Quantitative Medical Physics National Job Data Distribution Analysis.

PURPOSE: The purpose of this study was to investigate the contemporary distribution of medical physics (MP) employment opportunities across the United States. METHODS AND MATERIALS: An annual record (2018-2019) of advertised full-time MP jobs was created using publicly available information from the American Association of Physicists in Medicine and Indeed websites. Listed jobs were categorized based on position name, work experience, job function, and geographic region. To account for regional population differences, a preponderance of employment opportunities per 10 million people was computed. Using Commission on Accreditation of Medical Physics Education Programs residency accreditation data, the nationwide locations of the MP training centers and the number of residency positions per annum were identified. A chi-square goodness-of-fit test was used for statistical analysis. RESULTS: A total of 441 unique MP jobs were identified nationwide per annum (2018-2019). The highest percentage of MP jobs was reported from the South region (33.6%), and the lowest (17.2%) was from the West. Analysis revealed that 148 jobs (33.6%) were academic and 293 (66.4%) were nonacademic. The South had the most academic jobs overall (31.8%), whereas the West had the fewest (13.5%). Regionally, the highest percentage of academic jobs (46.9%) was reported from the Northeast, whereas the West had the lowest percentage (26.3%). The analysis of academic versus nonacademic job comparison by regions showed statistically significant differences (P = .0133). The Midwest and the West regions, respectively, showed the highest (18.2) and lowest (10.24) number of jobs per unit population, measured in 10 million. CONCLUSIONS: To our knowledge, this is one of the first national quantitative job data analyses of MP job distributions. This study revealed the level of demand for qualified candidates in 2018 to 2019, showing an imbalance between academic and nonacademic positions across the regions of the United States. Moreover, the geographic distribution of job listings deviated significantly from expectation given the relative population of each region.

Use of a laser-guided collimation system to perform direct kilovoltage x-ray spectra measurements on a linear accelerator onboard imager.

PURPOSE: The increased use of image-guided radiation therapy (IGRT) has led to increased use of kV on board imaging (OBI) devices. At present, directly measured OBI beam quality data have only been reported in terms of half-value layers (HVL). However, the HVL metric alone does not give the full OBI energy spectra as needed for accurate beam modeling. Although direct kV spectrometer devices exist they typically suffer from detector pile-up when used with OBI sources. We therefore present, for the first time, a novel laser-guided collimation system that allows direct measurement of the full energy spectrum for clinical OBI systems. METHODS: Several clinically relevant spectra (80, 100, and 125 kVp), with and without the half bow-tie filter, were measured using a thermoelectric cooled cadmium telluride (CdTe) detector paired with a multichannel analyzer. To prevent detector saturation, the photon flux at the detector was reduced by use of an in-house designed laser-guided collimation system. After applying energy bin corrections, direct spectroscopic measurements were compared to Monte Carlo (MC) simulated spectra in order to verify accuracy of collected data. Both percent depth dose (PDD) curves and digitally reconstructed radiographs (DRR) were compared using the measured vs MC spectra. RESULTS: Measured and MC spectra agree with RMSD between 1.96% and 3.29%. PDD curves generated from the measured and MC spectra were found to match except for in the small buildup region, with an overall match for the six beams ranging between 0.3% and 2.7% RMSD. DRRs matched well with a maximum difference in contrast of 1.1% and RMSD of 0.46% contrast for various materials in DRRs. CONCLUSIONS: The use of a laser-guided collimation system provided a method for quickly obtaining highly accurate kV spectrum data from OBI sources. For kV dose or DRR calculation, it was found that both spectra produced similar results.

A feasibility study of using advanced external beam techniques to create a vaginal cuff brachytherapy-like endometrial boost plan.

The purpose of this study was to explore the feasibility of using advanced external beam radiation therapy (EBRT) planning techniques for creating plans that could be used as a possible alternative for high-dose rate (HDR) vaginal cuff brachytherapy (VCBT) boost in treating endometrial cancer. The computed tomography (CT) images of a total of 4 female patients who had endometrial cancer treated with HDR-VCBT were selected for this study. A typical HDR-VCBT target volume, 0.5-cm-thick shell volume around the cylinder applicator in the prescribed treatment length was contoured and used as the planning target volume (PTV) in both the HDR VCBT and the EBRT VCBT-like plans. HDR-VCBT plans were made based on the clinical protocol, 6 Gy given at the cylinder surface. The EBRT plans were generated using either a 7-field intensity-modulated radiation therapy (IMRT) or a 2-arc volumetric-modulated arc therapy (VMAT) techniques for different cylinder sizes and treatment lengths, with the prescription dose of 5 Gy. Organs at risk (OARs) such as bladder, femoral heads, rectum, and sigmoid were also contoured and used in dosimetric evaluations. Dose-to-target metrics included mean dose, the dose covering 90% of target volume (D90) and the percentage of target volume covered by 90% of prescription dose (V90 or V13.5 Gy). Dose to OAR metrics included the maximum dose received by 0.1 cc (D0.1cc), 1.0 cc (D1.0cc), and 2.0 cc (D2.0cc) of OARs. These metrics were calculated and compared between all techniques. After the EBRT plans were normalized to achieve a comparable mean dose to target as HDR-VCBT, the EBRT plans were found to have superior target coverage and increased dose homogeneity compared with HDR-VCBT. V90s of EBRT plans were 95%, compared with 50% to 58% of the HDR plans. However, D0.1cc, D1.0cc, and D2.0cc of OARs were 2% to 38% lower in HDR-VCBT than in EBRT. Although HDR-VCBT plans demonstrated superior normal tissue sparing, both EBRT and HDR-VCBT plans produce plans that met clinical dose constraints on normal tissues. Advanced EBRT techniques such as IMRT and VMAT are capable of making plans, which closely resemble HDR-VCBT. Although the doses of OARs are greater in EBRT than in HDR-VCBT, the prescription dose coverage and dose homogeneity of the EBRT plans are greater than that of HDR-VCBT plans at the similar mean dose, and the OAR dose is still acceptable with EBRT plans. The detailed dosimetric approaches are provided in the study.

Use of proximal operator graph solver for radiation therapy inverse treatment planning.

PURPOSE: Most radiation therapy optimization problems can be formulated as an unconstrained problem and solved efficiently by quasi-Newton methods such as the Limited-memory Broyden-Fletcher-Goldfarb-Shanno (L-BFGS) algorithm. However, several next generation planning techniques such as total variation regularization- based optimization and MV+kV optimization, involve constrained or mixed-norm optimization, and cannot be solved by quasi-Newton methods. Using standard optimization algorithms on such problems often leads to prohibitively long optimization times and large memory requirements. This work investigates the use of a recently developed proximal operator graph solver (POGS) in solving such radiation therapy optimization problems. METHODS: Radiation therapy inverse treatment planning was formulated as a graph form problem, and the proximal operators of POGS for quadratic optimization were derived. POGS was exploited for the first time to impose hard dose constraints along with soft constraints in the objective function. The solver was applied to several clinical treatment sites (TG119, liver, prostate, and head&neck), and the results were compared to the solutions obtained by other commercial and non-commercial optimizers. RESULTS: For inverse planning optimization with nonnegativity box constraints on beamlet intensity, the speed of POGS can compete with that of LBFGSB in some situations. For constrained and mixed-norm optimization, POGS is about one or two orders of magnitude faster than the other solvers while requiring less computer memory. CONCLUSIONS: POGS was used for solving inverse treatment planning problems involving constrained or mixed-norm formulation on several example sites. This approach was found to improve upon standard solvers in terms of computation speed and memory usage, and is capable of solving traditionally difficult problems, such as total variation regularization-based optimization and combined MV+kV optimization.

Spatial and rotational quality assurance of 6DOF patient tracking systems.

PURPOSE: External tracking systems used for patient positioning and motion monitoring during radiotherapy are now capable of detecting both translations and rotations. In this work, the authors develop a novel technique to evaluate the 6 degree of freedom 6(DOF) (translations and rotations) performance of external motion tracking systems. The authors apply this methodology to an infrared marker tracking system and two 3D optical surface mapping systems in a common tumor 6DOF workspace. METHODS: An in-house designed and built 6DOF parallel kinematics robotic motion phantom was used to perform motions with sub-millimeter and subdegree accuracy in a 6DOF workspace. An infrared marker tracking system was first used to validate a calibration algorithm which associates the motion phantom coordinate frame to the camera frame. The 6DOF positions of the mobile robotic system in this space were then tracked and recorded independently by an optical surface tracking system after a cranial phantom was rigidly fixed to the moveable platform of the robotic stage. The calibration methodology was first employed, followed by a comprehensive 6DOF trajectory evaluation, which spanned a full range of positions and orientations in a 20 × 20 × 16 mm and 5° × 5° × 5° workspace. The intended input motions were compared to the calibrated 6DOF measured points. RESULTS: The technique found the accuracy of the infrared (IR) marker tracking system to have maximal root-mean square error (RMSE) values of 0.18, 0.25, 0.07 mm, 0.05°, 0.05°, and 0.09° in left-right (LR), superior-inferior (SI), anterior-posterior (AP), pitch, roll, and yaw, respectively, comparing the intended 6DOF position and the measured position by the IR camera. Similarly, the 6DOF RSME discrepancy for the HD optical surface tracker yielded maximal values of 0.46, 0.60, 0.54 mm, 0.06°, 0.11°, and 0.08° in LR, SI, AP, pitch, roll, and yaw, respectively, over the same 6DOF evaluative workspace. An earlier generation 3D optical surface tracking unit was observed to have worse tracking capabilities than both the IR camera unit and the newer 3D surface tracking system with maximal RMSE of 0.69, 0.74, 0.47 mm, 0.28°, 0.19°, and 0.18°, in LR, SI, AP, pitch, roll, and yaw, respectively, in the same 6DOF evaluation space. CONCLUSIONS: The proposed technique was found to be effective at evaluating the performance of 6DOF patient tracking systems. All observed optical tracking systems were found to exhibit tracking capabilities at the sub-millimeter and subdegree level within a 6DOF workspace.

Robotic real-time translational and rotational head motion correction during frameless stereotactic radiosurgery.

PURPOSE: To develop a control system to correct both translational and rotational head motion deviations in real-time during frameless stereotactic radiosurgery (SRS). METHODS: A novel feedback control with a feed-forward algorithm was utilized to correct for the coupling of translation and rotation present in serial kinematic robotic systems. Input parameters for the algorithm include the real-time 6DOF target position, the frame pitch pivot point to target distance constant, and the translational and angular Linac beam off (gating) tolerance constants for patient safety. Testing of the algorithm was done using a 4D (XY Z + pitch) robotic stage, an infrared head position sensing unit and a control computer. The measured head position signal was processed and a resulting command was sent to the interface of a four-axis motor controller, through which four stepper motors were driven to perform motion compensation. RESULTS: The control of the translation of a brain target was decoupled with the control of the rotation. For a phantom study, the corrected position was within a translational displacement of 0.35 mm and a pitch displacement of 0.15° 100% of the time. For a volunteer study, the corrected position was within displacements of 0.4 mm and 0.2° over 98.5% of the time, while it was 10.7% without correction. CONCLUSIONS: The authors report a control design approach for both translational and rotational head motion correction. The experiments demonstrated that control performance of the 4D robotic stage meets the submillimeter and subdegree accuracy required by SRS.

Development of a 6DOF robotic motion phantom for radiation therapy.

PURPOSE: The use of medical technology capable of tracking patient motion or positioning patients along 6 degree-of-freedom (6DOF) has steadily increased in the field of radiation therapy. However, due to the complex nature of tracking and performing 6DOF motion, it is critical that such technology is properly verified to be operating within specifications in order to ensure patient safety. In this study, a robotic motion phantom is presented that can be programmed to perform highly accurate motion along any X (left-right), Y (superior-inferior), Z (anterior-posterior), pitch (around X), roll (around Y), and yaw (around Z) axes. In addition, highly synchronized motion along all axes can be performed in order to simulate the dynamic motion of a tumor in 6D. The accuracy and reproducibility of this 6D motion were characterized. METHODS: An in-house designed and built 6D robotic motion phantom was constructed following the Stewart-Gough parallel kinematics platform archetype. The device was controlled using an inverse kinematics formulation, and precise movements in all 6 degrees-of-freedom (X, Y, Z, pitch, roll, and yaw) were performed, both simultaneously and separately for each degree-of-freedom. Additionally, previously recorded 6D cranial and prostate motions were effectively executed. The robotic phantom movements were verified using a 15 fps 6D infrared marker tracking system and the measured trajectories were compared quantitatively to the intended input trajectories. The workspace, maximum 6D velocity, backlash, and weight load capabilities of the system were also established. RESULTS: Evaluation of the 6D platform demonstrated translational root mean square error (RMSE) values of 0.14, 0.22, and 0.08 mm over 20 mm in X and Y and 10 mm in Z, respectively, and rotational RMSE values of 0.16°, 0.06°, and 0.08° over 10° of pitch, roll, and yaw, respectively. The robotic stage also effectively performed controlled 6D motions, as well as reproduced cranial trajectories over 15 min, with a maximal RMSE of 0.04 mm translationally and 0.04° rotationally, and a prostate trajectory over 2 min, with a maximal RMSE of 0.06 mm translationally and 0.04° rotationally. CONCLUSIONS: This 6D robotic phantom has proven to be accurate under clinical standards and capable of reproducing tumor motion in 6D. Such functionality makes the robotic phantom usable for either quality assurance or research purposes.

Combined MV + kV inverse treatment planning for optimal kV dose incorporation in IGRT.

Despite the existence of real-time kV intra-fractional tumor tracking strategies for many years, clinical adoption has been held back by concern over the excess kV imaging dose cost to the patient when imaging in continuous fluoroscopic mode. This work aims to solve this problem by investigating, for the first time, the use of convex optimization tools to optimally integrate this excess kV imaging dose into the MV therapeutic dose in order to make real-time kV tracking clinically feasible. Phase space files modeling both a 6 MV treatment beam and a kV on-board-imaging beam of a commercial LINAC were generated with BEAMnrc, and used to generate dose influence matrices in DOSXYZnrc for ten previously treated lung cancer patients. The dose optimization problem for IMRT, formulated as a quadratic problem, was modified to include additional constraints as required for real-time kV intra-fractional tracking. An interior point optimizer was used to solve the modified optimization problem. It was found that when using large kV imaging apertures during fluoroscopic tracking, combined MV + kV optimization lead to a 0.5%-5.17% reduction in the total number of monitor units assigned to the MV beam due to inclusion of the kV dose over the ten patients. This was accompanied by a reduction of up to 42% of the excess kV dose compared to standard MV IMRT with kV tracking. For all kV field sizes considered, combined MV + kV optimization provided prescription dose to the treatment volume coverage equal to the no-imaging case, yet superior to standard MV IMRT with non-optimized kV fluoroscopic tracking. With combined MV + kV optimization, it is possible to quantify in a patient specific way the dosimetric effect of real-time imaging on the patient. Such information is necessary when substantial kV imaging is performed.

Spatial and temporal performance of 3D optical surface imaging for real-time head position tracking.

PURPOSE: The spatial and temporal tracking performance of a commercially available 3D optical surface imaging system is evaluated for its potential use in frameless stereotactic radiosurgery head tracking applications. METHODS: Both 3D surface and infrared (IR) marker tracking were performed simultaneously on a head phantom mounted on an xyz motion stage and on four human subjects. To allow spatial and temporal comparison on human subjects, three points were simultaneously monitored, including the upper facial region (3D surface), a dental plate (IR markers), and upper forehead (IR markers). RESULTS: For both static and dynamic phantom studies, the 3D surface tracker was found to have a root mean squared error (RMSE) of approximately 0.30 mm for region-of-interest (ROI) surface sizes greater than 1000 vertex points. Although, the processing period (1/fps) of the 3D surface system was found to linearly increase as a function of the number of ROI vertex points, the tracking accuracy was found to be independent of ROI size provided that the ROI was sufficiently large and contained features for registration. For human subjects, the RMSE between 3D surface tracking and IR marker tracking modalities was 0.22 mm left-right (x-axis), 0.44 mm superior-inferior (y-axis), 0.27 mm anterior-posterior (z-axis), 0.29° pitch (around x-axis), 0.18° roll (around y-axis), and 0.15° yaw (around z-axis). CONCLUSIONS: 3D surface imaging has the potential to provide submillimeter level head motion tracking. This is provided that a highly accurate camera-to-LINAC frame of reference calibration can be performed and that the reference ROI is of sufficient size and contains suitable surface features for registration.

An EPID based method for performing high accuracy calibration between an optical external marker tracking device and the LINAC reference frame.

PURPOSE: With the increasing use of external 3D optical tracking cameras to guide modern radiation therapy procedures, it has become vitally important to have an accurate camera to linear accelerator (LINAC) reference frame calibration. To eliminate errors present in current calibration procedures based on the manual hand alignment of a device using the light field crosshairs and in room guidance lasers, a semiautomated quantitative calibration approach requiring only use of an electronic portal imaging device (EPID) was developed. METHODS: A phantom comprised of seven highly IR reflective plastic BBs was placed on the LINAC treatment couch and imaged with both a 3D stereoscopic IR imager and the on board megavoltage (MV) EPID imager. Having knowledge of the optically determined 3D positions and projected EPID images of the BBs, simulated annealing was used to optimize the location of the BBs in the LINAC frame using four different optimization functions. Singular value decomposition was then used to calculate the transformation matrix between the camera and LINAC reference frames. Results were then compared to a traditional camera calibration method for overall accuracy. RESULTS: Using modeled data, the simulated annealing process was able to determine the actual locations of the BBs with a RMSE of 0.23 mm. Using projection images acquired with an MV imager, the process was able to determine locations of BBs within .26 mm. The results depend on the choice of optimization function. CONCLUSIONS: Results show that the method can be used to provide highly accurate spatial registration between an external 3D imaging reference frame and the LINAC frame. The experimental MV imager results, while not as precise as the simulated results, exceed 1 mm accuracy and the current accepted AAPM TG-142 standard of ≤2 mm positioning accuracy.