Jenghwa Chang

Background: AAPM Report No. 365 recommends that medical physics graduate programs offer courses covering both mathematical and statistical methods (Section 3.1.7) as well as computational methods and medical informatics (Section 3.1.8). While our program seeks to incorporate both of these essential areas into the curriculum, various financial and programmatic constraints have necessitated a more streamlined approach. Accordingly, this work presents a single 2‐semester‐hour course designed to address these topics in an integrated format. Purpose: In this paper, we present our efforts in developing a new teaching approach that addresses both AAPM Report No. 365 recommendations in one course. Methods: The major challenge of designing this course was the insufficient number of semester hours allocated to teaching both topics. To overcome this obstacle, we implemented a novel approach to homework assignments. Unlike the traditional approach in which students complete homework manually, students in this class were asked to write computer programs to solve most homework questions. These carefully designed assessments not only enhanced students’ understanding of the course materials but also required them to utilize appropriate computational methods. Recognizing that students had varying levels of coding experience from their undergraduate studies, the program instructed them, prior to starting the program, to acquire foundational Python skills through self‐guided learning to prepare for this course. In addition, basic Python programming guidance was provided with each homework assignment to support students with less coding experience. Results: The final course covered the following key mathematical concepts: signals and systems, Fourier series and transform, probability, statistical inference, image quality, optimization methods, and an introduction to artificial intelligence with an emphasis on machine learning. To complete the homework assignments, students developed coding skills in data visualization, numerical integration, convolution, continuous‐time/discrete‐time/fast Fourier transforms, random number generator, point estimation, confidence interval, hypothesis testing, linear models, DICOM, the Rose model, conjugate‐gradient descent, iterative methods for solving systems of linear equations, and support vector machines. The course was offered in the Spring 2024 and Spring 2025 semesters and received generally positive evaluations, with some noted challenges per the Course and Teacher Rating reports. Overall, students reported that the course was educationally beneficial; however, some indicated that the coding‐based assignments were demanding. Conclusion: We successfully developed and implemented a course that covers mathematical and statistical methods as well as computational methods and medical informatics as recommended in AAPM Report No. 365.

Objective. To evaluate proton Bragg peak FLASH for ocular treatments to enhance normal tissue sparing and enable dose escalation via FLASH biological optimization (FBO). Approach. The FLASH-sparing factors for normal tissues were derived from the literature in modeling the phenomenological FLASH normal tissue sparing effect. Using the single-energy BP-FLASH technique (SEBP-FLASH), an in-house treatment planning system was implemented with the FLASH FBO module. Ten consecutive ocular patients who were treated using conventional dose rate intensity-modulated proton therapy (CONV-IMPT) to 50 Gy in 5 fractions were replanned using the FLASH technique. The dose metrics for the OARs were compared using the two different techniques. The fraction dose was then intentionally escalated from 10 to 12 Gy through FBO to assess whether the plans still met clinical constraints. Main results. In the FLASH regimen without FBO (50 Gy/5 fractions), all ipsilateral OAR dosimetric metrics met clinical objectives with safe margins. While the clinical CONV-IMPT approach demonstrated slightly better dosimetric performance than SEBP-FLASH plans, the incorporation of FBO improved all OAR dose metrics beyond those of CONV- IMPT, except for the mean dose to the cornea (no difference). When the target dose was increased from 50 to 60 Gy using FBO, all OARs remained within clinical limits. The mean and maximum doses to the cornea increased from 11.7 to 15.4 Gy and from 22.8 to 23.6 Gy, respectively, when transitioning from 50 Gy CONV-IMPT to 60 Gy FBO. However, in the 60 Gy FBO plans, the maximum doses were reduced for the eye (102.0%–87.0%), optic nerves (98.7%–74.0%), retina (100.5%–81.8%), lacrimal gland (84.9%–73.2%), and conjunctiva (91%–72.3%). Significance. SEBP-FLASH achieves plan quality comparable to CONV-IMPT using 50 Gy/5 fractions and enables dose escalation via FLASH FBO while meeting clinical standards, potentially improving tumor control with acceptable toxicity.

Bragg peak FLASH radiotherapy (RT) uses a distal tracking method to eliminate exit doses and can achieve superior OAR sparing. This study explores the application of this novel method in stereotactic body radiotherapy prostate FLASH-RT. An in-house platform was developed to enable intensity-modulated proton therapy (IMPT) planning using a single-energy Bragg peak distal tracking method. The patients involved in the study were previously treated with proton stereotactic body radiotherapy (SBRT) using the pencil beam scanning (PBS) technique to 40 Gy in five fractions. FLASH plans were optimized using a four-beam arrangement to generate a dose distribution similar to the conventional opposing beams. All of the beams had a small angle of two degrees from the lateral direction to increase the dosimetry quality. Dose metrics were compared between the conventional PBS and the Bragg peak FLASH plans. The dose rate histogram (DRVH) and FLASH metrics of 40 Gy/s coverage (V_40Gy/s) were investigated for the Bragg peak plans. There was no significant difference between the clinical and Bragg peak plans in rectum, bladder, femur heads, large bowel, and penile bulb dose metrics, except for D_max. For the CTV, the FLASH plans resulted in a higher D_max than the clinical plans (116.9% vs. 103.3%). For the rectum, the V_40Gy/s reached 94% and 93% for 1 Gy dose thresholds in composite and single-field evaluations, respectively. Additionally, the FLASH ratio reached close to 100% after the application of the 5 Gy threshold in composite dose rate assessment. In conclusion, the Bragg peak distal tracking method can yield comparable plan quality in most OARs while preserving sufficient FLASH dose rate coverage, demonstrating that the ultra-high dose technique can be applied in prostate FLASH SBRT.

Purpose: To calculate the dose distribution using Monte Carlo simulations for a novel high‐dose‐rate Yttrium‐90 (Y‐90) disc source recently developed for episcleral brachytherapy and provide a lookup table for treatment planning.Methods: Monte Carlo simulations were performed to calculate the in‐water dose distribution of the Y‐90 disc source using the “GATE”, a software based on the “Geant4” Monte Carlo simulation toolkit developed by the international OpenGATE collaboration. The geometry of this novel beta source, its capsule, and the surrounding water medium were accurately modeled in the simulation input files. The standard Y‐90 element beta spectrum from ICRU 72 was used, and the physics processes for beta and photon interactions with matters were all included. The dose distribution of this Y‐90 disc source was measured in a separate study using Gafchromic EBT‐3 films and the results were reported elsewhere. To match the setup of the experiment, a Gafchromic EBT‐3 film was also included in the simulation geometry. The simulated dose profiles were exported from the 3D dose distribution results and compared with the measured dose profiles. Transverse dose profiles at different distances from the seed surface were also obtained to study the lateral coverage of the source.Results: The measured percent depth dose (PDD) curves along the central axis perpendicular to the surface of the Y‐90 disc were constructed from the experimental and simulated data, and normalized to the reference point at 1 mm from the source capsule. Both PDD curves agreed well up to 4 mm from the source surface (maximum difference ± 10%) but deviated from each other beyond 4 mm. The deviation might be caused by the increased measurement uncertainty in the low‐dose region. The dose rate at the reference point calculated from the Monte Carlo simulation was 1.09 cGy/mCi‐s and agreed very well with the measured dose rate of 1.05 cGy/mCi‐s. If the 80% isodose line is selected as the lateral coverage, the lateral dose coverage is maximal (∼4.5 mm) at the plane next to the source surface, and gradually decreases with the increasing distance, approaching 3.5 mm when the plane is 5 mm from the 6‐mm diameter source surface.Conclusion: Monte Carlo simulations were successfully performed to confirm the measured PDD curve of the novel Y‐90 disc source. This simulation work laid a solid foundation for characterizing the full dosimetry parameters of this source for episcleral brachytherapy applications.

Bragg peak FLASH-RT can deliver highly conformal treatment and potentially offer improved normal tissue protection for radiotherapy patients. This study focused on developing ultra-high dose rate (≥40 Gy × RBE/s) intensity-modulated proton therapy (IMPT) for hypofractionated treatment of early-stage breast cancer. A novel tracking technique was developed to enable pencil beaming scanning (PBS) of single-energy protons to adapt the Bragg peak (BP) to the target distally. Standard-of-care PBS treatment plans of consecutively treated early-stage breast cancer patients using multiple energy layers were reoptimized using this technique, and dose metrics were compared between single-energy layer BP FLASH and conventional IMPT plans. FLASH dose rate coverage by volume (V_40Gy/s) was also evaluated for the FLASH sparing effect. Distal tracking can precisely stop BP at the target distal edge. All plans (n = 10) achieved conformal IMPT-like dose distributions under clinical machine parameters. No statistically significant differences were observed in any dose metrics for heart, ipsilateral lung, most ipsilateral breast, and CTV metrics (p > 0.05 for all). Conventional plans yielded slightly superior target and skin dose uniformities with 4.5% and 12.9% lower dose maxes, respectively. FLASH-RT plans reached 46.7% and 61.9% average-dose rate FLASH coverage for tissues receiving more than 1 and 5 Gy plan dose total under the 250 minimum MU condition. Bragg peak FLASH-RT techniques achieved comparable plan quality to conventional IMPT while reaching adequate dose rate ratios, demonstrating the feasibility of early-stage breast cancer clinical applications.

To improve the efficiency of frame-based and frameless Gamma Knife® Icon™ (GKI) treatments by analyzing the workflows of both treatment approaches and identifying steps that lead to prolonged patient in-clinic or treatment time. The treatment processes of 57 GKI patients, 16 frame-based and 41 frameless cases were recorded and analyzed. For frame-based treatments, time points were recorded for various steps in the process, including check-in, magnetic resonance imaging (MRI) completion, plan approval, and treatment start/end times. The time required for completing each step was calculated and investigated. For frameless treatments, the actual and planned treatment times were compared to evaluate the patient tolerance of the treatment. In addition, the time spent on room cleaning and preparation between treatments was also recorded and analyzed. For frame-based cases, the average in-clinic time was 6.3 hours (ranging from 4 to 8.7 hours). The average time from patient check-in to plan approval was 4.2 hours (ranging from 2.8 to 5.5 hours), during which the frame was placed, stereotactic reference MRI images were taken, target volumes were contoured, and the treatment plan was developed and second-checked. For patients immobilized with a mask, treatment pauses triggered by the intra-fractional motion monitoring system resulted in a significantly longer actual treatment time than the planned time. In 50 (or 55%) of the 91 frameless treatments, the patient on-table time was longer than the planned treatment time by more than 10 minutes, and in 19 (or 21%) of the treatments the time difference was larger than 20 minutes. Major treatment interruptions, defined as pauses leading to a longer than 10-minute delay, were more commonly encountered in patients with a planned treatment time longer than 40 minutes, which accounted for 64% of the recorded major interruptions. For frame-based cases, the multiple pretreatment steps (from patient check-in to plan approval) in the workflow were time-consuming and resulted in prolonged patient in-clinic time. These pretreatment steps may be shortened by performing some of these steps before the treatment day, e.g., pre-planning the treatment using diagnostic MRI scans acquired a few days earlier. For frameless patients, we found that a longer planned treatment time is associated with a higher chance of treatment interruption. For patients with a long treatment time, a planned break or consideration of fractionated treatments (i.e., 3 to 5 fractionated stereotactic radiosurgery) may optimize the workflow and improve patient satisfaction.

The objective of this work was to identify the exact location of the effective point of measurement (EPM) of four different active detectors to compare the relative collimator output factors (ROF) of Leksell Gamma Knife (LGK) according to IAEA TRS‐483 recommendations. ROF was measured at the center of the spherical LGK‐Solid Water (LGK‐SW) Phantom for three (4‐, 8‐, and 16‐mm in diameter) collimators using four (PTW‐TN60008, PTW‐TN60016, PTW‐TN60017, and PTW‐60019 diode/diamond) detectors. Since diode detectors have a much smaller sensitive volume than the PTW‐31010 ion chamber used for reference dosimetry, its EPM might not be at the center of the phantom, or (100, 100, 100) of the Leksell Coordinate System, particularly in the z‐direction. Hence for each diode detector, a CBCT image was acquired after it was inserted into the phantom, from which the z‐Leksell coordinate of EPM was determined. Relative collimator output factors was then measured by focusing GK beams on the determined EPM of each diode. Measured ROFs were compared with the vendor‐provided values in GK treatment planning system. For validation, a plan was generated to measure the output of 4‐mm collimator for PTW‐TN60017 at various couch locations along the z‐axis. For PTW‐TN60008, the percentage variations were 0.6 ± 0.4%, and −0.8 ± 0.2% for 4 and 8‐mm collimators, respectively. For PTW‐TN60016, the percentage variations were 0.8 ± 0.0%, and 0.2 ± 0.1%, respectively. The percentage variations were −3.3 ± 0.0% and −0.9 ± 0.1%, respectively, for PTW‐TN60017, and −0.5 ± 0.0% and −0.8 ± 0.2%, respectively, for PTW‐TN60019. Center of the measured profile for PTW‐TN60017 was only 0.1 mm different from that identified using the CBCT. In conclusion, we have developed a simple and effective method to determine the EPMs of diode detectors when inserted into the existing LGK‐SW phantom. With the acquired positional information and using TRS‐483 protocol, good agreements were obtained between the measured ROFs and manufacturer recommended values.