Combined clinical and research training in medical physics in a multi-institutional setting: 13-year experience of Harvard Medical Physics Residency Program.

PURPOSE: This manuscript describes the structure, management and outcomes of a multi-institutional clinical and research medical physics residency program (Harvard Medical Physics Residency Program, or HMPRP) to provide potentially useful information to the centers considering a multi-institutional approach for their training programs. METHODS: Data from the program documents and public records was used to describe HMPRP and obtain statistics about participating faculty, enrolled residents, and graduates. Challenges associated with forming and managing a multi-institutional program and developed solutions for effective coordination between several clinical centers are described. RESULTS: HMPRP was formed in 2009 and was accredited by the Commission on Accreditation of Medical Physics Education Programs (CAMPEP) in 2011. It is a 3-year therapy program, with a dedicated year of research and the 2 years of clinical training at three academic hospitals. A CAMPEP-accredited Certificate Program is embedded in HMPRP to allow enrolled residents to complete a formal didactic training in medical physics if necessary. The clinical training covers the material required by CAMPEP. In addition, training in protons, CyberKnife, MR-linac, and at network locations is included. The clinical training and academic record of the residents is outstanding. All graduates have found employment within clinical medical physics, mostly at large academic centers and graduates had a 100% pass rate at the oral American Board of Radiology exams. On average, three manuscripts per resident are published during residency, and multiple abstracts are presented at conferences. CONCLUSIONS: A multi-institutional medical physics residency program can be successfully formed and managed. With a collaborative administrative structure, the program creates an environment for high-quality clinical training of the residents and high productivity in research. The main advantage of such program is access to a wide variety of resources. The main challenge is creating a structure for efficient management of multiple resources at different locations. This report may provide valuable information to centers considering starting a multi-institutional residency program.

Improving Management and Compliance of Radiation Therapy Linear Accelerator Quality Assurance Program With Automated Tracking Tools.

Purpose: Although quality assurance (QA) is crucial in radiation oncology departments, substantial efforts are required to monitor and ensure compliance to high standards. This work aims to analyze the impact of implementing a centralized and automated tracking dashboard on compliance and variance observed in radiation therapy QA results for linear accelerators. Methods and Materials: The study was performed in a large academic center with 7 linear accelerators. An in-house QA Dashboard was implemented in 2019 with design specifications including automated monitoring and visualization of QA progress, ease of use and accessibility, ease of integration, adaptability to new technologies, and facilitation of automated reminders. Monthly QA data were collected between 2016 and 2022 to analyze compliance pre- and post-dashboard implementation. Compliance was characterized as the percentage of tests completed on time. In addition, variance trends for linear accelerator dosimetry measurements and imaging were analyzed over 7 years. Results: In total, 76,066 records were analyzed. Of these records, 73,187 QA measurements were completed on time. Overall QA compliance increased from 75% in 2016 to >99% in 2019 after successful implementation of the QA Dashboard. The main improvement was observed for tests that were implemented more recently, eg, imaging QA and formal recording of daily QA review by physicists. The coefficient of variation was reduced by approximately a factor of 2 for imaging QA after the implementation of the QA Dashboard. For recorded dosimetry measurements, no substantial change in variance was observed. Conclusions: Implementation of the QA Dashboard resulted in a distinct increase in QA compliance. Reduction in the variance in QA measurements was observed for all imaging modalities. These findings demonstrate high impact of automated tracking QA tools on improved compliance and accuracy of QA.

Impact of Prepectoral vs. Subpectoral Tissue Expander Placement on Post-mastectomy Radiation Therapy Delivery: A Retrospective Cohort Study.

Background: Implant-based reconstruction is the most common method of postmastectomy reconstruction. Many patients require postmastectomy radiation (PMRT). Tissue expanders (TEs), typically inserted as a first stage, have historically been placed subpectorally. More recently, prepectoral reconstruction has gained popularity, but its impact on PMRT is unknown. Prior studies focus on complication rates and aesthetic outcomes. This study examines whether there is a difference in radiation dosimetry among patients undergoing prepectoral versus subpectoral TE reconstruction. Methods: Electronic medical records and radiation plans of 50 patients (25 prepectoral, 25 subpectoral) who underwent mastectomy with immediate TE reconstruction at our institution or affiliate site were reviewed. Pectoralis major muscle and chest wall structures were contoured and mean percentage volumes of these structures receiving less than 95%, 100%, and more than 105% target radiation dose were calculated, as were heart and ipsilateral lung doses. Welch two sample t test, Fisher exact test, and Pearson chi-squared tests were performed. Results: The groups had comparable patient and tumor characteristics and underwent similar ablative and reconstructive procedures and radiation dosimetry. Subpectoral patients had larger mean areas receiving less than 95% target dose ("cold spots"); prepectoral patients had larger mean areas receiving greater than 105% ("hot spots") and 100% target doses. There were no differences in chest wall, heart, and lung doses. Conclusions: Our results demonstrate an increased mean percentage area of pectoralis cold spots with subpectoral reconstruction and increased area of hot spots and 100% dose delivery to the pectoralis in prepectoral patients. Larger studies should analyze long-term effects of prepectoral reconstruction on radiation dosing and recurrence rates.

Optimal parameters for clinical implementation of breast cancer patient setup using Varian DTS software.

Digital tomosynthesis (DTS) was evaluated as an alternative to cone-beam computed tomography (CBCT) for patient setup. DTS is preferable when there are constraints with setup time, gantry-couch clearance, and imaging dose using CBCT. This study characterizes DTS data acquisition and registration parameters for the setup of breast cancer patients using nonclinical Varian DTS software. DTS images were reconstructed from CBCT projections acquired on phantoms and patients with surgical clips in the target volume. A shift-and-add algorithm was used for DTS volume reconstructions, while automated cross-correlation matches were performed within Varian DTS software. Triangulation on two short DTS arcs separated by various angular spread was done to improve 3D registration accuracy. Software performance was evaluated on two phantoms and ten breast cancer patients using the registration result as an accuracy measure; investigated parameters included arc lengths, arc orientations, angular separation between two arcs, reconstruction slice spacing, and number of arcs. The shifts determined from DTS-to-CT registration were compared to the shifts based on CBCT-to-CT registration. The difference between these shifts was used to evaluate the software accuracy. After findings were quantified, optimal parameters for the clinical use of DTS technique were determined. It was determined that at least two arcs were necessary for accurate 3D registration for patient setup. Registration accuracy of 2 mm was achieved when the reconstruction arc length was > 5° for clips with HU ≥ 1000; larger arc length (≥ 8°) was required for very low HU clips. An optimal arc separation was found to be ≥ 20° and optimal arc length was 10°. Registration accuracy did not depend on DTS slice spacing. DTS image reconstruction took 10-30 seconds and registration took less than 20 seconds. The performance of Varian DTS software was found suitable for the accurate setup of breast cancer patients. Optimal data acquisition and registration parameters were determined.

Less Time Is Less Motion: Analysis of Practical Efficiencies Gained With a Modified Workflow Integrating Planar kV Midimaging With CBCT for Spine Stereotactic Body Radiation Therapy.

Purpose: Our purpose was to optimize an image guided radiation therapy (IGRT) workflow to achieve practical setup accuracy in spine stereotactic body radiation therapy (SBRT). We assessed the time-saving efficiencies gained from incorporating planar kV midimaging as a surrogate for cone beam computed tomography (CBCT) for intrafraction motion monitoring. Methods and Materials: We selected 5 thoracic spine SBRT patients treated in 5 fractions and analyzed patient shifts captured by a modified IGRT workflow using planar kV midimaging integrated with CBCT to maintain a tolerance of 1 mm and 1°. We determined the frequency at which kV midimaging captured intrafraction motion as validated on repeat CBCT and assessed the potential time and dosimetric advantages of our modified IGRT workflow. Results: Patient motion, detected as out-of-tolerance shifts on planar kV midimaging, occurred during 6 of 25 fractions (24%) and were validated on repeat CBCT 100% of the time. Observed intrafraction absolute shifts (mean ± standard deviation) for the 25 fractions were 0.39 ± 0.21, 0.56 ± 0.22, and 0.45 ± 0.21 mm for lateral-longitude-vertical translations and 0.38 ± 0.12°, 0.32 ± 0.09°, and 0.47 ± 0.14° for pitch-roll-yaw rotation, which if uncorrected, could have significantly affected target coverage and increased spinal cord dose. The average times for pretreatment imaging, midtreatment verification, and total treatment time were 8.94, 2.81, and 16.21 minutes. Our modified IGRT workflow reduced the total number of CBCTs required from 120 to 35 (70%) and imaging dose from 126.2 to 43.4 cGy (65.6%) while maintaining high fidelity for our patient population. Conclusions: Accurate patient positioning was effectively achieved with use of multiple 2-dimensional-3-dimensional kV images and an average of 1 verification CBCT scan per fraction. Integration of planar kV midimaging can effectively reduce treatment time associated with spine SBRT delivery and minimize the potential dosimetric effect of intrafraction motion on target coverage and spinal cord dose.

Evaluation of MatriXX for IMRT and VMAT dose verifications in peripheral dose regions.

PURPOSE: MatriXX is a two-dimensional ion chamber array designed for IMRT/VMAT (RapidArc, IMAT, etc.) dose verifications. Its dosimetric properties have been characterized for megavoltage beams in a number of studies; however, to the best of the authors' knowledge, there is still a lack of an investigation into its performance in the peripheral or low dose regions. In this work, the authors have carried out a systematic study on this issue. METHODS: The authors compare the performance of MatriXX with a cylindrical ion chamber in solid water phantoms in the peripheral dose regions. The comparisons are performed for a number of typical irradiation conditions that involve different gantry and/or MLC motions, field sizes, and distances to the target including static gantry/open fields, static gantry/sweeping MLC gap (mimicking an IMRT delivery), dynamic gantry/oscillating sweeping MLC gap (mimicking a VMAT delivery), as well as clinical IMRT and VMAT plans. RESULTS: MatriXX, when used according to the manufacturer's recommendations, is found to disagree with an ion chamber in peripheral dose regions. This disagreement has been attributed to four types of MatriXX errors, namely, positive bias, over-response to scattered doses, round-off error, and angular dependence, all of which contribute to dose inaccuracies in the peripheral regions. The positive bias, which is independent of the dose level, is cumulative when MatriXX operates in the movie mode. The accumulation is proportional to the number of movie frames (snaps) when the sampling time is greater than 500 ms and is proportional to the overall movie time for a sampling time shorter than 500 ms. This behavior suggests multiple sources of the bias. MatriXX is also found to over-respond to peripheral doses by about 2.0% for the regions investigated in this work (3-15 cm from the field edge), where phantom scatter and collimator scatter dominate. Round-off error is determined to be due to insufficient precision in conversion of the raw signals to MatriXX software data for low doses. Angular dependence is defined as the dose response of MatriXX at different gantry angles. Up to 8% difference in detector response has been observed between 0 degree and 180 degrees. Possible sources of these errors are discussed and a correction method is suggested. With corrections, MatriXX shows good agreement with the ion chamber in all cases involving different gantry and/or MLC dynamics, as well as the clinical plans. For both primary and peripheral doses, MatriXX shows dose linearity down to 2 cGy with an accuracy of within 1% of the local dose. CONCLUSIONS: The performance of MatriXX has been systematically evaluated in the peripheral dose regions. Major sources of error associated with MatriXX are identified and a correction method is suggested. This method has been successfully tested using both experimental and clinical plans. In all cases, good agreements between MatriXX and an ion chamber are achieved after corrections. The authors conclude that with proper corrections, MatriXX can be reliably used for peripheral dose measurements within the ranges studied.

Clinical application of varian OBI CBCT system and dose reduction techniques in breast cancer patients setup.

PURPOSE: To characterize the standard modes of Varian on board imaging (OBI) v1.4 system and identify techniques to further optimize imaging parameters, in particular, for breast treatment setup. METHODS: A male anthropomorphic torso phantom was used for image quality assessment and a simpler thorax phantom for dose measurements. Both phantoms had artificial breasts attached. Doses were measured with an ion chamber in seven locations in the thorax and the breast. Evaluation of image quality was performed in terms of contrast-to-noise ratio (CNR) and in combination with the dose-to-contrast-to-noise (CNRD) parameter. The effect of kVp and mAs on the image quality, dose, and CNRD parameter was analyzed. In addition, image geometry with noncentral isocenter location with start and stop imaging angles adjusted for greater sparing of the contralateral breast was evaluated in terms of image quality and dose. RESULTS: The measurements showed doses between 0.02 and 1.6 cGy for the three full-fan modes and 0.6-3.2 cGy for half-fan modes. This is a reduction of over 80% and 30%-50% compared to OBI v.1.3 modes for full-fan and half-fan modes, respectively. The CNRD is the highest for both low dose modes (low dose thorax and low dose head). Optimal ranges for an averaged sized thorax are tube voltages not higher than 100 kVp and current-time products between 100 and 400 mAs. For the contralateral breast and lung, a dose less than 0.03 cGy per scan was measured for the optimized image geometry with the noncentral isocenter location. CONCLUSIONS: The OBI v1.4 system allows for imaging with a larger variety of imaging parameters compared to previous OBI v1.3 systems. The largest doses (up to 4 cGy) were measured in a phantom when OBI v1.4 system was used for imaging with half-fan modes. Using full-fan modes resulted in the doses less than 1.6 cGy. Further decrease in dose may be achieved by reducing mAs while preserving acceptable image quality. Organ specific sparing (e.g., contralateral breast) may be achieved by proper selection of the start and stop angles. For thorax imaging, the use of Low Dose Thorax mode is recommended.

Evaluation of radiation dose delivered by cone beam CT and tomosynthesis employed for setup of external breast irradiation.

A systematic set of measurements is reported for evaluation of doses to critical organs resulting from cone-beam CT (CB-CT) and cone-beam tomosynthesis (CB-TS) as applied to breast setup for external beam irradiation. The specific focus of this study was on evaluation of doses from these modalities in a setting of volumetric breast imaging for target localization in radiotherapy treatments with the goal of minimizing radiation to healthy organs. Ion chamber measurements were performed in an anthropomorphic female thorax phantom at the center of each breast and lung and on the phantom surface at one anterior and two lateral locations (seven points total). The measurements were performed for three different isocenters located at the center of the phantom and at offset locations of the right and left breast. The dependence of the dose on angle selection for the CB-TS arc was also studied. For the most typical situation of centrally located CB-CT isocenter the measured doses ranged between 3 and 7 cGy, in good agreement with previous reports. Dose measurements were performed for a range of start/stop angles commonly used for CB-TS and the impact of direct and scatter dose on organs at risk was analyzed. All measured CB-TS doses were considerably lower than CB-CT doses, with greater decrease in dose for the organs outside of the beam (up to 98% decrease in dose). Remarkably, offsetting the isocenter towards the ipsilateral breast resulted on average to additional 46% dose reduction to organs at risk. The lowest doses to the contralateral breast and lung were less than 0.1 cGy when they were measured for the offset isocenter. The biggest reduction in dose was obtained by using CB-TS beams that completely avoid the critical organ. For points inside the CB-TS beam, the dose was reduced in a linear relation with distance from the center of the imaging arc. The data indicate that it is possible to reduce substantially radiation doses to the contralateral organs by proper selection of CB-TS angles and imaging field sizes. Our results provide the first systematic study on CB-TS doses from setup imaging for external breast irradiation and can be a useful resource for estimating anticipated radiation doses as a function of the conditions chosen for imaging breast setup.

Evaluation of clip localization for different kilovoltage imaging modalities as applied to partial breast irradiation setup.

Surgical clip localization and image quality were evaluated for different types of kilovoltage cone beam imaging modalities as applied to partial breast irradiation (PBI) setup. These modalities included (i) clinically available radiographs and cone beam CT (CB-CT) and (ii) various alternative modalities based on partial/sparse/truncated CB-CT. An anthropomorphic torso-breast phantom with surgical clips was used for the imaging studies. The torso phantom had artificial lungs, and the attached breast phantom was a mammographic phantom with realistic shape and tissue inhomogeneities. Three types of clips of variable size were used in two orthogonal orientations to assess their in-/cross-plane characteristics for image-guided setup of the torso-breast phantom in supine position. All studies were performed with the Varian on-board imaging (OBI, Varian) system. CT reconstructions were calculated with the standard Feldkamp-Davis-Kress algorithm. First, the radiographs were studied for a wide range of viewing angles to characterize image quality for various types of body anatomy in the foreground/background of the clips. Next, image reconstruction quality was evaluated for partial/sparse/truncated CB-CT. Since these modalities led to reconstructions with strong artifacts due to insufficient input data, a knowledge-based CT reconstruction method was also tested. In this method, the input data to the reconstruction algorithm were modified by combining complementary data sets selected from the treatment and reference projections. Different partial/sparse/truncated CB-CT scan types were studied depending on the total are angle, angular increment between the consequent views (CT projections), orientation of the arc center with respect to the imaged breast and chest wall, and imaging field size. The central angles of the viewing arcs were either tangential or orthogonal to the chest wall. Several offset positions of the phantom with respect to the reference position were studied. The acquired and reconstructed image data sets were analyzed using home-built software focusing on the ability to localize clips in 3D. Streaking and leakage reconstruction artifacts and spatial distortions of breast surface were analyzed as well. Advantages and disadvantages of each kilovoltage CB imaging modality as applied to partial breast setup evaluation based on clips are presented. Because clips were found to be difficult to recognize in radiographs, 3D reconstructions were preferred. Even though it was possible to localize clips with about +/-1 mm accuracy based on reconstructions for short arcs of 40 degrees and incremental angle up to about 5 degrees, identification of clips in such reconstructions is difficult. Reconstructions obtained for arcs of as low as 80 degrees and incremental angle of as high as 3 degrees were suggested for easier clip identification. For more severely undersampled data, iterative CB-CT reconstruction is recommended to decrease the artifacts.

Conditional random fields for phase-based lung feature tracking with ultra-low-dose x-rays.

PURPOSE: During radiation therapy, a continuous internal tumor monitoring without additional imaging dose is desirable. In this study, a sequential feature-based position estimation with ultra-low-dose (ULD) kV x rays using linear-chain conditional random fields (CRFs) is performed. METHODS: Four-dimensional computed tomography (4D-CTs) of eight patients serve as a-priori information from which ULD projections are simulated using a Monte Carlo method. CRFs are trained with Local Energy-based Shape Histogram features extracted from the ULD images to estimate one out of ten breathing phases from the 4D-CT associated with the tumor position. RESULTS: Compared to a mean accuracy for ±1 breathing phase of 0.867 using a support vector machine (SVM), a mean accuracy of 0.958 results for the CRF with ten incident photons per pixel. This corresponds to a position estimation with a discretization error of 2.4-5.3 mm assuming a linear displacement relation between the breathing phases and a systematic error of 2.0-4.4 mm due to motion underestimation of the 4D-CT. CONCLUSIONS: The tumor position estimation is comparable to state-of-the-art methods despite its low imaging dose. Training CRFs further allows a prediction of the following phase and offers a precise post-treatment evaluation tool when decoding the full image sequence.

Infrared-guided patient setup for lung cancer patients.

PURPOSE: To evaluate the utility of an infrared-guided patient setup (iGPS) system to reduce the uncertainties in the setup of lung cancer patients. METHODS AND MATERIALS: A total of 15 patients were setup for lung irradiation using skin tattoos and lateral leveling marks. Daily electronic portal device images and iGPS marker locations were acquired and retrospectively reviewed. The iGPS-based shifts were compared with the daily electronic portal device image shifts using both the central axis iGPS marker and all five iGPS markers. For shift calculation using the five markers, rotational misalignment was included. The level of agreement between the iGPS and portal imaging to evaluate the setup was evaluated as the frequency of the shift difference in the range of 0-5 mm, 5-10 mm, and >10 mm. RESULTS: Data were obtained for 450 treatment sessions for 15 patients. The difference in the isocenter shifts between the weekly vs. daily images was 0-5 mm in 42%, 5-10 mm in 30%, and >10 mm in 10% of the images. The shifts seen using the iGPS data were 0-5 mm in 81%, 5-10 mm in 14%, and >10 mm in 5%. Using only the central axis iGPS marker, the difference between the iGPS and portal images was <5 mm in 77%, 5-10 mm in 16%, and >10 mm in 7% in the left-right direction and 73%, 18%, and 9% in the superoinferior direction, respectively. When all five iGPS markers were used, the disagreements between the iGPS and portal image shifts >10 mm were reduced from 7% to 2% in the left-right direction and 9% to 3% in the superoinferior direction. Larger reductions were also seen (e.g., a reduction from 50% to 0% in 1 patient). CONCLUSION: The daily iGPS-based shifts correlated well with the daily electronic portal device-based shifts. When patient movement has nonlinear rotational components, a combination of surface markers and portal images might be particularly beneficial to improve the setup for lung cancer patients.

Evaluation of a new IR-guided system for mechanical QA of linear accelerators.

The authors report the development of a new procedure for mechanical quality assurance of linear accelerators using an infrared-guided system. The system consists of an infrared (IR) camera and an IR-reflective marker that can be attached to a gantry, a collimator, or a treatment table. The trace of this marker can be obtained in three dimensions (3D) for a full or partial rotation of the mechanical devices. The software is written to localize rotational axes of the gantry, collimator, and the treatment table based on the marker traces. The separation of these axes characterizes the size of the sphere defining the mechanical isocenter. Additional information on anomalies in gantry movement such as degree of gantry sag and hysteresis can also be obtained. An intrinsic uncertainty of the system to localize rotational axis is 0.35 mm or less. Tests on a linear accelerator demonstrated the ability of this system to detect the separation between rotational axes of less than 1 mm and to confirm orthogonality of the planes of gantry, collimator, and table rotation.

Inter fractional variability of breathing phase definition as determined by fiducial location.

Reconstruction of four-dimensional (4D) imaging typically requires an externally measurable surrogate to represent the real-time relative phase of respiration. A common method is to use a reflective marker on the external surface of the patient which moves with respiration and can be tracked in real time. The location of the marker is often chosen to maximize the observable motion, though this location may not be at the region of interest. We evaluate the importance of infrared (IR) marker placement location on breathing phase definition for the purpose of respiratory gating and 4D computed tomography (CT) image reconstruction. Data were collected for ten patients enrolled on an approved IRB protocol. Real-time position data were collected during CT imaging and daily treatments for two external IR reflective markers: one placed near the xyphoid and another at the approximate location of the treatment isocenter. Motion traces from the markers were compared using cross-correlation coefficient and by estimating the relative respiratory phase, based on either marker, as would be used for 4D-CT reconstruction. Cross-correlation analysis revealed differences in the motion waveform, as well as phase differences, both of which were variable between patients as well as day to day for the same patient. Estimated relative phases from each marker were compared by the percentage amount of time the estimated phase for each marker was different, binned based on increments of 10% of a full cycle. For all collected data combined, the frequency with which breathing phase mismatch led to different bin allocation in steps of 10% was as follows: T0%-10% = 65.1%, T10%-20% = 25.3%, T20%-30% = 7.8%, T30%-40% = 1.5% and T40%-50% = 0.4%. Based on ten images per cycle, this indicates that 4D reconstructions would be influenced, depending on which marker was used, by at least 1 bin 34.9% of the time. This number was noticeably higher for some patients; the maximum was 71% of the time for one patient of ten. In conclusion, the respiratory amplitude and relative phase depend significantly on the location of the IR marker used to monitor respiration. For some patients the xyphoid and isocentric markers may be completely out of phase. More importantly, this relationship varies day to day, suggesting that a single marker may be inadequate for the purposes of respiratory gating.

Feasibility of using single photon counting X-ray for lung tumor position estimation based on 4D-CT.

PURPOSE: In stereotactic body radiation therapy of lung tumors, reliable position estimation of the tumor is necessary in order to minimize normal tissue complication rate. While kV X-ray imaging is frequently used, continuous application during radiotherapy sessions is often not possible due to concerns about the additional dose. Thus, ultra low-dose (ULD) kV X-ray imaging based on a single photon counting detector is suggested. This paper addresses the lower limit of photons to locate the tumor reliably with an accuracy in the range of state-of-the-art methods, i.e. a few millimeters. METHOD: 18 patient cases with four dimensional CT (4D-CT), which serves as a-priori information, are included in the study. ULD cone beam projections are simulated from the 4D-CTs including Poisson noise. The projections from the breathing phases which correspond to different tumor positions are compared to the ULD projection by means of Poisson log-likelihood (PML) and correlation coefficient (CC), and template matching under these metrics. RESULTS: The results indicate that in full thorax imaging five photons per pixel suffice for a standard deviation in tumor positions of less than half a breathing phase. Around 50 photons per pixel are needed to achieve this accuracy with the field of view restricted to the tumor region. Compared to CC, PML tends to perform better for low photon counts and shifts in patient setup. Template matching only improves the position estimation in high photon counts. The quality of the reconstruction is independent of the projection angle. CONCLUSIONS: The accuracy of the proposed ULD single photon counting system is in the range of a few millimeters and therefore comparable to state-of-the-art tumor tracking methods. At the same time, a reduction in photons per pixel by three to four orders of magnitude relative to commercial systems with flatpanel detectors can be achieved. This enables continuous kV image-based position estimation during all fractions since the additional dose to the patient is negligible.

Performance and characteristics of an IR localizing system for radiation therapy.

We report the development of a new system for interactive patient posture, position and respiratory control during radiation therapy treatment. The system consists of an infrared camera, retro-reflective markers and dedicated software that makes it practical to use in the clinic. The system is designed to be used with multiple retro-reflective markers to monitor not only position, but also the posture of the patient in real time. Specific features of the system include: 1. The system reports an absolute misalignment at several points on a patients, and also provides feedbacks on any necessary adjustments in terms of site specific set-up parameters, such as focus to surface distance (PIN), superior and inferior alignment, chest-wall angle, etc. 2. The system is based on the set of predefined templates containing number and position of control points and feedback parameters developed for different treatment sites. 3. A "virtual portal vision" procedure is developed to project organ contours in the beams-eye-view (BEV) based on the marker locations obtained in real time and compare them with digitally reconstructed radiographs (DRRs) from CT simulation. Assuming good correlation between external markers and internal anatomy, the system offers the possibility of mimicking a verification procedure without taking port-films, which can potentially reduce the setup time. In this paper, we concentrate on system properties and performance, while initial applications on a number of clinical sites is ongoing. Accuracy and precision of this system are evaluated in the context of breast/chest treatments using rigid phantoms. The system has an intrinsic uncertainty of +/- 1 mm; and when two systems in different rooms (CT and treatment room) are used for correlating positional information, the uncertainty is less than 2 mm.

Prospective assessment of deep inspiration breath-hold using 3-dimensional surface tracking for irradiation of left-sided breast cancer.

PURPOSE: Deep inspiration breath hold (DIBH) is used to decrease cardiac irradiation during radiation therapy (RT) for breast cancer. The patients most likely to benefit and the impact on treatment time remain largely unknown. We sought to identify predictors for the use of DIBH and to quantify differences in dosimetry and treatment time using a prospective registry. METHODS AND MATERIALS: A total of 150 patients with left breast cancer were enrolled. All patients were simulated with both free breathing (FB) and DIBH. RT was delivered by either modality. Alternate scans were planned with use of deformable registration to include identical RT volumes. DIBH patients were monitored by a real-time surface tracking system, AlignRT (Vision RT, Ltd, London, United Kingdom). Baseline characteristics and treatment times were compared by Fisher exact test and Wilcoxon rank sum test. Dosimetric endpoints were analyzed by Wilcoxon signed rank test, and linear regression identified predictors for change in mean heart dose (∆MHD). RESULTS: We treated 38 patients with FB and 110 with DIBH. FB patients were older, more likely to have heart and lung disease, and less likely to receive chemotherapy or immediate reconstruction (all P < .05). Treatment times were not significantly different, but DIBH patients had greater variability in times (P = .0002). Of 146 evaluable patients, DIBH resulted in >20 cGy improvement in MHD in 107 patients but a >20 cGy increase in MHD in 14. Both MHD and lung V20 were significantly lower in DIBH than in paired FB plans. On multivariate analysis, younger age (4.18 cGy per year; P < .0001), higher body mass index (6.06 cGy/kg/m(2); P = .0018), and greater change in lung volumes (130 cGy/L; P = .003) were associated with greater ∆MHD. CONCLUSIONS: DIBH improves cardiac dosimetry without significantly impacting treatment time in most patients. Greater inspiratory lung volumes augment this benefit. Because the improvement with DIBH was not uniform, patients should be scanned with both FB and DIBH.

Automation of clip localization in Digital Tomosynthesis for setup of breast cancer patients.

The objective of this study is to develop an automatic clip localization procedure for breast cancer patient setup based on Digital Tomosynthesis (DTS) and to characterize its performance with respect to the overall registration accuracy and robustness. The study was performed under an IRB-approved protocol for 12 breast cancer patients with surgical clips implanted around the tumor cavity. The registration of DTS images to planning CTs was performed using an automatic algorithm developed to overcome specific challenges of localization and registration of clips in the breast setup images. The automatic method consisted of auto-segmentation (intensity-based thresholding with a priori knowledge about clip size and location to distinguish clips from bony features) and auto-registration of the segmented clip clusters. To determine the inherent accuracy and robustness of the registration algorithm, additional simulated DTS data was analyzed. The developed algorithm is efficient in removing false positives and negatives and provides an accuracy of better than 2.3mm for 60° and 3.3mm for 40° DTS. When incorporated in clinical software, this algorithm helps to facilitate fast and accurate setup evaluation with minimal dose delivered to patients.

Localization of a portion of an endorectal balloon for prostate image-guided radiation therapy using cone-beam tomosynthesis: a feasibility study.

PURPOSE: To assess the feasibility of using cone-beam tomosynthesis (CBTS) to localize the air-tissue interface for the application of prostate image-guided radiation therapy using an endorectal balloon for immobilization and localization. METHODS AND MATERIALS: A Feldkamp-David-Kress-based CBTS reconstruction was applied to selected sets of cone-beam computed tomography (CBCT) projection data to simulate volumetric imaging achievable from tomosynthesis for a limited range of scan angles. Projection data were calculated from planning CT images of 10 prostate cancer patients treated with an endorectal balloon, as were experimental CBCT projections for a pelvic phantom in two patients. More than 50 points at the air-tissue interface were objectively identified by an intensity-based interface-finding algorithm. Using three-dimensional point sets extracted from CBTS images compared with points extracted from corresponding CBCT images, the relative shift resulting from a reduced scan angle was determined. Because the CBCT and CBTS images were generated from the same projection data set, shift identified was presumed to be due to distortions introduced by the tomosynthesis technique. RESULTS: Scans of ≥60° were shown to be able to localize an air-tissue interface near the isocenter with accuracy on the order of a millimeter. The accuracy was quantified in terms of the mean discrepancy as a function of reconstruction angle. CONCLUSION: This work provides an understanding of the effect of scan angle used in localization of a portion of an endorectal balloon by means of CBTS. CBTS with relatively small scan angles is capable of accurately localizing an extended interface near the isocenter and may provide clinically relevant measurements to guide IGRT treatments while reducing imaging radiation to the patient.

A phenomenological kV beam model for cone-beam imaging.

A phenomenological kV beam model was developed to address attenuation and scatter in radiographic images for the purpose of cone-beam imaging. Characterization of a kV beam in terms of the minimal number of parameters and calculation of attenuation and scatter in radiographs of scanned objects are the main applications of this model. Model parameters are derived from radiographs of homogeneous solid water phantoms for various depths and field sizes. The response of the cone-beam detector to kV beams is factorized into different contributions such as output factor, tissue-air ratio and off-axis ratio, with each contribution having an analytical representation. The formulas which are used to characterize the beam model in uniform phantoms are then extended to arbitrary objects using the concept of the water-equivalent pathlength. A weighted sum of three Gaussians in each direction models the dose deposition kernel. Detector response arising from the first Gaussian term can be interpreted as the primary signal while the second and third Gaussians constitute short- and long-range scatter. The model is then applied to predict the primary and scatter signals for arbitrary objects. A technique of scatter removal from the measured radiographs is investigated. The model accurately predicts detector response of varying-thickness phantoms such as multi-step and cylindrical phantoms. The scatter contributes over 90% to the total signal for 20 cm thick phantoms. The calculated scatter-to-primary ratio as a function of spatial coordinates agrees with Monte Carlo studies reported in the literature. Water-equivalent thickness related to primary and scatter contributions calculated from an analysis of radiographs results in an improved calibration technique suitable for CB-CT reconstruction. The kV beam model and the associated theoretical formulations can be utilized to characterize any kV beam line; however, for the specific study the OBI system (Varian) was used to obtain experimental radiographs.

Optimal gantry angles and field sizes in kilovoltage cone-beam tomosynthesis for set-up of women with breast cancer undergoing radiotherapy treatment.

Optimal beam geometry for kilovoltage cone-beam tomosynthesis (CB-TS) imaging for the set-up of 20 patients with breast cancer was analyzed and presented for prone and supine scenarios. Gantry angles and field sizes that allow maximum clearance and minimum dose to normal organs from kilovoltage imaging were determined for medial and lateral arcs. Optimal CB-TS arc span may vary depending on patient geometry, particularly on the separation between the cavity and the chest wall. CB-TS arc angles 40 degrees were found to be optimal for the cavity to chest wall separation d>3 cm for supine set-up and d>7 cm for prone set-up. Maximum required jaw openings were 20 x 14 cm(2). Recommendations are provided for clinical implementation of CB-TS imaging for breast cancer patients.