keeping. Different from the traditional film dosimetry workflow, where a film developing time of at least 16 hours is recommended, a near real‐time dose readout can be achieved with the Pnt‐Dos system using a novel calibration procedure. This involves an automated scanning process at user‐specified time intervals, utilizing auto‐region of interest (ROI) detection and triple‐channel calibration to capture the time‐resolved post‐irradiation growth. Two standard Epson scanner models (V600/13000XL) were used to cross‐validate readouts and accommodate users who may prefer to utilize existing 13000XL scanners rather than acquire an additional V600 for in vivo dosimetry. The dosimetric accuracy was evaluated over a range of 15–400 cGy. Angular dependence was studied in 45° increments over 360°, normalized to the response at 0°, at 250 cGy using a cylindrical phantom. Energy dependence was evaluated for four photon energies (6 MV, 6 MV FFF, 10 MV FFF, 15 MV) and five electron energies (6 MeV, 9 MeV, 12 MeV, 16 MeV, and 20 MeV). Long‐term reproducibility/stability were assessed with nine devices with different doses under identical conditions, alongside daily scans of quality control (QC) devices over three months. The system provides accurate dose measurements across high‐ and low‐dose ranges. All readings were within specification: accuracy was < ± 5 cGy for doses ≤ 80 cGy doses (max discrepancy 6.0 cGy), and < ± 5% for doses > 80 cGy on average (max discrepancy 5.1%). Angular dependence showed a maximum variation of 2.6% ± 2.1% when the beam passed through the posterior oblique side of the device. Daily QC/reproducibility tests confirmed system constancy of 0.1% average day‐to‐day variation. Energy dependence analysis revealed deviations of up to 4.9% ± 2.3% for all photon and electron energies compared to 6 MV photons, indicating the need for energy correction during commissioning. Film readings were compared with ion chamber measurements at 10 × 10 cm², d_max, 100 cm SAD (photons) or 100 cm SSD (electrons). Both scanners provided comparable readouts, within 1.3 cGy for doses ≤ 80 cGy and 0.6% for doses > 80 cGy. Based on these findings, user guidelines were established to ensure optimal performance and accuracy. The new film‐based in vivo dosimetry system provides an automated workflow that enables consistent, time‐independent, and near real‐time readout with a user‐friendly design that simplifies handling and analysis, thereby streamlining in vivo dosimetry measurements. It also provides a traceable record of the patient dosimetry.

Purpose: A comprehensive 4DCT QA program that includes the assessment of spatial integrity and image quality of 4D phantom scans can be resource‐intensive, especially because the analysis burden scales with the number of motion traces used for QA. This work presents a streamlined and scalable workflow, enabled by the use of a widely available phantom and an automated analysis tool.Methods: For 4DCT imaging QA, the Catphan was placed on the QUASAR motion platform, driven with sinusoidal motion traces of various amplitudes and frequencies. The acquired image sets were automatically analyzed using a newly designed 4DCT analysis module in TotalQA. The metrics of interest for 4DCT QA were analyzed for image sets of all motion bins and compared to the stationary reference image set. Three broad categories of imaging tests for 4DCT QA were defined: spatial integrity, HU constancy, and image quality. The sensitometry plugs in the Catphan were used as motion surrogates for evaluating spatial integrity, including the dimensions of the plugs and the observed motion amplitude in the scans. HU values of the sensitometry plugs in different phases were evaluated for consistency. For image quality, spatial resolution, low contrast resolution, and image noise were evaluated.Results: Compared to the stationary reference CT, spatial integrity was within ±1 mm and the HU values were within two HU in the 4DCT for all phases. The spatial resolution was consistent while slightly higher noise was observed in the 4DCT images. The automated analysis in TotalQA was completed in approximately 20 min per motion trace, improving efficiency by more than 80% compared to the manual workflow.Conclusions: A streamlined 4DCT imaging QA workflow with automated analysis offers a robust assessment of imaging quality and motion accuracy in 4DCT scans, enabling an efficient and consistent QA process in large healthcare networks.

Objective . To investigate the effects of respiratory motion on the delivered dose in the context of proton pencil beam scanning (PBS) transmission FLASH radiotherapy (FLASH-RT) by simulation and phantom measurements. Approach . An in-house simulation code was employed to perform in silico simulation of 2D dose distributions for clinically relevant proton PBS transmission FLASH-RT treatments. A moving simulation grid was introduced to investigate the impacts of various respiratory motion and treatment delivery parameters on the dynamic PBS dose delivery. A strip-ionization chamber array detector and an IROC motion platform were employed to perform phantom measurements of the 2D dose distribution for treatment fields similar to those used for simulation. Main results . Clinically relevant respiratory motion and treatment delivery parameters resulted in degradation of the delivered dose compared to the static delivery as translation and distortion. Simulation showed that the gamma passing rates (2 mm/2% criterion) and target coverage could drop below 50% and 80%, respectively, for certain scenarios if no mitigation strategy was used. The gamma passing rates and target coverage could be restored to more than 95% and 98%, respectively, for short beams delivered at the maximal inhalation or exhalation phase. The simulation results were qualitatively confirmed in phantom measurements with the motion platform. Significance . Respiratory motion could cause dose quality degradation in a clinically relevant proton PBS transmission FLASH-RT treatment if no mitigation strategy is employed, or if an adequate margin is not given to the target. Besides breath-hold, gated delivery can be an alternative motion management strategy to ensure high consistency of the delivered dose while maintaining minimal dose to the surrounding normal tissues. To the best of our knowledge, this is the first study on motion impacts in the context of proton transmission FLASH radiotherapy.