Brendan Whelan

Comp Plan: A computer program to generate dose and radiobiological metrics from dose-volume histogram files.

Treatment planning studies often require the calculation of a large number of dose and radiobiological metrics. To streamline these calculations, a computer program called Comp Plan was developed using MATLAB. Comp Plan calculates common metrics, including equivalent uniform dose, tumor control probability, and normal tissue complication probability from dose-volume histogram data. The dose and radiobiological metrics can be calculated for the original data or for an adjusted fraction size using the linear quadratic model. A homogeneous boost dose can be added to a given structure if desired. The final output is written to an Excel file in a format convenient for further statistical analysis. Comp Plan was verified by independent calculations. A lung treatment planning study comparing 45 plans for 7 structures using up to 6 metrics for each structure was successfully analyzed within approximately 5 minutes with Comp Plan. The code is freely available from the authors on request.

A novel electron accelerator for MRI-Linac radiotherapy

PURPOSE MRI guided radiotherapy is a rapidly growing field; however, current electron accelerators are not designed to operate in the magnetic fringe fields of MRI scanners. As such, current MRI-Linac systems require magnetic shielding, which can degrade MR image quality and limit system flexibility. The purpose of this work was to develop and test a novel medical electron accelerator concept which is inherently robust to operation within magnetic fields for in-line MRI-Linac systems. METHODS Computational simulations were utilized to model the accelerator, including the thermionic emission process, the electromagnetic fields within the accelerating structure, and resulting particle trajectories through these fields. The spatial and energy characteristics of the electron beam were quantified at the accelerator target and compared to published data for conventional accelerators. The model was then coupled to the fields from a simulated 1 T superconducting magnet and solved for cathode to isocenter distances between 1.0 and 2.4 m; the impact on the electron beam was quantified. RESULTS For the zero field solution, the average current at the target was 146.3 mA, with a median energy of 5.8 MeV (interquartile spread of 0.1 MeV), and a spot size diameter of 1.5 mm full-width-tenth-maximum. Such an electron beam is suitable for therapy, comparing favorably to published data for conventional systems. The simulated accelerator showed increased robustness to operation in in-line magnetic fields, with a maximum current loss of 3% compared to 85% for a conventional system in the same magnetic fields. CONCLUSIONS Computational simulations suggest that replacing conventional DC electron sources with a RF based source could be used to develop medical electron accelerators which are robust to operation in in-line magnetic fields. This would enable the development of MRI-Linac systems with no magnetic shielding around the Linac and reduce the requirements for optimization of magnetic fringe field, simplify design of the high-field magnet, and increase system flexibility.

An MRI-Compatible patient rotation system – Design, construction, and first organ deformation results

Purpose: Conventionally in radiotherapy, a very heavy beam forming apparatus (gantry) is rotated around a patient. From a mechanical perspective, a more elegant approach is to rotate the patient within a stationary beam. Key obstacles to this approach are patient tolerance and anatomical deformation. Very little information on either aspect is available in the literature. The purpose of this work was therefore to design and test an MRI‐compatible patient rotation system such that the feasibility of a patient rotation workflow could be tested. Methods: A patient rotation system (PRS) was designed to fit inside the bore of a 3T MRI scanner (Skyra, Siemens) such that 3D images could be acquired at different rotation angles. Once constructed, a pelvic imaging study was carried out on a healthy volunteer. T2‐weighted MRI images were taken every 45° between 0° and 360°, (with 0° equivalent to supine). The prostate, bladder, and rectum were segmented using atlas‐based auto contouring. The images from each angle were registered back to the 0° image in three steps: (a) Rigid registration was based on MRI visible markers on the couch. (b) Rigid registration based on the prostate contour (equivalent to a rigid shift to the prostate). (c) Nonrigid registration. The Dice similarity coefficient (DSC) and mean average surface distance (MASD) were calculated for each organ at each step. Results: The PRS met all design constraints and was successfully integrated with the MRI scanner. Phantom images showed minimal difference in signal or noise with or without the PRS in the MRI scanner. For the MRI images, the DSC (mean ± standard deviation) over all angles in the prostate, rectum, and bladder was 0.60 ± 0.11, 0.56 ± 0.15, and 0.76 ± 0.06 after rigid couch registration, 0.88 ± 0.03, 0.81 ± 0.08, and 0.86 ± 0.03 after rigid prostate guided registration, and 0.85 ± 0.03, 0.88 ± 0.02, 0.87 ± 0.02 after nonrigid registration. Conclusions: An MRI‐compatible patient rotation system has been designed, constructed, and tested. A pelvic study was carried out on a healthy volunteer. Rigid registration based on the prostate contour yielded DSC overlap statistics in the prostate superior to interobserver contouring variability reported in the literature.

Dynamic keyhole: A novel method to improve MR images in the presence of respiratory motion for real-time MRI

PURPOSE In this work, the authors present a novel magnetic resonance imaging reconstruction method to improve the quality of MR images in the presence of respiratory motion for real-time thoracic image-guided radiotherapy. METHODS This new reconstruction method is called dynamic keyhole and utilizes a library of previously acquired, peripheral k-space datasets from the same (or similar) respiratory state in conjunction with central k-space datasets acquired in real-time. Internal or external respiratory signals are utilized to sort, match, and combine the two separate peripheral and central k-space datasets with respect to respiratory displacement, thereby reducing acquisition time and improving image quality without respiratory-related artifacts. In this study, the dynamic keyhole, conventional keyhole, and zero-filling methods were compared to full k-space acquisition (ground truth) for 60 coronal datasets acquired from 15 healthy human subjects. RESULTS For the same image-quality difference from the ground-truth image, the dynamic keyhole method reused 79% of the prior peripheral phase-encoding lines, while the conventional keyhole reused 73% and zero-filling 63% (p-value < 0.0001), corresponding to faster acquisition speed of dynamic keyhole for real-time imaging applications. CONCLUSIONS This study demonstrates that the dynamic keyhole method is a promising technique for clinical applications such as image-guided radiotherapy requiring real-time MR monitoring of the thoracic region. Based on the results from this study, the dynamic keyhole method could increase the temporal resolution by a factor of five compared with full k-space methods.

Development and testing of a database of NIH research funding of AAPM members: A report from the AAPM Working Group for the Development of a Research Database (WGDRD)

Purpose To produce and maintain a database of National Institutes of Health (NIH) funding of the American Association of Physicists in Medicine (AAPM) members, to perform a top‐level analysis of these data, and to make these data (hereafter referred to as the AAPM research database) available for the use of the AAPM and its members. Methods NIH‐funded research dating back to 1985 is available for public download through the NIH exporter website, and AAPM membership information dating back to 2002 was supplied by the AAPM. To link these two sources of data, a data mining algorithm was developed in Matlab. The false‐positive rate was manually estimated based on a random sample of 100 records, and the false‐negative rate was assessed by comparing against 99 member‐supplied PI_ID numbers. The AAPM research database was queried to produce an analysis of trends and demographics in research funding dating from 2002 to 2015. Results A total of 566 PI_ID numbers were matched to AAPM members. False‐positive and ‐negative rates were respectively 4% (95% CI: 1–10%, N = 100) and 10% (95% CI: 5–18%, N = 99). Based on analysis of the AAPM research database, in 2015 the NIH awarded

Patient reported outcomes of slow, single arc rotation: Do we need rotating gantries?

USD 110M to members of the AAPM. The four NIH institutes which historically awarded the most funding to AAPM members were the National Cancer Institute, National Institute of Biomedical Imaging and Bioengineering, National Heart Lung and Blood Institute, and National Institute of Neurological Disorders and Stroke. In 2015, over 85% of the total NIH research funding awarded to AAPM members was via these institutes, representing 1.1% of their combined budget. In the same year, 2.0% of AAPM members received NIH funding for a total of

Performance of a clinical gridded electron gun in magnetic fields: Implications for MRI‐linac therapy

116M, which is lower than the historic mean of

Passive magnetic shielding in MRI-Linac systems

120M (in 2015 USD). Conclusions A database of NIH‐funded research awarded to AAPM members has been developed and tested using a data mining approach, and a top‐level analysis of funding trends has been performed. Current funding of AAPM members is lower than the historic mean. The database will be maintained by members of the Working group for the development of a research database (WGDRD) on an annual basis, and is available to the AAPM, its committees, working groups, and members for download through the AAPM electronic content website. A wide range of questions regarding financial and demographic funding trends can be addressed by these data. This report has been approved for publication by the AAPM Science Council.

MRI-guidance for motion management in external beam radiotherapy: current status and future challenges

Patient rotation could greatly simplify radiation therapy delivery, with particularly important ramifications for fixed beam treatment with protons, heavy ions, MRI‐Linacs, and low cost Linacs. Patient tolerance is often cited as a barrier to widespread implementation to patient rotation; however, no quantitative data addressing this issue exists. In this study, patient reported experiences of slow, single arc rotation in upright (sitting) and lying orientations are reported.

An update of NIH research funding of AAPM members from 1985 to 2017

PURPOSE MRI-linac therapy is a rapidly growing field, and requires that conventional linear accelerators are operated with the fringe field of MRI magnets. One of the most sensitive accelerator components is the electron gun, which serves as the source of the beam. The purpose of this work was to develop a validated finite element model (FEM) model of a clinical triode (or gridded) electron gun, based on accurate geometric and electrical measurements, and to characterize the performance of this gun in magnetic fields. METHODS The geometry of a Varian electron gun was measured using 3D laser scanning and digital calipers. The electric potentials and emission current of these guns were measured directly from six dose matched true beam linacs for the 6X, 10X, and 15X modes of operation. Based on these measurements, a finite element model (FEM) of the gun was developed using the commercial software opera/scala. The performance of the FEM model in magnetic fields was characterized using parallel fields ranging from 0 to 200 G in the in-line direction, and 0-35 G in the perpendicular direction. RESULTS The FEM model matched the average measured emission current to within 5% across all three modes of operation. Different high voltage settings are used for the different modes; the 6X, 10X, and 15X modes have an average high voltage setting of 15, 10, and 11 kV. Due to these differences, different operating modes show different sensitivities in magnetic fields. For in line fields, the first current loss occurs at 40, 20, and 30 G for each mode. This is a much greater sensitivity than has previously been observed. For perpendicular fields, first beam loss occurred at 8, 5, and 5 G and total beam loss at 27, 22, and 20 G. CONCLUSIONS A validated FEM model of a clinical triode electron gun has been developed based on accurate geometric and electrical measurements. Three different operating modes were simulated, with a maximum mean error of 5%. This gun shows greater sensitivity to in-line magnetic fields than previously presented models, and different operating modes show different sensitivity.