Dosimetric evaluation of irradiation geometry and potential air gaps in an acrylic miniphantom used for external audit of absolute dose calibration for a hybrid 1.5 T MR-linac system.

INTRODUCTION: To investigate the impact of partial lateral scatter (LS), backscatter (BS) and presence of air gaps on optically stimulated luminescence dosimeter (OSLD) measurements in an acrylic miniphantom used for dosimetry audit on the 1.5 T magnetic resonance-linear accelerator (MR-linac) system. METHODS: The following irradiation geometries were investigated using OSLDs, A26 MR/A12 MR ion chamber (IC), and Monaco Monte Carlo system: (a) IC/OSLD in an acrylic miniphantom (partial LS, partial BS), (b) IC/OSLD in a miniphantom placed on a solid water (SW) stack at a depth of 1.5 cm (partial LS, full BS), (c) IC/OSLD placed at a depth of 1.5 cm inside a 3 cm slab of SW/buildup (full LS, partial BS), and (d) IC/OSLD centered inside a 3 cm slab of SW/buildup at a depth of 1.5 cm placed on top of a SW stack (full LS, full BS). Average of two irradiated OSLDs with and without water was used at each setup. An air gap of 1 and 2 mm, mimicking presence of potential air gap around the OSLDs in the miniphantom geometry was also simulated. The calibration condition of the machine was 1 cGy/MU at SAD = 143.5 cm, d = 5 cm, G90, and 10 × 10 cm2 . RESULTS: The Monaco calculation (0.5% uncertainty and 1.0 mm voxel size) for the four setups at the measurement point were 108.2, 108.1, 109.4, and 110.0 cGy. The corresponding IC measurements were 109.0 ± 0.03, 109.5 ± 0.06, 110.2 ± 0.02, and 109.8 ± 0.03 cGy. Without water, OSLDs measurements were ∼10% higher than the expected. With added water to minimize air gaps, the measurements were significantly improved to within 2.2%. The dosimetric impacts of 1 and 2 mm air gaps were also verified with Monaco to be 13.3% and 27.9% higher, respectively, due to the electron return effect. CONCLUSIONS: A minimal amount of air around or within the OSLDs can cause measurement discrepancies of 10% or higher when placed in a high b-field MR-linac system. Care must be taken to eliminate the air from within and around the OSLD.

A phantom study to evaluate three different registration platform of 3D/3D, 2D/3D, and 3D surface match with 6D alignment for precise image-guided radiotherapy.

PURPOSE: To evaluate two three-dimensional (3D)/3D registration platforms, one two-dimensional (2D)/3D registration method, and one 3D surface registration method (3DS). These three technologies are available to perform six-dimensional (6D) registrations for image-guided radiotherapy treatment. METHODS: Fiducial markers were asymmetrically placed on the surfaces of an anthropomorphic head phantom (n = 13) and a body phantom (n = 8), respectively. The point match (PM) solution to the six-dimensional (6D) transformation between the two image sets [planning computed tomography (CT) and cone beam CT (CBCT)] was determined through least-square fitting of the fiducial positions using singular value decomposition (SVD). The transformation result from SVD was verified and was used as the gold standard to evaluate the 6D accuracy of 3D/3D registration in Varian's platform (3D3DV), 3D/3D and 2D/3D registration in the BrainLab ExacTrac system (3D3DE and 2D3D), as well as 3DS in the AlignRT system. Image registration accuracy from each method was quantitatively evaluated by root mean square of target registration error (rmsTRE) on fiducial markers and by isocenter registration error (IRE). The Wilcoxon signed-rank test was utilized to compare the difference of each registration method with PM. A P < 0.05 was considered significant. RESULTS: rmsTRE was in the range of 0.4 mm/0.7 mm (cranial/body), 0.5 mm/1 mm, 1.0 mm/1.5 mm, and 1.0 mm/1.2 mm for PM, 3D3D, 2D3D, and 3DS, respectively. Comparing to PM, the mean errors of IRE were 0.3 mm/1 mm for 3D3D, 0.5 mm/1.4 mm for 2D3D, and 1.6 mm/1.35 mm for 3DS for the cranial and body phantoms respectively. Both of 3D3D and 2D3D methods differed significantly in the roll direction as compared to the PM method for the cranial phantom. The 3DS method was significantly different from the PM method in all three translation dimensions for both the cranial (P = 0.003-P = 0.03) and body (P < 0.001-P = 0.008) phantoms. CONCLUSION: 3D3D using CBCT had the best image registration accuracy among all the tested methods. 2D3D method was slightly inferior to the 3D3D method but was still acceptable as a treatment position verification device. 3DS is comparable to 2D3D technique and could be a substitute for X-ray or CBCT for pretreatment verification for treatment of anatomical sites that are rigid.

Dosimetric evaluation of an atlas-based synthetic CT generation approach for MR-only radiotherapy of pelvis anatomy.

PURPOSE: To investigate the potential of an atlas-based approach in generation of synthetic CT for pelvis anatomy. METHODS: Twenty-three matched pairs of computed tomography (CT) and magnetic resonance imaging (MRI) scans were selected from a pool of prostate cancer patients. All MR scans were preprocessed to reduce scanner- and patient-induced intensity inhomogeneities and to standardize their intensity histograms. Ten (training dataset) of 23 pairs were then utilized to construct the coregistered CT-MR atlas. The synthetic CT for a new patient is generated by appropriately weighting the deformed atlas of CT-MR onto the new patient MRI. The training dataset was used as an atlas to generate the synthetic CT for the rest of the patients (test dataset). The mean absolute error (MAE) between the deformed planning CT and synthetic CT was computed over the entire CT image, bone, fat, and muscle tissues. The original treatment plans were also recomputed on the new synthetic CTs and dose-volume histogram metrics were compared. The results were compared with a commercially available synthetic CT Software (MRCAT) that is routinely used in our clinic. RESULTS: MAE errors (±SD) between the deformed planning CT and our proposed synthetic CTs in the test dataset were 47 ± 5, 116 ± 12, 36 ± 6, and 47 ± 5 HU for the entire image, bone, fat, and muscle tissues respectively. The MAEs were 65 ± 5, 172 ± 9, 43 ± 7, and 42 ± 4 HU for the corresponding tissues in MRCAT CT. The dosimetric comparison showed consistent results for all plans using our synthetic CT, deformed planning CT and MRCAT CT. CONCLUSION: We investigated the potential of a multiatlas approach to generate synthetic CT images for the pelvis. Our results demonstrate excellent results in terms of HU value assignment compared to the original CT and dosimetric consistency.

Dual-input tracer kinetic modeling of dynamic contrast-enhanced MRI in thoracic malignancies.

Pulmonary perfusion with dynamic contrast-enhanced (DCE-) MRI is typically assessed using a single-input tracer kinetic model. Preliminary studies based on perfusion CT are indicating that dual-input perfusion modeling of lung tumors may be clinically valuable as lung tumors have a dual blood supply from the pulmonary and aortic system. This study aimed to investigate the feasibility of fitting dual-input tracer kinetic models to DCE-MRI datasets of thoracic malignancies, including malignant pleural mesothelioma (MPM) and nonsmall cell lung cancer (NSCLC), by comparing them to single-input (pulmonary or systemic arterial input) tracer kinetic models for the voxel-level analysis within the tumor with respect to goodness-of-fit statistics. Fifteen patients (five MPM, ten NSCLC) underwent DCE-MRI prior to radiotherapy. DCE-MRI data were analyzed using five different single- or dual-input tracer kinetic models: Tofts-Kety (TK), extended TK (ETK), two compartment exchange (2CX), adiabatic approximation to the tissue homogeneity (AATH) and distributed parameter (DP) models. The pulmonary blood flow (BF), blood volume (BV), mean transit time (MTT), permeability-surface area product (PS), fractional interstitial volume (vI ), and volume transfer constant (KTrans ) were calculated for both single- and dual-input models. The pulmonary arterial flow fraction (γ), pulmonary arterial blood flow (BFPA ) and systemic arterial blood flow (BFA ) were additionally calculated for only dual-input models. The competing models were ranked and their Akaike weights were calculated for each voxel according to corrected Akaike information criterion (cAIC). The optimal model was chosen based on the lowest cAIC value. In both types of tumors, all five dual-input models yielded lower cAIC values than their corresponding single-input models. The 2CX model was the best-fitted model and most optimal in describing tracer kinetic behavior to assess microvascular properties in both MPM and NSCLC. The dual-input 2CX-model-derived BFA was the most significant parameter in differentiating adenocarcinoma from squamous cell carcinoma histology for NSCLC patients.

Simultaneous MV-kV imaging for intrafractional motion management during volumetric-modulated arc therapy delivery.

The purpose of this study was to evaluate the accuracy and clinical feasibility of a motion monitoring method employing simultaneously acquired MV and kV images during volumetric-modulated arc therapy (VMAT). Short-arc digital tomosynthesis (SA-DTS) is used to improve the quality of the MV images that are then combined with orthogonally acquired kV images to assess 3D motion. An anthropomorphic phantom with implanted gold seeds was used to assess accuracy of the method under static, typical prostatic, and respiratory motion scenarios. Automatic registra-tion of kV images and single MV frames or MV SA-DTS reconstructed with arc lengths from 2° to 7° with the appropriate reference fiducial template images was performed using special purpose-built software. Clinical feasibility was evaluated by retrospectively analyzing images acquired over four or five sessions for each of three patients undergoing hypofractionated prostate radiotherapy. The standard deviation of the registration error in phantom using MV SA-DTS was similar to single MV images for the static and prostate motion scenarios (σ = 0.25 mm). Under respiratory motion conditions, the standard deviation of the registration error increased to 0.7mm and 1.7 mm for single MV and MV SA-DTS, respectively. Registration failures were observed with the respiratory scenario only and were due to motion-induced fiducial blurring. For the three patients studied, the mean and standard deviation of the difference between automatic registration using 4° MV SA-DTS and manual registration using single MV images results was 0.07±0.52mm. The MV SA-DTS results in patients were, on average, superior to single-frame MV by nearly 1 mm - significantly more than what was observed in phantom. The best MV SA-DTS results were observed with arc lengths of 3° to 4°. Registration failures in patients using MV SA-DTS were primarily due to blockage of the gold seeds by the MLC. The failure rate varied from 2% to 16%. Combined MV SA-DTS and kV imaging is feasible for intratreatment motion monitoring during VMAT of anatomic sites where limited motion is expected, and improves registration accuracy compared to single MV/kV frames. To create a clinically robust technique, further improvements to ensure visualization of fiducials at the desired control points without degradation of the treatment plan are needed.

Automated and Clinically Optimal Treatment Planning for Cancer Radiotherapy.

Each year, approximately 18 million new cancer cases are diagnosed worldwide, and about half must be treated with radiotherapy. A successful treatment requires treatment planning with the customization of penetrating radiation beams to sterilize cancerous cells without harming nearby normal organs and tissues. This process currently involves extensive manual tuning of parameters by an expert planner, making it a time-consuming and labor-intensive process, with quality and immediacy of critical care dependent on the planner's expertise. To improve the speed, quality, and availability of this highly specialized care, Memorial Sloan Kettering Cancer Center developed and applied advanced optimization tools to this problem (e.g., using hierarchical constrained optimization, convex approximations, and Lagrangian methods). This resulted in both a greatly improved radiotherapy treatment planning process and the generation of reliable and consistent high-quality plans that reflect clinical priorities. These improved techniques have been the foundation of high-quality treatments and have positively impacted over 4,000 patients to date, including numerous patients in severe pain and in urgent need of treatment who might have otherwise required longer hospital stays or undergone unnecessary surgery to control the progression of their disease. We expect that the wide distribution of the system we developed will ultimately impact patient care more broadly, including in resource-constrained countries.

Clinical Experience of Automated SBRT Paraspinal and Other Metastatic Tumor Planning With Constrained Hierarchical Optimization.

PURPOSE: We report on the clinical performance of a fully automated approach to treatment planning based on a Pareto optimal, constrained hierarchical optimization algorithm, named Expedited Constrained Hierarchical Optimization (ECHO). METHODS AND MATERIALS: From April 2017 to October 2018, ECHO produced 640 treated plans for 523 patients who underwent stereotactic body radiation therapy (RT) for paraspinal and other metastatic tumors. A total of 182 plans were for 24 Gy in a single fraction, 387 plans were for 27 Gy in 3 fractions, and the remainder were for other prescriptions or fractionations. Of the plans, 84.5% were for paraspinal tumors, with 69, 302, and 170 in the cervical, thoracic, and lumbosacral spine, respectively. For each case, after contouring, a template plan using 9 intensity modulated RT fields based on disease site and tumor location was sent to ECHO through an application program interface plug-in from the treatment planning system. ECHO returned a plan that satisfied all critical structure hard constraints with optimal target volume coverage and the lowest achievable normal tissue doses. Upon ECHO completion, the planner received an e-mail indicating the plan was ready for review. The plan was accepted if all clinical criteria were met. Otherwise, a limited number of parameters could be adjusted for another ECHO run. RESULTS: The median planning target volume size was 84.3 cm3 (range, 6.9-633.2). The median time to produce 1 ECHO plan was 63.5 minutes (range, 11-340 minutes) and was largely dependent on the field sizes. Of the cases, 79.7% required 1 run to produce a clinically accepted plan, 13.3% required 1 additional run with minimal parameter adjustments, and 7.0% required ≥2 additional runs with significant parameter modifications. All plans met or bettered the institutional clinical criteria. CONCLUSIONS: We successfully implemented automated stereotactic body RT paraspinal and other metastatic tumors planning. ECHO produced high-quality plans, improved planning efficiency and robustness, and enabled expedited treatment planning at our clinic.

Deep learning-based auto-segmentation of targets and organs-at-risk for magnetic resonance imaging only planning of prostate radiotherapy.

BACKGROUND AND PURPOSE: Magnetic resonance (MR) only radiation therapy for prostate treatment provides superior contrast for defining targets and organs-at-risk (OARs). This study aims to develop a deep learning model to leverage this advantage to automate the contouring process. MATERIALS AND METHODS: Six structures (bladder, rectum, urethra, penile bulb, rectal spacer, prostate and seminal vesicles) were contoured and reviewed by a radiation oncologist on axial T2-weighted MR image sets from 50 patients, which constituted expert delineations. The data was split into a 40/10 training and validation set to train a two-dimensional fully convolutional neural network, DeepLabV3+, using transfer learning. The T2-weighted image sets were pre-processed to 2D false color images to leverage pre-trained (from natural images) convolutional layers' weights. Independent testing was performed on an additional 50 patient's MR scans. Performance comparison was done against a U-Net deep learning method. Algorithms were evaluated using volumetric Dice similarity coefficient (VDSC) and surface Dice similarity coefficient (SDSC). RESULTS: When comparing VDSC, DeepLabV3+ significantly outperformed U-Net for all structures except urethra (P < 0.001). Average VDSC was 0.93 ± 0.04 (bladder), 0.83 ± 0.06 (prostate and seminal vesicles [CTV]), 0.74 ± 0.13 (penile bulb), 0.82 ± 0.05 (rectum), 0.69 ± 0.10 (urethra), and 0.81 ± 0.1 (rectal spacer). Average SDSC was 0.92 ± 0.1 (bladder), 0.85 ± 0.11 (prostate and seminal vesicles [CTV]), 0.80 ± 0.22 (penile bulb), 0.87 ± 0.07 (rectum), 0.85 ± 0.25 (urethra), and 0.83 ± 0.26 (rectal spacer). CONCLUSION: A deep learning-based model produced contours that show promise to streamline an MR-only planning workflow in treating prostate cancer.

Automated intensity modulated treatment planning: The expedited constrained hierarchical optimization (ECHO) system.

PURPOSE: To develop and implement a fully automated approach to intensity modulated radiation therapy (IMRT) treatment planning. METHOD: The optimization algorithm is developed based on a hierarchical constrained optimization technique and is referred internally at our institution as expedited constrained hierarchical optimization (ECHO). Beamlet contributions to regions-of-interest are precomputed and captured in the influence matrix. Planning goals are of two classes: hard constraints that are strictly enforced from the first step (e.g., maximum dose to spinal cord), and desirable goals that are sequentially introduced in three constrained optimization problems (better planning target volume (PTV) coverage, lower organ at risk (OAR) doses, and smoother fluence map). After solving the optimization problems using external commercial optimization engines, the optimal fluence map is imported into an FDA-approved treatment planning system (TPS) for leaf sequencing and accurate full dose calculation. The dose-discrepancy between the optimization and TPS dose calculation is then calculated and incorporated into optimization by a novel dose correction loop technique using Lagrange multipliers. The correction loop incorporates the leaf sequencing and scattering effects into optimization to improve the plan quality and reduce the calculation time. The resultant optimal fluence map is again imported into TPS for leaf sequencing and final dose calculation for plan evaluation and delivery. The workflow is automated using application program interface (API) scripting, requiring user interaction solely to prepare the contours and beam arrangement prior to launching the ECHO plug-in from the TPS. For each site, parameters and objective functions are chosen to represent clinical priorities. The first site chosen for clinical implementation was metastatic paraspinal lesions treated with stereotactic body radiotherapy (SBRT). As a first step, 75 ECHO paraspinal plans were generated retrospectively and compared with clinically treated plans generated by planners using VMAT (volumetric modulated arc therapy) with 4 to 6 partial arcs. Subsequently, clinical deployment began in April, 2017. RESULTS: In retrospective study, ECHO plans were found to be dosimetrically superior with respect to tumor coverage, plan conformity, and OAR sparing. For example, the average PTV D95%, cord and esophagus max doses, and Paddick Conformity Index were improved, respectively, by 1%, 6%, 14%, and 15%, at a negligible 3% cost of the average skin D10cc dose. CONCLUSION: Hierarchical constrained optimization is a powerful and flexible tool for automated IMRT treatment planning. The dosimetric correction step accurately accounts for detailed dosimetric multileaf collimator and scattering effects. The system produces high-quality, Pareto optimal plans and avoids the time-consuming trial-and-error planning process.

Use of a constrained hierarchical optimization dataset enhances knowledge-based planning as a quality assurance tool for prostate bed irradiation.

PURPOSE: To investigate whether building a knowledge-based planning (KBP) model with prostate bed plans constructed from constrained hierarchical optimization (CHO) would result in more efficient model construction with more consistent output than a model built using plans from a traditional, trial-and-error-based optimization (TEO) technique. METHODS: Three KBP models were constructed from plans from subsets of 58 post-prostatectomy patients treated with intensity-modulated radiation therapy. TEO54 was built from 54 TEO plans, selected to represent typical clinical variations in target and organ-at-risk sizes and shapes. CHO30 and TEO30 were built from the same 30 patients populated with CHO and TEO plans, respectively. The three models were each applied to a new set of 18 patient scans and dose-volume histogram estimates (DVHEs) were generated for rectal and bladder walls and compared for each patient. RESULTS: CHO30 resulted in a significantly tighter range in DVHEs (P < 0.01) for both the rectal and bladder walls compared with either of the TEO models, indicating less uncertainty in the dose estimation. Plans resulting from KBP optimization using each model were very similar. CONCLUSION: Populating a KBP model with CHO data resulted in a high quality model. Since CHO plans can be generated automatically offline in a process that necessitates little to no user interaction, a CHO-KBP model can quickly adapt to changes in plan evaluation criteria or planning techniques without the need to wait to accrue sufficient numbers of clinical TEO plans. This may facilitate the use of KBP approaches for initial or ongoing quality assurance procedures and plan quality audits.

Correlation Between Tumor Metabolism and Semiquantitative Perfusion Magnetic Resonance Imaging Metrics in Non-Small Cell Lung Cancer.

PURPOSE: To correlate semiquantitative parameters derived from dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and 18F-fluorodeoxyglucose positron emission tomography (18F-FDG-PET) for non-small cell lung cancer (NSCLC). METHODS AND MATERIALS: Twenty-four NSCLC patients who underwent pretreatment 18F-FDG-PET and DCE-MRI were analyzed. The maximum standardized uptake value (SUVmax) was measured from 18F-FDG-PET. Dynamic contrast-enhanced MRI was obtained on a 3T MRI scanner using 4-dimensional T1-weighted high-resolution imaging with a volume excitation sequence. The DCE-MRI parameters, consisting of mean, median, standard deviation (SD), and median absolute deviation (MAD) of peak enhancement, time to peak (TTP), time to half peak (TTHP), wash-in slope (WIS), wash-out slope (WOS), initial gradient, wash-out gradient, signal enhancement ratio, and initial area under the relative signal enhancement curve taken up to 30, 60, 90, 120, 150, and 180 seconds, TTP, and TTHP (IAUCtthp), were calculated for each lesion. Univariate analysis (UVA) was performed using Spearman correlation. A linear regression model to predict SUVmax from DCE-MRI parameters was developed by multivariate analysis (MVA) using least absolute shrinkage selection operator in combination with leave-one-out cross-validation (LOOCV). RESULTS: In UVA, mean(WOS) (ρ = -0.456, P = .025), mean(IAUCtthp) (ρ = -0.439, P = .032), median(IAUCtthp) (ρ = -0.543, P = .006), and MAD(IAUCtthp) (ρ = -0.557, P = .005) were statistically significant; all these parameters were negatively correlated with SUVmax. In MVA, a linear combination of SD(WIS), SD(TTP), MAD(TTHP), and MAD(IAUCtthp) was statistically significant for predicting SUVmax (LOOCV-based adjusted R2 = 0.298, P = .0006). A decrease in SD(WIS), MAD(TTHP), and MAD(IAUCtthp) and an increase in SD(TTP) were associated with a significant increase in SUVmax. CONCLUSION: An association was found between SUVmax, the SD, and MAD of DCE-MRI metrics derived during contrast uptake in NSCLC, reflecting that intratumoral heterogeneity in wash-in contrast kinetics is associated with tumor metabolism. Although MAD(IAUCtthp) was a significant feature in both UVA and MVA, the LASSO-based multivariate regression model yielded better predictability of SUVmax than a univariate regression model using MAD(IAUCtthp). This study will facilitate understanding of the complex relationship between tumor vascularization and metabolism and eventually help in guiding targeted therapy.

Long-term clinical outcomes of whole-breast irradiation delivered in the prone position.

PURPOSE: The aim of this study was to evaluate retrospectively the effectiveness and toxicity of post-lumpectomy whole-breast radiation therapy delivered with prone positioning. METHODS AND MATERIALS: Between September 1992 and August 2004, 245 women with 248 early-stage invasive or in situ breast cancers were treated using a prone breast board. Photon fields treated the whole breast to 46 to 50.4 Gy with standard fractionation. The target volume was clinically palpable breast tissue; no attempt was made to irradiate chest wall lymphatics. Tumor bed boosts were delivered in 85% of cases. Adjuvant chemotherapy and hormonal therapy were administered to 42% and 62% of patients, respectively. RESULTS: After a median follow-up of 4.9 years, the 5 year actuarial true local and elsewhere ipsilateral breast tumor recurrence rates were 4.8% and 1.3%, respectively. The 5-year actuarial rates of regional nodal recurrence and distant metastases were 1.6% and 7.4%. Actuarial disease-free, disease-specific, and overall survival rates at 5 years were 89.4%, 97.3%, and 93%, respectively. Treatment breaks were required by 2.4% of patients. Grade 3 acute dermatitis and edema were each limited to 2% of patients. Only 4.9% of patients complained of acute chest wall discomfort. Chronic Grade 2 to 3 skin and subcutaneous tissue toxicities were reported in 4.4% and 13.7% of patients, respectively. CONCLUSIONS: Prone position breast radiation results in similar long-term disease control with a favorable toxicity profile compared with standard supine tangents. The anatomic advantages of prone positioning may contribute to improving the therapeutic ratio of post-lumpectomy radiation by improving dose homogeneity and minimizing incidental cardiac and lung dose.

Using an onboard kilovoltage imager to measure setup deviation in intensity-modulated radiation therapy for head-and-neck patients.

The purpose of the present study was to use a kilovoltage imaging device to measure interfractional and intrafractional setup deviations in patients with head-and-neck or brain cancers receiving intensity-modulated radiotherapy (IMRT) treatment. Before and after IMRT treatment, approximately 3 times weekly, 7 patients were imaged using the Varian On-Board Imager (OBI: Varian Medical Systems, Palo Alto, CA), a kilovoltage imaging device permanently mounted on the gantry of a Varian 21EX LINAC (Varian Medical Systems). Because of commissioning of the remote couch correction of the OBI during the study, online setup corrections were performed on 2 patients. For the other 5 patients, weekly corrections were made based on a sliding average of the measured data. From these data, we determined the interfractional setup deviation (defined as the shift from the original setup position suggested by the daily image), the residual error associated with the weekly correction protocol, and the intrafractional setup deviation, defined as the difference between the post-treatment and pretreatment images. We also used our own image registration software to determine interfractional and intrafractional rotational deviations from the images based on the template-matching method. In addition, we evaluated the influence of inter-observer variation on our results, and whether the use of various registration techniques introduced differences. Finally, translational data were compared with rotational data to search for correlations. Translational setup errors from all data were 0.0 +/- 0.2 cm, -0.1 +/- 0.3 cm, and -0.2 +/- 0.3 cm in the right-left (RL), anterior-posterior (AP), and superior-inferior (SI) directions respectively. Residual error for the 5 patients with a weekly correction protocol was -0.1 +/- 0.2 cm (RL), 0.0 +/- 0.3 cm (AP), and 0.0 +/- 0.2 cm (SI). Intrafractional translation errors were small, amounting to 0.0 +/- 0.1 cm, -0.1 +/- 0.2 cm, and 0.0 +/- 0.1 cm in the RL, AP, and SI directions respectively. In the sagittal and coronal views respectively, interfractional rotational errors were -1.1 +/- 1.7 degrees and -0.5 +/- 0.9 degrees, and intrafractional rotational errors were 0.3 +/- 0.6 degrees and 0.2 +/- 0.5 degrees. No significant correlation was seen between translational and rotational data. The OBI image data were used to study setup error in the head-and-neck patients. Nonzero systematic errors were seen in the interfractional translational and rotational data, but not in the intrafractional data, indicating that the mask is better at maintaining head position than at reproducing it.

Radiation therapy of large intact breasts using a beam spoiler or photons with mixed energies.

Radiation treatment of large intact breasts with separations of more than 24 cm is typically performed using x-rays with energies of 10 MV and higher, to eliminate high-dose regions in tissue. The disadvantage of the higher energy beams is the reduced dose to superficial tissue in the buildup region. We evaluated 2 methods of avoiding this underdosage: (1) a beam spoiler: 1.7-cm-thick Lucite plate positioned in the blocking tray 35 cm from the isocenter, with 15-MV x-rays; and (2) combining 6- and 15-MV x-rays through the same portal. For the beam with the spoiler, we measured the dose distribution for normal and oblique incidence using a film and ion chamber in polystyrene, as well as a scanning diode in a water tank. In the mixed-energy approach, we calculated the dose distributions in the buildup region for different proportions of 6- and 15-MV beams. The dose enhancement due to the beam spoiler exhibited significant dependence upon the source-to-skin distance (SSD), field size, and the angle of incidence. In the center of a 20 x 20-cm(2) field at 90-cm SSD, the beam spoiler raises the dose at 5-mm depth from 77% to 87% of the prescription, while maintaining the skin dose below 57%. Comparison of calculated dose with measurements suggested a practical way of treatment planning with the spoiler--usage of 2-mm "beam" bolus--a special option offered by in-house treatment planning system. A second method of increasing buildup doses is to mix 6- and 15-MV beams. For example, in the case of a parallel-opposed irradiation of a 27-cm-thick phantom, dose to D(max) for each energy, with respect to midplane, is 114% for pure 6-, 107% for 15-MV beam with the spoiler, and 108% for a 3:1 mixture of 15- and 6-MV beams. Both methods are practical for radiation therapy of large intact breasts.

Multiatlas approach with local registration goodness weighting for MRI-based electron density mapping of head and neck anatomy.

PURPOSE: The growing use of magnetic resonance imaging (MRI) as a substitute for computed tomography-based treatment planning requires the development of effective algorithms to generate electron density maps for treatment planning and patient setup verification. The purpose of this work was to develop a method to synthesize computerized tomography (CT) for MR-only radiotherapy of head and neck cancer patients. METHODS: The algorithm is based on registration of multiple patient datasets containing both MRI and CT images (a "multiatlas" algorithm). Twelve matched pairs of good quality CT and MRI scans (those without apparent motion and blurring artifacts) were selected from a pool of head and neck cancer patients to form the atlas. All atlas MRI scans were preprocessed to reduce scanner- and patient-induced intensity inhomogeneities and to standardize their intensity histograms. Atlas CT and MRIs were coregistered using a novel bone-to-air replacement technique applied to the CT scans that improves the similarity between CTs and MRIs and facilitates the registration process. For each new patient, all atlas MRIs are deformed initially onto the new patients' MRI. We introduce a generalized registration error (GRE) metric that automatically measures the goodness of local registration between MRI pairs. The final synthetic CT value at each point is a nonlinear GRE-weighted average of the atlas CTs. For evaluation, the leave-one-out technique was used for synthetic CT generation and the mean absolute error (MAE) between the original and synthetic CT was computed over the entire CT image. The impact of our proposed CT-MR registration scheme on the accuracy of the final synthetic CT was also studied. The original treatment plans were also recomputed on the new synthetic CTs and dose-volume histogram metrics were compared. In addition, the two-dimensional (2D) gamma analysis at 1%/1 mm and 2%/2 mm dose difference/distance to agreement was also performed to study the dose distribution at the isocenter. RESULTS: MAE error (± standard deviation) between the original and the synthetic CTs was 64 ± 10, 113 ± 12, and 130 ± 28 Hounsfield Unit (HU) for the entire image, air, and bone regions respectively. Our results showed that our proposed bone-suppression based CT-MR fusion and GRE-weighted strategy could lower the overall MAE error between the original and synthetic CTs by ~69% and ~34% respectively. Dose recalculation comparison showed highly consistent results between plans based on the synthetic vs. the original CTs. The 2D gamma analysis revealed the pass rate of 95.44 ± 2.5 and 99.36 ± 0.71 for 1%/1 mm and 2%/2 mm criteria respectively. Due to local registration weighting, the method is robust with respect to MRI imaging artifacts. CONCLUSION: We developed a novel image analysis technique to synthesize CT for head and neck anatomy. Novel methods were introduced to accurately register atlas CTs and MRIs as well as to weight the final electron density maps using local registration goodness estimates. The resulting accuracy is clinically acceptable, at least for these atlas patients.

Geometric factors influencing dosimetric sparing of the parotid glands using IMRT.

PURPOSE/OBJECTIVE: To determine the relationship between the parotid volume, parotid-planning target volume (PTV) overlap, and dosimetric sparing of the parotid with intensity-modulated radiation therapy (IMRT). METHODS AND MATERIALS: Parotid data were collected retrospectively for 51 patients treated with simultaneous boost IMRT. Unresectable patients received 54 or 59.4 Gy to subclinical disease, 70 Gy to gross disease. Patients treated postoperatively received 54, 60, and 66 Gy to low-risk, high-risk, and tumor bed regions. Volume and mean dose of each gland and gland segments outside of and overlapping the PTV were collected. Proximity of each gland to each PTV was recorded. RESULTS: Dosimetric sparing (mean dose 21% overlap (p = <0.0001). Among spared glands, the median mean dose in the overlap region was 55.0 Gy in glands with < or =21% overlap, but only 45.4 Gy when overlap >21%. Median mean dose was 25.9 Gy to glands overlapping PTV(54) or PTV(59) alone and 30.0 Gy to those abutting PTV(70) (p < 0.001). Although proximity to PTV(70) was associated with higher parotid dose, satisfactory sparing was achieved in 24 of 43 ipsilateral glands. CONCLUSIONS: Dosimetric sparing of the parotid is feasible when the parotid-PTV overlap is less than approximately 20%. With more overlap, sparing may result in low doses within the overlap region, possibly leading to inadequate PTV coverage. Gland proximity to the high-dose PTV is associated with higher mean dose but does not always preclude dosimetric sparing.

Intensity-modulated radiotherapy as the boost or salvage treatment of nasopharyngeal carcinoma: the appropriate parameters in the inverse planning and the effect of patient's anatomic factors on the planning results.

The current study demonstrates that the large increase in normal tissue penalty often degrades target dose uniformity without a concomitant large improvement in normal tissue dose, especially in anatomically unfavorable patients. The excessively large normal tissue penalties do not improve treatment plans for patients having unfavorable geometry.

Visual Analysis of the Daily QA Results of Photon and Electron Beams of a Trilogy Linac over a Five-year Period.

Data visualization technique was applied to analyze the daily QA results of photon and electron beams. Special attention was paid to any trend the beams might display. A Varian Trilogy Linac equipped with dual photon energies and five electron energies was commissioned in early 2010. Daily Linac QA tests including the output constancy, beam flatness and symmetry (radial and transverse directions) were performed with an ionization chamber array device (QA BeamChecker Plus, Standard Imaging). The data of five years were collected and analyzed. For each energy, the measured data were exported and processed for visual trending using an in-house Matlab program. These daily data were cross-correlated with the monthly QA and annual QA results, as well as the preventive maintenance records. Majority of the output were within 1% of variation, with a consistent positive/upward drift for all seven energies (~+0.25% per month). The baseline of daily device is reset annually right after the TG-51 calibration. This results in a sudden drop of the output. On the other hand, the large amount of data using the same baseline exhibits a sinusoidal behavior (cycle = 12 months; amplitude = 0.8%, 0.5% for photons, electrons, respectively) on symmetry and flatness when normalization of baselines is accounted for. The well known phenomenon of new Linac output drift was clearly displayed. This output drift was a result of the air leakage of the over-pressurized sealed monitor chambers for the specific vendor. Data visualization is a new trend in the era of big data in radiation oncology research. It allows the data to be displayed visually and therefore more intuitive. Based on the visual display from the past, the physicist might predict the trend of the Linac and take actions proactively. It also makes comparisons, alerts failures, and potentially identifies causalities.

Dosimetric analysis of a simplified intensity modulation technique for prone breast radiotherapy.

PURPOSE: Prone-position breast radiotherapy (RT) has been described as an alternative technique to improve dose homogeneity for women with large, pendulous breasts. We report the feasibility and dosimetric analysis of a simplified intensity-modulated RT (IMRT) technique, previously reported for women in the supine treatment position, to plan prone-position RT to the intact breast. METHODS AND MATERIALS: Twenty patients with clinical Stage TisN0-T1bN1 breast cancer undergoing breast-conserving therapy underwent whole breast RT using a prone position technique. The treatment plans were developed using both conventional tangents and a simplified intensity-modulated tangential beam technique based on optimization of the intensity distributions across the breast. The plans were compared with regard to the dose-volume parameters. RESULTS: Dose heterogeneity within the breast planning target volume was significantly greater for the conventional tangent plans. Of 20 patients, 16 (80%) received maximal doses of > or =110% using the conventional tangents vs. only 1 (5%) using the IMRT plan. The isodose level encompassing 5% of the planning target volume was reduced from an average of 110% with conventional tangents to 105% with IMRT. The maximal dose within the planning target volume was reduced from an average of 114% with conventional tangents to 107% with IMRT. The greatest improvement was seen in the patients with the most pendulous breasts. CONCLUSION: An IMRT planning approach is feasible for prone-position breast RT and improves dose homogeneity, particularly in women with larger, pendulous breasts. Additional follow-up is necessary to determine whether the improvements in dose homogeneity impact acute toxicity and cosmetic outcome in this cohort of women who have historically suffered from poor cosmesis after breast-conserving therapy.

Evaluation of concave dose distributions created using an inverse planning system.

PURPOSE: To evaluate and develop optimum inverse treatment planning strategies for the treatment of concave targets adjacent to normal tissue structures. METHODS AND MATERIALS: Optimized dose distributions were designed using an idealized geometry consisting of a cylindrical phantom with a concave kidney-shaped target (PTV) and cylindrical normal tissues (NT) placed 5-13 mm from the target. Targets with radii of curvature from 1 to 2.75 cm were paired with normal tissues with radii between 0.5 and 2.25 cm. The target was constrained to a prescription dose of 100% and minimum and maximum doses of 95% and 105% with relative penalties of 25. Maximum dose constraint parameters for the NT varied from 10% to 70% with penalties from 10 to 1000. Plans were evaluated using the PTV uniformity index (PTV D(max)/PTV D(95)) and maximum normal tissue doses (NT D(max)/PTV D(95)). RESULTS: In nearly all situations, the achievable PTV uniformity index and the maximum NT dose exceeded the corresponding constraints. This was particularly true for small PTV-NT separations (5-8 mm) or strict NT dose constraints (10%-30%), where the achievable doses differed from the requested by 30% or more. The same constraint parameters applied to different PTV-NT separations yielded different dose distributions. For most geometries, a range of constraints could be identified that would lead to acceptable plans. The optimization results were fairly independent of beam energy and radius of curvature, but improved as the number of beams increased, particularly for small PTV-NT separations or strict dose constraints. CONCLUSION: Optimized dose distributions are strongly affected by both the constraint parameters and target-normal tissue geometry. Standard site-specific constraint templates can serve as a starting point for optimization, but the final constraints must be determined iteratively for individual patients. A strategy whereby NT constraints and penalties are modified until the highest acceptable PTV uniformity index is achieved is discussed. This strategy can be used, in simple patient geometries, to ensure the lowest possible normal tissue dose. Strategies for setting the optimum dose constraints and penalties may vary for different optimization algorithms and objective functions. Increasing the number of beams can significantly improve normal tissue dose and target uniformity in situations where the PTV-NT separation is small or the normal tissue dose limits are severe. Setting unrealistically severe constraints in such situations often results in dose distributions that are inferior to plans achieved with more lenient constraints.