An Adaptive Low-Rank Modeling-Based Active Learning Method for Medical Image Annotation.

Active learning is an effective solution to interactively select a limited number of informative examples and use them to train a learning algorithm that can achieve its optimal performance for specific tasks. It is suitable for medical image applications in which unlabeled data are abundant but manual annotation could be very time-consuming and expensive. However, designing an effective active learning strategy for informative example selection is a challenging task, due to the intrinsic presence of noise in medical images, the large number of images, and the variety of imaging modalities. In this study, a novel low-rank modeling-based multi-label active learning (LRMMAL) method is developed to address these challenges and select informative examples for training a classifier to achieve the optimal performance. The proposed method independently quantifies image noise and integrates it with other measures to guide a pool-based sampling process to determine the most informative examples for training a classifier. In addition, an automatic adaptive cross entropy-based parameter determination scheme is proposed for further optimizing the example sampling strategy. Experimental results on varied medical image datasets and comparisons with other state-of-the-art multi-label active learning methods illustrate the superior performance of the proposed method.

Magnetic Resonance Image-Guided Radiotherapy (MRIgRT): A 4.5-Year Clinical Experience.

AIMS: Magnetic resonance image-guided radiotherapy (MRIgRT) has been clinically implemented since 2014. This technology offers improved soft-tissue visualisation, daily imaging, and intra-fraction real-time imaging without added radiation exposure, and the opportunity for adaptive radiotherapy (ART) to adjust for anatomical changes. Here we share the longest single-institution experience with MRIgRT, focusing on trends and changes in use over the past 4.5 years. MATERIALS AND METHODS: We analysed clinical information, including patient demographics, treatment dates, disease sites, dose/fractionation, and clinical trial enrolment for all patients treated at our institution using MRIgRT on a commercially available, integrated 0.35 T MRI, tri-cobalt-60 device from 2014 to 2018. For each patient, factors including disease site, clinical rationale for MRIgRT use, use of ART, and proportion of fractions adapted were summated and compared between individual years of use (2014-2018) to identify shifts in institutional practice patterns. RESULTS: Six hundred and forty-two patients were treated with 666 unique treatment courses using MRIgRT at our institution between 2014 and 2018. Breast cancer was the most common disease, with use of cine MRI gating being a particularly important indication, followed by abdominal sites, where the need for cine gating and use of ART drove MRIgRT use. One hundred and ninety patients were treated using ART in 1550 fractions, 67.6% (1050) of which were adapted. ART was primarily used in cancers of the abdomen. Over time, breast and gastrointestinal cancers became increasingly dominant for MRIgRT use, hypofractionated treatment courses became more popular, and gastrointestinal cancers became the principal focus of ART. DISCUSSION: MRIgRT is widely applicable within the field of radiation oncology and new clinical uses continue to emerge. At our institution to date, applications such as ART for gastrointestinal cancers and accelerated partial breast irradiation (APBI) for breast cancer have become dominant indications, although this is likely to continue to evolve.

A framework for automated contour quality assurance in radiation therapy including adaptive techniques.

Contouring of targets and normal tissues is one of the largest sources of variability in radiation therapy treatment plans. Contours thus require a time intensive and error-prone quality assurance (QA) evaluation, limitations which also impair the facilitation of adaptive radiotherapy (ART). Here, an automated system for contour QA is developed using historical data (the 'knowledge base'). A pilot study was performed with a knowledge base derived from 9 contours each from 29 head-and-neck treatment plans. Size, shape, relative position, and other clinically-relevant metrics and heuristically derived rules are determined. Metrics are extracted from input patient data and compared against rules determined from the knowledge base; a computer-learning component allows metrics to evolve with more input data, including patient specific data for ART. Nine additional plans containing 42 unique contouring errors were analyzed. 40/42 errors were detected as were 9 false positives. The results of this study imply knowledge-based contour QA could potentially enhance the safety and effectiveness of RT treatment plans as well as increase the efficiency of the treatment planning process, reducing labor and the cost of therapy for patients.

SU-E-J-176: Clinical Evaluations of a Novel Metal Artifact Reduction Technique for Treatment Planning in Radiation Therapy.

PURPOSE: To evaluate the effects of a CT on-board commercially available metal artifact reduction (MAR) algorithm for the use in radiation therapy treatment planning. METHODS: Phantom and clinical data were used for evaluation. A CIRS electron density phantom (Model 062) was scanned with Philips Brilliance BigBore 16-slice CT simulator to establish ground truth for CT Hounsfield numbers. Titanium hip prostheses were subsequently inserted into the phantom to mimic single or double hip implants. The phantom were scanned, CT images were reconstructed with and without MAR correction. Dose distributions for a 6X or an 18X beam were calculated using the three datasets and compared. CT Hounsfield number and variations were evaluated on both MAR-corrected and uncorrected images of ten clinical cases with hip implants. Dose distributions for three patients based on MAR-corrected images were compared to those of the uncorrected datasets with artifact regions density-overridden to 1.0g/cc. RESULTS: Metal artifacts were reduced dramatically on MAR corrected images for all phantom and patient cases. The phantom study indicated a remarkable improvement of Hounsfield number accuracy with maximum percentage difference reduction of 45% compared to the ground truth. CT number standard variations of the critical organs for the clinical cases were reduced from 30% to 66.7%. The image geometries were not affected by the MAR algorithm. Both critical structures and targets on clinical cases went from invisible to clearly visible. For all examined phantom and clinical cases, dosimetry difference was within 3% (mostly within 1% of the target volume) of the prescription dose and was not clinical significant for dose calculations based on different image datasets. CONCLUSIONS: The MAR algorithm can be safely utilized in the radiation therapy treatment planning process with remarkable improvements in CT number accuracy and structure conspicuity. Dosimetry is not highly dependent on the datasets utilized for dose calculations.

WE-G-BRB-08: TG-51 Calibration of First Commercial MRI-Guided IMRT System in the Presence of 0.35 Tesla Magnetic Field.

PURPOSE: The first real-time-MRI-guided radiotherapy system has been installed in a clinic and it is being evaluated. Presence of magnetic field (MF) during radiation output calibration may have implications on ionization measurements and there is a possibility that standard calibration protocols may not be suitable for dose measurements for such devices. In this study, we evaluated whether a standard calibration protocol (AAPM- TG-51) is appropriate for absolute dose measurement in presence of MF. METHODS: Treatment delivery of the ViewRay (VR) system is via three 15,000Ci Cobalt-60 heads positioned 120-degrees apart and all calibration measurements were done in the presence of 0.35T MF. Two ADCL- calibrated ionization-chambers (Exradin A12, A16) were used for TG-51 calibration. Chambers were positioned at 5-cm depth, (SSD=105cm: VR's isocenter), and the MLC leaves were shaped to a 10.5cm × 10.5 cm field size. Percent-depth-dose (PDD) measurements were performed for 5 and 10 cm depths. Individual output of each head was measured using the AAPM- TG51 protocol. Calibration accuracy for each head was subsequently verified by Radiological Physics Center (RPC) TLD measurements. RESULTS: Measured ion-recombination (Pion) and polarity (Ppol) correction factors were less-than 1.002 and 1.006, respectively. Measured PDDs agreed with BJR-25 within ±0.2%. Maximum dose rates for the reference field size at VR's isocenter for heads 1, 2 and 3 were 1.445±0.005, 1.446±0.107, 1.431±0.006 Gy/minute, respectively. Our calibrations agreed with RPC- TLD measurements within ±1.3%, ±2.6% and ±2.0% for treatment-heads 1, 2 and 3, respectively. At the time of calibration, mean activity of the Co-60 sources was 10,800Ci±0.1%. CONCLUSIONS: This study shows that the TG- 51 calibration is feasible in the presence of 0.35T MF and the measurement agreement is within the range of results obtainable for conventional treatment machines. Drs. Green, Goddu, and Mutic served as scientific consultants for ViewRay, Inc. Dr. Mutic is on the clinical focus group for ViewRay, Inc., and his spouse holds shares in ViewRay, Inc.

WE-G-213CD-04: A Triggering System to Guide 4DMRI Image Acquisition.

PURPOSE: To develop a practical triggering system based on pre-selected respiratory amplitudes to guide prospectively the image acquisition of 4DMRI to track abdominal tumor motion for radiotherapy treatment planning. METHODS: The proposed triggering scheme consists of a preparation stage and an acquisition stage. Immediately prior to MRI acquisition, the preparation stage monitors the respiration via an external respiratory belt. Based on the respiratory amplitude and status (i.e., inhalation↑ or exhalation↓), the respiration cycle is equally divided into N respiratory states. For example, in the 4-state case, the 4 respiratory states are 5%↓″, 50%↑, 95%↑ and 50%↓. The 5%↓ and 95%↑ are used instead of 0% and 100% to improve the robustness of the triggering system. Each state is associated with a trigger which starts image acquisition for one and only one slice. A complete 4DMRI imageset requires N dynamic scans. In each dynamic scan, all slices are acquired once and each is in a specific respiratory state. In different dynamic scans, each slice is associated with different respiratory states to form a 4D dataset. For proof-of-principle, the triggering system was integrated into a T2-weighted turbo spin echo sequence (TE=100ms, TR=7122ms) on a clinical 1.5T MRI (Philips Achieva) scanner. The 4-state case was tested on a healthy subject. RESULTS: Plots of the physiology data recorded and exported from the scanner clearly verified that triggers occurred at the expected locations. Four T2-weighted images from a representative slice recorded the liver motion corresponding to the 4 respiratory states. CONCLUSIONS: Our results confirmed that the newly-developed system could trigger prospectively to guide 4DMRI image acquisition to achieve T2 weighting, which has a better tumor-tissue contrast than those offered by previous 4DMRI techniques with T1 or T2/T1 weighting. This is a first report of a pure T2-weighted 4DMRI to track respiration induced abdominal motion.

WE-E-BRB-10: DosCheck - an Electronic Chart Checking Tool for Dosimetrists.

PURPOSE: In addition to treatment planning, dosimetrists have to prepare documentation and manually enter data in treatment management system (TMS) which did not transfer or setup automatically. The required documents and data are dependent on the disease site, treatment machine and clinical workflow. Errors and inconsistencies can cause redundant work, treatment delays and potentially treatment errors. To address these issues, an electronic checking software tool, DosCheck was clinically implemented to check the existence of necessary documentations and the integrity of manually-entered data. The purpose of this software is to reduce the frequency of human errors and to improve efficiency. METHODS: DosCheck reads data and documents from 1) TMS, 2) Pinnacle TPS, and 3) DICOM plan files stored in a DICOM-RT PACS. It processes documents in Word and PDF format, treatment plan data in Pinnacle native format and DICOM format, and Mosaiq data in database records. The software cross-checks data accuracy and consistency by following rules that are pre-defined according to the clinical requirements and treatment sties. It interacts with dosimetrists and presents instantaneous results via graphical user interface. RESULTS: DosCheck has been implemented in C#. It performs a full check for a patient with 20 seconds. It has been clinically commissioned and is used daily by all dosimetrists at our institution. Retrospective analysis shows that DosCheck identifies 30% to 40% of previously reported dosimetrist human errors. Additional ∼30% errors are checked by other tools that could be integrated DosCheck in the near future. CONCLUSIONS: As an electronic data checking tool, DosCheck can obtain and process data and documents from multiple clinical computer systems in the radiation oncology department, and perform checks according to clinical rules. It is able to improve the accuracy and efficiency of clinical data and document process, and therefore to reduce any potential inconsistencies and errors.

WE-E-BRB-07: Direct 3D Fluence Calculation from Machine Beam Parameters for VMAT Delivery Verification.

PURPOSE: MLC dynamic log files have been clinically used for quality assurance for years. The logged machine parameters and the derived beam 2D fluence maps can be compared to the ones obtained in the treatment plans in order to evaluate the accuracy and consistency of IMRT beam deliveries. In this study, we propose a computationally efficient method, called Direct 3D Fluence Calculation or D3DFC, to extend 2D fluence map derivation to 3D fluence volume computation. The aim is to extend dynalog-based QA from fixed-gantry IMRT to rotational-gantry VMAT. METHODS: D3DFC calculates the 3D volume of photon fluence distribution directly from the machine parameters (gantry angles, jaw positions, MLC positions, collimator rotation angle, MU) contained in the dynamic log files or DICOM plans, without converting to 2D fluence maps per gantry angle in order to allow higher computation speed and accuracy. For testing, results were verified with film-in-air measurements. 3D fluence volumes computed from VMAT delivery records (with artificially introduced delivery errors) are compared to ones computed from the treatment plans to determine if these delivery errors can be identified. RESULTS: D3DFC is implemented in MATLAB and supports the DICOM plans, Varian MLC dynamic logs and Varian Truebeam machine logs. Computation takes 10 to 20 seconds for a single-arc VMAT plan or delivery records. The results showed that 1 mm MLC errors can be clearly detected using delivery-to-plan fluence volume comparison. CONCLUSIONS: Direct computation from machine parameters allows higher computation speed and accuracy. These advantages are useful for beam delivery verification purposes for which (slower) full patient CT based dose computation is less necessary. The calculated 3D photon fluence volume is useful to detect and visually present the VMAT delivery discrepancies.

SU-E-T-305: Limitations of Using DICOM Data for BrachyVision Treatment Plan Evaluation.

PURPOSE: To evaluate the accuracy of a real-time automated method of performing dosimetric quality assurance using Eclipse DICOM files for patients receiving HDR-brachytherapy and IMRT. METHODS: GYN patients are treated with concurrent high-dose rate brachtherapy and IMRT. The dosimetric parameters were obtained through an in-house QA program developed using Matlab. The DICOM files containing DVH data for organsat-risk (OAR) were analyzed Dosimetric data for 7 patients (total 42 fractions) were collected for bladder, rectum and sigmoid. The accuracy of the dosimetric parameters was estimated by comparing the parameters obtained from the DICOM based QA program and those in BrachyVision. RESULTS: The maximal dose values (Dmax) for the OARs obtained using the DICOM-based program are significantly smaller than those valued reported in BrachyVision by 36.2%-48.3%. The mean dose has a deviation from 1% - 2.4%. The dose for the volume of 2cc (D2cc) has a difference up to 7.6% for structures with the volume larger than 200 cc. The average difference of D2cc is 0.5% for structures less than 200 cc. We found that Eclipse BrachyVision only exports DVH data down to a volume equivalent to 1% of the maximum volume for a given structure. Therefore, the reported maximal dose values obtained from DICOM RT dose file do not accurately reflect the maximum dose in a treatment plan. This will also slightly affect the mean dose calculation and D2cc when the structure volume is larger than 200cc. CONCLUSIONS: The automatic QA tool based on DICOM files provides a quick retrieval of dose to organs-at-risk and coverage of targets. However, maximal dose to structures is not accurate due to the truncationof the DVH information contained in DICOM files.

SU-E-T-159: Sensitivity of in Line Real Time Scintillating Fiber Detectors for External Beam Treatment Verification and Patient Safety.

PURPOSE: We studied the sensitivity of a novel transmission fiber scintillator array designed and built for in line treatment verification. The purpose of this project is to assess the capability of the fiber detector array technology to detect treatment errors in real time without false positives to enhance patient safety. METHODS: We developed a linear scintillator array detector using radiation hard scintillating fibers and high speed parallel signal conditioning and data acquisition to monitor external beam treatment fluence in real time. The detector captures and resolves the time and amplitude of each linac pulse at each MLC segment. The detector has 60 fibers aligned to each MLC leaf and two output channels per fiber. The data is captured by a high speed parallel digitizer to determine the IMRT beam output delivered to a patient in real time. We evaluated the detector peak pulse linearity according to dose rate, MLC positioning, and beam energy. We analyzed the detector sensitivity, signal to noise ratio, and pulse distribution statistics to determine beam output and fluence in real time. RESULTS: We analyzed the response of the detector to 6 MV and 10 MV photon beams. The statistical analysis of the detected linac pulses indicates that a minimum of 20 pulses are required to evaluate MLC positioning and fluence with 3 mm and 3% resolution, respectively. During testing, no false positives were detected. Linearity with respect to output rate, MLC or jaw opening, and fluence is within 2%. CONCLUSIONS: Measured sensitivity and signal to noise ratio of a real time linear fiber array detector show that delivered beam fluence can be monitored every 55 msec, with no observed false positives during treatment to provide in vivo real time patient safety and beam monitoring.

SU-E-T-513: Clinical Implementation of a GPU Accelerated Pencil Beam Dose Calculation Algorithm.

PURPOSE: This work reports a clinical implementation of a GPU accelerated pencil beam dose calculation algorithm (GPU-PB). METHODS: Model parameters were determined using in-house scripts written in MATLAB. Dose distributions in a water phantom were calculated using a Pinnacle TPS for various open field sizes. Lateral profiles at 2-mm incremental depths were used to calculate PB kernel parameters. Weighted sum of squares were used for least-squares parameter fitting utilizing a Levenberg-marquardt algorithm. Weightings were adjusted based on goodness of fitting and in accordance with field size to suppress deviations due to horn effects. The scale factor was fitted iteratively. The calculated doses for two patient cases were analyzed with a 3D gamma method (3%/3mm). RESULTS: Excellent agreement between Pinnacle calculation and GPU-PB calculation was achieved regarding PDD, profiles and output factor. For a head-neck 10-field step-and-shoot IMRT case, gamma passing rate was over 99% and maximum absolute dose value is 75.6 Gy vs. 73.9 Gy, differing by 2.3%. Similar results were obtained for a SBRT lung case. Gamma passing rate is a function of PB kernel cut-off distance and beamlet resolution. It appears that if the highest accuracy is desired, a resolution of 10 mm or better in the direction parallel to MLC travel and a cutoff distance of 10 cm or better should be used. The calculation time increases with both cut-off distance and beamlet resolution. For example, GPU calculation time is 1.36 seconds for 5 cm cut-off distance, and increases to 4.84 s for 15 cm cut-off distance. CONCLUSIONS: A GPU accelerated PB dose calculation algorithm has been implemented using clinical measurement data. Excellent agreement with Pinnacle TPS has been achieved. Beamlet size and PB cut-off distance should be chosen according to desired dose calculation accuracy and speed.

MicroRT-small animal conformal irradiator.

A novel small animal conformal radiation therapy system has been designed and prototyped: MicroRT. The microRT system integrates multimodality imaging, radiation treatment planning, and conformal radiation therapy that utilizes a clinical 192Ir isotope high dose rate source as the radiation source (teletherapy). A multiparameter dose calculation algorithm based on Monte Carlo dose distribution simulations is used to efficiently and accurately calculate doses for treatment planning purposes. A series of precisely machined tungsten collimators mounted onto a cylindrical collimator assembly is used to provide the radiation beam portals. The current design allows a source-to-target distance range of 1-8 cm at four beam angles: 0 degrees (beam oriented down), 90 degrees, 180 degrees, and 270 degrees. The animal is anesthetized and placed in an immobilization device with built-in fiducial markers and scanned using a computed tomography, magnetic resonance, or positron emission tomography scanner prior to irradiation. Treatment plans using up to four beam orientations are created utilizing a custom treatment planning system-microRTP. A three-axis computer-controlled stage that supports and accurately positions the animals is programmed to place the animal relative to the radiation beams according to the microRTP plan. The microRT system positioning accuracy was found to be submillimeter. The radiation source is guided through one of four catheter channels and placed in line with the tungsten collimators to deliver the conformal radiation treatment. The microRT hardware specifications, the accuracy of the treatment planning and positioning systems, and some typical procedures for radiobiological experiments that can be performed with the microRT device are presented.

Functional imaging in treatment planning in radiation therapy: a review

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Multimodality image registration quality assurance for conformal three-dimensional treatment planning.

PURPOSE: We present a quality assurance methodology to determine the accuracy of multimodality image registration and fusion for the purpose of conformal three-dimensional and intensity-modulated radiation therapy treatment planning. Registration and fusion accuracy between any combination of computed tomography (CT), magnetic resonance (MR), and positron emission computed tomography (PET) imaging studies can be evaluated. METHODS AND MATERIALS: A commercial anthropomorphic head phantom filled with water and containing CT, MR, and PET visible targets was modified to evaluate the accuracy of multimodality image registration and fusion software. For MR and PET imaging, the water inside the phantom was doped with CuNO(3) and 18F-fluorodeoxyglucose (18F-FDG), respectively. Targets consisting of plastic spheres and pins were distributed throughout the cranium section of the phantom. Each target sphere had a conical-shaped bore with its apex at the center of the sphere. The pins had a conical extension or indentation at the free end. The contours of the spheres, sphere centers, and pin tips were used as anatomic landmark models for image registration, which was performed using affine coordinate-transformation tools provided in a commercial multimodality image registration/fusion software package. Four sets of phantom image studies were obtained: primary CT, secondary CT with different phantom immobilization, MR, and PET study. A novel CT, MR, and PET external fiducial marking system was also tested. RESULTS: The registration of CT/CT, CT/MR, and CT/PET images allowed correlation of anatomic landmarks to within 2 mm, verifying the accuracy of the registration software and spatial fidelity of the four multimodality image sets. CONCLUSIONS: This straightforward phantom-based quality assurance of the image registration and fusion process can be used in a routine clinical setting or for providing a working image set for development of the image registration and fusion process and new software.

Characterization of a commercial multileaf collimator used for intensity modulated radiation therapy.

The characteristics of a commercial multileaf collimator (MLC) to deliver static and dynamic multileaf collimation (SMLC and DMLC, respectively) were investigated to determine their influence on intensity modulated radiation therapy (IMRT) treatment planning and quality assurance. The influence of MLC leaf positioning accuracy on sequentially abutted SMLC fields was measured by creating abutting fields with selected gaps and overlaps. These data were also used to measure static leaf positioning precision. The characteristics of high leaf-velocity DMLC delivery were measured with constant velocity leaf sequences starting with an open field and closing a single leaf bank. A range of 1-72 monitor units (MU) was used providing a range of leaf velocities. The field abutment measurements yielded dose errors (as a percentage of the open field max dose) of 16.7+/-0.7% mm(-1) and 12.8+/-0.7% mm(-1) for 6 MV and 18 MV photon beams, respectively. The MLC leaf positioning precision was 0.080+/-0.018 mm (single standard deviation) highlighting the excellent delivery hardware tolerances for the tested beam delivery geometry. The high leaf-velocity DMLC measurements showed delivery artifacts when the leaf sequence and selected monitor units caused the linear accelerator to move the leaves at their maximum velocity while modulating the accelerator dose rate to deliver the desired leaf and MU sequence (termed leaf-velocity limited delivery). According to the vendor, a unique feature to their linear accelerator and MLC is that the dose rate is reduced to provide the correct cm MU(-1) leaf velocity when the delivery is leaf-velocity limited. However, it was found that the system delivered roughly 1 MU per pulse when the delivery was leaf-velocity limited causing dose profiles to exhibit discrete steps rather than a smooth dose gradient. The root mean square difference between the steps and desired linear gradient was less than 3% when more than 4 MU were used. The average dose per MU was greater and less than desired for closing and opening leaf patterns, respectively, when the delivery was leaf-velocity limited. The results indicated that the dose delivery artifacts should be minor for most clinical cases, but limit the assumption of dose linearity when significantly reducing the delivered dose for dosimeter characterization studies or QA measurements.

Abutment dosimetry for serial tomotherapy.

The dose distributions at the abutment region for serial tomotherapy are reviewed. While tomotherapy provides unparalleled dose distributions, precise couch motion and good patient immobilization are required because the dose in the abutment region changes by 25% for each millimeter of misalignment. The process of delivering intensity-modulated radiation therapy using sequentially delivered modulated arcs yields hot spots below and cold spots above the machine isocenter when arc angles of less than 360 degrees are used. The magnitude of the hot and cold spots increases significantly as the arc angle is reduced 180 degrees such as when limited by couch clearance restrictions. Placement of isocenter also significantly affects the dose heterogeneity in the abutment region, with the hot and cold spots increasing nearly linearly with off-axis distance in the vertical direction. Reduction of the magnitude of the abutment region dose heterogeneities is possible if helical delivery is provided by moving the couch during arc delivery. The dose heterogeneity can also be reduced by creating 2 treatment plans, each with slightly different abutment region positions, or by using multiple couch angles.

Room shielding for intensity-modulated radiation therapy treatment facilities.

PURPOSE: The traditional assumptions used in room-shielding calculations are reassessed for intensity-modulated radiation therapy (IMRT). IMRT makes relatively inefficient use of monitor units (MUs) when compared to conventional radiation therapy, affecting the assumptions used in room-shielding calculations. For the same single-fraction tumor dose delivered, the total number of MUs for IMRT is much greater than for a conventional treatment. Therefore, the exposure contribution from the linear accelerator head leakage will be significantly greater than with conventional treatments. METHODS AND MATERIALS: We propose a shielding calculation model that decouples the concepts of workload, MUs, and target dose when determining primary and secondary barrier thicknesses. The workload for primary barrier calculations for conventional multileaf collimator (MLC) IMRT treatments is determined according to patient tumor doses. The same calculation for accelerator-based serial tomotherapy IMRT requires scaling by the average number of treatment slices. However, rotational therapy yields a small use factor that compensates for this increase. We further define a series of efficiency factors to account for the small field sizes employed in IMRT. For secondary barrier calculations, the patient-scattered radiation is assumed to be the same for all IMRT modalities as for conventional therapy. The accelerator head leakage contribution is proportional to the number of MUs. Knowledge of the average number of MUs per patient is required to estimate the head leakage contribution. We used a 6-MV linear accelerator photon beam to guide the development of this technique and to evaluate the adequacy of conventional barriers for IMRT. Average weekly IMRT workload estimates were made based on our experience with 180 serial tomotherapy patients and published data for both "step and shoot" and dynamic MLC delivered treatments. RESULTS: We found that conventional primary barriers are adequate for both dynamic MLC and serial tomotherapy IMRT. However, the excessive head leakage produced by these modalities requires an increase in secondary barrier shielding. CONCLUSION: When designing shielding for an IMRT facility, increases in accelerator head leakage must be taken into account for secondary shielding. Adequacy of secondary shielding will depend on the IMRT patient load. For conventional facilities that are being assessed for IMRT therapy, existing primary barriers will typically prove adequate.

A novel approach to overcome hypoxic tumor resistance: Cu-ATSM-guided intensity-modulated radiation therapy.

PURPOSE: Locoregional tumor control for locally advanced cancers with radiation therapy has been unsatisfactory. This is in part associated with the phenomenon of tumor hypoxia. Assessing hypoxia in human tumors has been difficult due to the lack of clinically noninvasive and reproducible methods. A recently developed positron emission tomography (PET) imaging-based hypoxia measurement technique which employs a Cu(II)-diacetyl-bis(N(4)-methylthiosemicarbazone) (Cu-ATSM) tracer is of great interest. Oxygen electrode measurements in animal experiments have demonstrated a strong correlation between low tumor pO(2) and excess (60)Cu-ATSM accumulation. Intensity-modulated radiation therapy (IMRT) allows selective targeting of tumor and sparing of normal tissues. In this study, we examined the feasibility of combining these novel technologies to develop hypoxia imaging (Cu-ATSM)-guided IMRT, which may potentially deliver higher dose of radiation to the hypoxic tumor subvolume to overcome inherent hypoxia-induced radioresistance without compromising normal tissue sparing. METHODS AND MATERIALS: A custom-designed anthropomorphic head phantom containing computed tomography (CT) and positron emitting tomography (PET) visible targets consisting of plastic balls and rods distributed throughout the "cranium" was fabricated to assess the spatial accuracy of target volume mapping after multimodality image coregistration. For head-and-neck cancer patients, a CT and PET imaging fiducial marker coregistration system was integrated into the thermoplastic immobilization head mask with four CT and PET compatible markers to assist image fusion on a Voxel-Q treatment-planning computer. This system was implemented on head-and-neck cancer patients, and the gross tumor volume (GTV) was delineated based on physical and radiologic findings. Within GTV, regions with a (60)Cu-ATSM uptake twice that of contralateral normal neck muscle were operationally designated as ATSM-avid or hypoxic tumor volume (hGTV) for this feasibility study. These target volumes along with other normal organs contours were defined and transferred to an inverse planning computer (Corvus, NOMOS) to create a hypoxia imaging-guided IMRT treatment plan. RESULTS: A study of the accuracy of target volume mapping showed that the spatial fidelity and imaging distortion after CT and PET image coregistration and fusion were within 2 mm in phantom study. Using fiducial markers to assist CT/PET imaging fusion in patients with carcinoma of the head-and-neck area, a heterogeneous distribution of (60)Cu-ATSM within the GTV illustrated the success of (60)Cu-ATSM PET to select an ATSM-avid or hypoxic tumor subvolume (hGTV). We further demonstrated the feasibility of Cu-ATSM-guided IMRT by showing an example in which radiation dose to the hGTV could be escalated without compromising normal tissue (parotid glands and spinal cord) sparing. The plan delivers 80 Gy in 35 fractions to the ATSM-avid tumor subvolume and the GTV simultaneously receives 70 Gy in 35 fractions while more than one-half of the parotid glands are spared to less than 30 Gy. CONCLUSION: We demonstrated the feasibility of a novel Cu-ATSM-guided IMRT approach through coregistering hypoxia (60)Cu-ATSM PET to the corresponding CT images for IMRT planning. Future investigation is needed to establish a clinical-pathologic correlation between (60)Cu-ATSM retention and radiation curability, to understand tumor re-oxygenation kinetics, and tumor target uncertainty during a course of radiation therapy before implementing this therapeutic approach to patients with locally advanced tumor.

Experimental validation of dose calculation algorithms for the GliaSite RTS, a novel 125I liquid-filled balloon brachytherapy applicator.

This paper compares experimentally measured and calculated dose-rate distributions for a novel 125I liquid-filled brachytherapy balloon applicator (the GliaSite RTS), designed for the treatment of malignant brain-tumor resection-cavity margins. This work is intended to comply with the American Association of Physicists in Medicine (AAPM) Radiation Therapy Committee's recommendations [Med. Phys. 25, 2269-2270 (1998)] for dosimetric characterization of low-energy photon interstitial brachytherapy sources. Absolute low dose-rate radiochromic film (RCF) dosimetry measurements were performed in coronal planes about the applicator. The applicator was placed in a solid water phantom, machined to conform to the inflated applicator's surface. The results were used to validate the accuracy of Monte Carlo photon transport (MCPT) simulations and a point-source dose-kernel algorithm in predicting dose to water. The absolute activity of the 125I solution was determined by intercomparing a National Institute of Standards and Technology (NIST) 125I standard with a known mass of radiotherapy solution (Iotrex) in an identical vial and geometry. For the two films not in contact with applicator, the average agreement between RCF and MCPT (specified as the mean absolute deviation in successive 4 mm rings) was found to be within +/-5% at distances 0.2-25 mm from the film centers. For the two films touching the catheter, the mean agreement was +/-14.5% and 7.5% near the balloon surface but improving to 7.5% and 6% by 3.5 mm from the surface. These errors, as large as 20% in isolated pixels, are likely due to trim damage, 125I contamination, and poor conformance with the balloon. At larger distances where the radiation doses were very low, the observed discrepancies were significantly larger than expected. We hypothesize that they are due to a dose-rate dependence of the RCF response. A 1%-10% average difference between a simple one-dimensional path-length semiempirical dose-kernel model and the MCPT calculations was observed over clinically relevant distances.

Validation of a precision radiochromic film dosimetry system for quantitative two-dimensional imaging of acute exposure dose distributions.

We present an evaluation of the precision and accuracy of image-based radiochromic film (RCF) dosimetry performed using a commercial RCF product (Gafchromic MD-55-2, Nuclear Associates, Inc.) and a commercial high-spatial resolution (100 microm pixel size) He-Ne scanning-laser film-digitizer (Personal Densitometer, Molecular Dynamics, Inc.) as an optical density (OD) imaging system. The precision and accuracy of this dosimetry system are evaluated by performing RCF imaging dosimetry in well characterized conformal external beam and brachytherapy high dose-rate (HDR) radiation fields. Benchmarking of image-based RCF dosimetry is necessary due to many potential errors inherent to RCF dosimetry including: a temperature-dependent time evolution of RCF dose response; nonuniform response of RCF; and optical-polarization artifacts. In addition, laser-densitometer imaging artifacts can produce systematic OD measurement errors as large as 35% in the presence of high OD gradients. We present a RCF exposure and readout protocol that was developed for the accurate dosimetry of high dose rate (HDR) radiation sources. This protocol follows and expands upon the guidelines set forth by the American Association of Physicists in Medicine (AAPM) Task Group 55 report. Particular attention is focused on the OD imaging system, a scanning-laser film digitizer, modified to eliminate OD artifacts that were not addressed in the AAPM Task Group 55 report. RCF precision using this technique was evaluated with films given uniform 6 MV x-ray doses between 1 and 200 Gy. RCF absolute dose accuracy using this technique was evaluated by comparing RCF measurements to small volume ionization chamber measurements for conformal external-beam sources and an experimentally validated Monte Carlo photon-transport simulation code for a 192Ir brachytherapy source. Pixel-to-pixel standard deviations of uniformly irradiated films were less than 1% for doses between 10 and 150 Gy; between 1% and 5% for lower doses down to 1 Gy and 1% and 1.5% for higher doses up to 200 Gy. Pixel averaging to form 200-800 microm pixels reduces these standard deviations by a factor of 2 to 5. Comparisons of absolute dose show agreement within 1.5%-4% of dose benchmarks, consistent with a highly accurate dosimeter limited by its observed precision and the precision of the dose standards to which it is compared. These results provide a comprehensive benchmarking of RCF, enabling its use in the commissioning of novel HDR therapy sources.