Design and implementation of a practical quality control program for dual-energy CT.

A novel routine dual-energy computed tomography (DECT) quality control (QC) program was developed to address the current deficiency of routine QC for this technology. The dual-energy quality control (DEQC) program features (1) a practical phantom with clinically relevant materials and concentrations, (2) a clinically relevant acquisition, reconstruction, and postprocessing protocol, and (3) a fully automated analysis software to extract quantitative data for database storage and trend analysis. The phantom, designed for easy set up for standalone or adjacent imaging next to the ACR phantom, was made in collaboration with an industry partner and informed by clinical needs to have four iodine inserts (0.5, 1, 2, and 5 mg/ml) and one calcium insert (100 mg/ml) equally spaced in a cylindrical water-equivalent background. The imaging protocol was based on a clinical DECT abdominal protocol capable of producing material specific concentration maps, virtual unenhanced images, and virtual monochromatic images. The QC automated analysis software uses open-source technologies which integrates well with our current automated CT QC database. The QC program was tested on a GE 750 HD scanner and two Siemens SOMATOM FLASH scanners over a 3-month period. The automated algorithm correctly identified the appropriate region of interest (ROI) locations and stores measured values in a database for monitoring and trend analysis. Slight variations in protocol settings were noted based on manufacturer. Overall, the project proved to provide a convenient and dependable clinical tool for routine oversight of DE CT imaging within the clinic.

Real-time motion-adaptive-optimization (MAO) in TomoTherapy.

IMRT delivery follows a planned leaf sequence, which is optimized before treatment delivery. However, it is hard to model real-time variations, such as respiration, in the planning procedure. In this paper, we propose a negative feedback system of IMRT delivery that incorporates real-time optimization to account for intra-fraction motion. Specifically, we developed a feasible workflow of real-time motion-adaptive-optimization (MAO) for TomoTherapy delivery. TomoTherapy delivery is characterized by thousands of projections with a fast projection rate and ultra-fast binary leaf motion. The technique of MAO-guided delivery calculates (i) the motion-encoded dose that has been delivered up to any given projection during the delivery and (ii) the future dose that will be delivered based on the estimated motion probability and future fluence map. These two pieces of information are then used to optimize the leaf open time of the upcoming projection right before its delivery. It consists of several real-time procedures, including 'motion detection and prediction', 'delivered dose accumulation', 'future dose estimation' and 'projection optimization'. Real-time MAO requires that all procedures are executed in time less than the duration of a projection. We implemented and tested this technique using a TomoTherapy research system. The MAO calculation took about 100 ms per projection. We calculated and compared MAO-guided delivery with two other types of delivery, motion-without-compensation delivery (MD) and static delivery (SD), using simulated 1D cases, real TomoTherapy plans and the motion traces from clinical lung and prostate patients. The results showed that the proposed technique effectively compensated for motion errors of all test cases. Dose distributions and DVHs of MAO-guided delivery approached those of SD, for regular and irregular respiration with a peak-to-peak amplitude of 3 cm, and for medium and large prostate motions. The results conceptually proved that the proposed method is applicable for real-time motion compensation in TomoTherapy delivery. Extension of the method to real-time adaptive radiation therapy (ART) that compensates for all kinds of delivery errors was proposed. Further validation and clinical implementation is underway.

Evaluation of geometric changes of parotid glands during head and neck cancer radiotherapy using daily MVCT and automatic deformable registration.

BACKGROUND AND PURPOSE: To assess and evaluate geometrical changes in parotid glands using deformable image registration and megavoltage CT (MVCT) images. METHODS: A deformable registration algorithm was applied to 330 daily MVCT images (10 patients) to create deformed parotid contours. The accuracy and robustness of the algorithm was evaluated through visual review, comparison with manual contours, and precision analysis. Temporal changes in the parotid gland geometry were observed. RESULTS: The deformed parotid contours were qualitatively judged to be acceptable. Compared with manual contours, the uncertainties of automatically deformed contours were similar with regard to geometry and dosimetric endpoint. The day-to-day variations (1 standard deviation of errors) in the center-of-mass distance and volume were 1.61mm and 4.36%, respectively. The volumes tended to decrease with a median total loss of 21.3% (6.7-31.5%) and a median change rate of 0.7%/day (0.4-1.3%/day). Parotids migrated toward the patient center with a median total distance change of -5.26mm (0.00 to -16.35mm) and a median change rate of -0.22mm/day (0.02 to -0.56mm/day). CONCLUSION: The deformable image registration and daily MVCT images provide an efficient and reliable assessment of parotid changes over the course of a radiation therapy.

Assessment of parotid gland dose changes during head and neck cancer radiotherapy using daily megavoltage computed tomography and deformable image registration.

PURPOSE: To analyze changes in parotid gland dose resulting from anatomic changes throughout a course of radiotherapy in a cohort of head-and-neck cancer patients. METHODS AND MATERIALS: The study population consisted of 10 head-and-neck cancer patients treated definitively with intensity-modulated radiotherapy on a helical tomotherapy unit. A total of 330 daily megavoltage computed tomography images were retrospectively processed through a deformable image registration algorithm to be registered to the planning kilovoltage computed tomography images. The process resulted in deformed parotid contours and voxel mappings for both daily and accumulated dose-volume histogram calculations. The daily and cumulative dose deviations from the original treatment plan were analyzed. Correlations between dosimetric variations and anatomic changes were investigated. RESULTS: The daily parotid mean dose of the 10 patients differed from the plan dose by an average of 15%. At the end of the treatment, 3 of the 10 patients were estimated to have received a greater than 10% higher mean parotid dose than in the original plan (range, 13-42%), whereas the remaining 7 patients received doses that differed by less than 10% (range, -6-8%). The dose difference was correlated with a migration of the parotids toward the high-dose region. CONCLUSIONS: The use of deformable image registration techniques and daily megavoltage computed tomography imaging makes it possible to calculate daily and accumulated dose-volume histograms. Significant dose variations were observed as result of interfractional anatomic changes. These techniques enable the implementation of dose-adaptive radiotherapy.

Evaluation of coplanar partial left breast irradiation using tomotherapy-based topotherapy.

PURPOSE: To investigate the use of topotherapy for accelerated partial breast irradiation through field-design optimization and dosimetric comparison to linear accelerator-based three-dimensional conformal radiotherapy (3D-CRT) and intensity-modulated radiation therapy (IMRT). METHODS AND MATERIALS: Hypothetical 3-cm lumpectomy sites were contoured in each quadrant of a left breast by using dosimetric guidelines from the National Surgical Adjuvant Breast and Bowel Project B-39/Radiation Therapy Oncology Group 0413 protocol. Coplanar intensity-modulated topotherapy treatment plans were optimized by using two-, three-, four-, five-, and seven-field arrangements for delivery by the tomotherapy unit with fixed gantry angles. Optimized noncoplanar five-field 3D-CRT and IMRT were compared with corresponding topotherapy plans. RESULTS: On average, 99.5% +/- 0.5% of the target received 100% of the prescribed dose for all topotherapy plans. Average equivalent uniform doses ranged from 1.20-2.06, 0.79-1.76, and 0.10-0.29 Gy for heart, ipsilateral lung, and contralateral lung, respectively. Average volume of normal breast exceeding 90% of the prescription and average area of skin exceeding 35 Gy were lowest for five-field plans. Average uniformity indexes for five-field plans using 3D-CRT, IMRT, and topotherapy were 1.047, 1.050, and 1.040, respectively. Dose-volume histograms and calculated equivalent uniform doses of all three techniques illustrate clinically equivalent doses to ipsilateral breast, lung, and heart. CONCLUSIONS: This dosimetric evaluation for a single patient shows that coplanar partial breast topotherapy provides good target coverage with exceptionally low dose to organs at risk. Use of more than five fields provided no additional dosimetric advantage. A comparison of five-field topotherapy to 3D-CRT and IMRT for accelerated partial breast irradiation illustrates equivalent target conformality and uniformity.

Patient dose from megavoltage computed tomography imaging.

PURPOSE: Megavoltage computed tomography (MVCT) can be used daily for imaging with a helical tomotherapy unit for patient alignment before treatment delivery. The purpose of this investigation was to show that the MVCT dose can be computed in phantoms, and further, that the dose can be reported for actual patients from MVCT on a helical tomotherapy unit. METHODS AND MATERIALS: An MVCT beam model was commissioned and verified through a series of absorbed dose measurements in phantoms. This model was then used to retrospectively calculate the imaging doses to the patients. The MVCT dose was computed for five clinical cases: prostate, breast, head/neck, lung, and craniospinal axis. RESULTS: Validation measurements in phantoms verified that the computed dose can be reported to within 5% of the measured dose delivered at the helical tomotherapy unit. The imaging dose scaled inversely with changes to the CT pitch. Relative to a normal pitch of 2.0, the organ dose can be scaled by 0.67 and 2.0 for scans done with a pitch of 3.0 and 1.0, respectively. Typical doses were in the range of 1.0-2.0 cGy, if imaged with a normal pitch. The maximal organ dose calculated was 3.6 cGy in the neck region of the craniospinal patient, if imaged with a pitch of 1.0. CONCLUSION: Calculation of the MVCT dose has shown that the typical imaging dose is approximately 1.5 cGy per image. The uniform MVCT dose delivered using helical tomotherapy is greatest when the anatomic thickness is the smallest and the pitch is set to the lowest value.

A simple fixed-point approach to invert a deformation field.

Inversion of deformation fields is applied frequently to map images, dose, and contours between the reference frame and the study frame. A prevailing approach that takes the negative of the forward deformation as the inverse deformation is oversimplified and can cause large errors for large deformations or deformations that are composites of several deformations. Other approaches, including Newton's method and scatter data interpolation, either require the first derivative or are very inefficient. Here we propose an iterative approach that is easy to implement, converges quickly to the inverse when it does, and works for a majority of cases in practice. Our approach is rooted in fixed-point theory. We build a sequence to approximate the inverse deformation through iterative evaluation of the forward deformation. A sufficient but not necessary convergence condition (Lipschitz condition) and its proof are also given. Though this condition guarantees the convergence, it may not be met for an arbitrary deformation field. One should always check whether the inverse exists for the given forward deformation field by calculating its Jacobian. If nonpositive values of the Jacobian occur only for few voxels, this method will usually converge to a pseudoinverse. In case the iteration fails to converge, one should switch to other means of finding the inverse. We tested the proposed method on simulated 2D data and real 3D computed tomography data of a lung patient and compared our method with two implementations in the Insight Segmentation and Registration Toolkit (ITK). Typically less than ten iterations are needed for our method to get an inverse deformation field with clinically relevant accuracy. Based on the test results, our method is about ten times faster and yet ten times more accurate than ITK's iterative method for the same number of iterations. Simulations and real data tests demonstrated the efficacy and the accuracy of the proposed algorithm.

Investigation of accelerated partial breast patient alignment and treatment with helical tomotherapy unit.

PURPOSE: To determine the precision of megavoltage computed tomography (MVCT)-based alignment of the seroma cavity for patients undergoing partial breast irradiation; and to determine whether accelerated partial breast irradiation (APBI) plans can be generated for TomoTherapy deliveries that meet the National Surgical Adjuvant Breast and Bowel Project (NSABP) B-39/Radiation Therapy Oncology Group (RTOG) 0413 protocol guidelines for target coverage and normal tissue dose limitations. METHODS AND MATERIALS: We obtained 50 MVCT images from 10 patients. An interuser study was designed to assess the alignment precision. Using a standard helical and a fixed beam prototype ("topotherapy") optimizer, two APBI plans for each patient were developed. RESULTS: The precision of the MVCT-based seroma cavity alignment was better than 2 mm if averaged over the patient population. Both treatment techniques could be used to generate acceptable APBI plans for patients that fulfilled the recommended NSABP B-39/RTOG-0413 selection criteria. For plans of comparable treatment time, the conformation of the prescription dose to the target was greater for helical deliveries, while the ipsilateral lung dose was significantly reduced for the topotherapy plans. CONCLUSIONS: The inherent volumetric imaging capabilities of a TomoTherapy Hi-Art unit allow for alignment of patients undergoing partial breast irradiation that is determined from the visibility of the seroma cavity on the MVCT image. The precision of the MVCT-based alignment was better than 2 mm (+/-standard deviation) when averaged over the patient population. Using the NSABP B-39/RTOG-0413 guidelines, acceptable APBI treatment plans can be generated using helical- or topotherapy-based delivery techniques.

The management of imaging dose during image-guided radiotherapy: report of the AAPM Task Group 75.

Radiographic image guidance has emerged as the new paradigm for patient positioning, target localization, and external beam alignment in radiotherapy. Although widely varied in modality and method, all radiographic guidance techniques have one thing in common--they can give a significant radiation dose to the patient. As with all medical uses of ionizing radiation, the general view is that this exposure should be carefully managed. The philosophy for dose management adopted by the diagnostic imaging community is summarized by the acronym ALARA, i.e., as low as reasonably achievable. But unlike the general situation with diagnostic imaging and image-guided surgery, image-guided radiotherapy (IGRT) adds the imaging dose to an already high level of therapeutic radiation. There is furthermore an interplay between increased imaging and improved therapeutic dose conformity that suggests the possibility of optimizing rather than simply minimizing the imaging dose. For this reason, the management of imaging dose during radiotherapy is a different problem than its management during routine diagnostic or image-guided surgical procedures. The imaging dose received as part of a radiotherapy treatment has long been regarded as negligible and thus has been quantified in a fairly loose manner. On the other hand, radiation oncologists examine the therapy dose distribution in minute detail. The introduction of more intensive imaging procedures for IGRT now obligates the clinician to evaluate therapeutic and imaging doses in a more balanced manner. This task group is charged with addressing the issue of radiation dose delivered via image guidance techniques during radiotherapy. The group has developed this charge into three objectives: (1) Compile an overview of image-guidance techniques and their associated radiation dose levels, to provide the clinician using a particular set of image guidance techniques with enough data to estimate the total diagnostic dose for a specific treatment scenario, (2) identify ways to reduce the total imaging dose without sacrificing essential imaging information, and (3) recommend optimization strategies to trade off imaging dose with improvements in therapeutic dose delivery. The end goal is to enable the design of image guidance regimens that are as effective and efficient as possible.

Daily variations in delivered doses in patients treated with radiotherapy for localized prostate cancer.

PURPOSE: The aim of this work was to study the variations in delivered doses to the prostate, rectum, and bladder during a full course of image-guided external beam radiotherapy. METHODS AND MATERIALS: Ten patients with localized prostate cancer were treated with helical tomotherapy to 78 Gy at 2 Gy per fraction in 39 fractions. Daily target localization was performed using intraprostatic fiducials and daily megavoltage pelvic computed tomography (CT) scans, resulting in a total of 390 CT scans. The prostate, rectum, and bladder were manually contoured on each CT by a single physician. Daily dosimetric analysis was performed with dose recalculation. The study endpoints were D95 (dose to 95% of the prostate), rV2 (absolute rectal volume receiving 2 Gy), and bV2 (absolute bladder volume receiving 2 Gy). RESULTS: For the entire cohort, the average D95 (+/-SD) was 2.02 +/- 0.04 Gy (range, 1.79-2.20 Gy). The average rV2 (+/-SD) was 7.0 +/- 8.1 cc (range, 0.1-67.3 cc). The average bV2 (+/-SD) was 8.7 +/- 6.8 cc (range, 0.3-36.8 cc). Unlike doses for the prostate, there was significant daily variation in rectal and bladder doses, mostly because of variations in volume and shape of these organs. CONCLUSION: Large variations in delivered doses to the rectum and bladder can be documented with daily megavoltage CT scans. Image guidance for the targeting of the prostate, even with intraprostatic fiducials, does not take into account the variation in actual rectal and bladder doses. The clinical impact of techniques that take into account such dosimetric parameters in daily patient set-ups should be investigated.

Real-time respiration monitoring using the radiotherapy treatment beam and four-dimensional computed tomography (4DCT)--a conceptual study.

Real-time knowledge of intra-fraction motion, such as respiration, is essential for four-dimensional (4D) radiotherapy. Surrogate-based and internal-fiducial-based methods may suffer from one or many drawbacks such as false correlation, being invasive, delivering extra patient radiation, and requiring complicated hardware and software development and implementation. In this paper we develop a simple non-surrogate, non-invasive method to monitor respiratory motion during radiotherapy treatments in real time. This method directly utilizes the treatment beam and thus imposes no additional radiation to the patient. The method requires a pre-treatment 4DCT and a real-time detector system. The method combines off-line processes with on-line processes. The off-line processes include 4DCT imaging and pre-calculating detector signals at each phase of the 4DCT based on the planned fluence map and the detector response function. The on-line processes include measuring detector signal from the treatment beam, and correlating the measured detector signal with the pre-calculated signals. The respiration phase is determined as the position of peak correlation. We tested our method with extensive simulations based on a TomoTherapy machine and a 4DCT of a lung cancer patient. Three types of simulations were implemented to mimic the clinical situations. Each type of simulation used three different TomoTherapy delivery sinograms, each with 800 to 1000 projections, as input fluences. Three arbitrary breathing patterns were simulated and two dose levels, 2 Gy/fraction and 2 cGy/fraction, were used for simulations to study the robustness of this method against detector quantum noise. The algorithm was used to determine the breathing phases and this result was compared with the simulated breathing patterns. For the 2 Gy/fraction simulations, the respiration phases were accurately determined within one phase error in real time for most projections of the treatment, except for a few projections at the start and end of the treatment in which beam intensities were extremely low. At 2 cGy/fraction dose level, the method can still determine the respiration phase very well with less than 10% of projections having more than two phases (approximately 1 s) error. This technique can also be applied in other delivery systems such as orthogonal x-ray systems, although in those cases it would entail the delivery of additional non-treatment radiation.

Deformable registration of the planning image (kVCT) and the daily images (MVCT) for adaptive radiation therapy.

The incorporation of daily images into the radiotherapy process leads to adaptive radiation therapy (ART), in which the treatment is evaluated periodically and the plan is adaptively modified for the remaining course of radiotherapy. Deformable registration between the planning image and the daily images is a key component of ART. In this paper, we report our researches on deformable registration between the planning kVCT and the daily MVCT image sets. The method is based on a fast intensity-based free-form deformable registration technique. Considering the noise and contrast resolution differences between the kVCT and the MVCT, an 'edge-preserving smoothing' is applied to the MVCT image prior to the deformable registration process. We retrospectively studied daily MVCT images from commercial TomoTherapy machines from different clinical centers. The data set includes five head-neck cases, one pelvis case, two lung cases and one prostate case. Each case has one kVCT image and 20-40 MVCT images. We registered the MVCT images with their corresponding kVCT image. The similarity measures and visual inspections of contour matches by physicians validated this technique. The applications of deformable registration in ART, including 'deformable dose accumulation', 'automatic re-contouring' and 'tumour growth/regression evaluation' throughout the course of radiotherapy are also studied.

Evaluation of two tomotherapy-based techniques for the delivery of whole-breast intensity-modulated radiation therapy.

PURPOSE: To evaluate two different techniques for whole-breast treatments delivered using the Hi-ART II tomotherapy device. METHODS AND MATERIALS: Tomotherapy uses the standard rotational helical delivery. Topotherapy uses a stationary gantry while delivering intensity-modulated treatments. CT scans from 5 breast cancer patients were used. The prescription dose was 50.4 Gy. RESULTS: On average, 99% of the target volume received 95% of prescribed dose with either technique. If treatment times are restricted to less than 9 min, the average percentage ipsilateral lung receiving > or =20 Gy was 22% for tomotherapy vs. 10% for topotherapy. The ipsilateral lung receiving > or =50.4 Gy was 4 cc for tomotherapy vs. 27 cc for topotherapy. The percentage of left ventricle receiving > or =30 Gy was 14% with tomotherapy vs. 4% for topotherapy. The average doses to the contralateral breast and lung were 0.6 and 0.8 Gy, respectively, for tomotherapy vs. 0.4 and 0.3 Gy for topotherapy. CONCLUSIONS: Tomotherapy provides improved target dose homogeneity and conformality over topotherapy. If delivery times are restricted, topotherapy reduces the amount of heart and ipsilateral lung volumes receiving low doses. For whole-breast treatments, topotherapy is an efficient technique that achieves adequate target uniformity while maintaining low doses to sensitive structures.

Automatic re-contouring in 4D radiotherapy.

Delineating regions of interest (ROIs) on each phase of four-dimensional (4D) computed tomography (CT) images is an essential step for 4D radiotherapy. The requirement of manual phase-by-phase contouring prohibits the routine use of 4D radiotherapy. This paper develops an automatic re-contouring algorithm that combines techniques of deformable registration and surface construction. ROIs are manually contoured slice-by-slice in the reference phase image. A reference surface is constructed based on these reference contours using a triangulated surface construction technique. The deformable registration technique provides the voxel-to-voxel mapping between the reference phase and the test phase. The vertices of the reference surface are displaced in accordance with the deformation map, resulting in a deformed surface. The new contours are reconstructed by cutting the deformed surface slice-by-slice along the transversal, sagittal or coronal direction. Since both the inputs and outputs of our automatic re-contouring algorithm are contours, it is relatively easy to cope with any treatment planning system. We tested our automatic re-contouring algorithm using a deformable phantom and 4D CT images of six lung cancer patients. The proposed algorithm is validated by visual inspections and quantitative comparisons of the automatic re-contours with both the gold standard segmentations and the manual contours. Based on the automatic delineated ROIs, changes of tumour and sensitive structures during respiration are quantitatively analysed. This algorithm could also be used to re-contour daily images for treatment evaluation and adaptive radiotherapy.

A novel method to correct for pitch and yaw patient setup errors in helical tomotherapy.

An accurate means of determining and correcting for daily patient setup errors is important to the cancer outcome in radiotherapy. While many tools have been developed to detect setup errors, difficulty may arise in accurately adjusting the patient to account for the rotational error components. A novel, automated method to correct for rotational patient setup errors in helical tomotherapy is proposed for a treatment couch that is restricted to motion along translational axes. In tomotherapy, only a narrow superior/inferior section of the target receives a dose at any instant, thus rotations in the sagittal and coronal planes may be approximately corrected for by very slow continuous couch motion in a direction perpendicular to the scanning direction. Results from proof-of-principle tests indicate that the method improves the accuracy of treatment delivery, especially for long and narrow targets. Rotational corrections about an axis perpendicular to the transverse plane continue to be implemented easily in tomotherapy by adjustment of the initial gantry angle.

Fast free-form deformable registration via calculus of variations.

In this paper, we present a fully automatic, fast and accurate deformable registration technique. This technique deals with free-form deformation. It minimizes an energy functional that combines both similarity and smoothness measures. By using calculus of variations, the minimization problem was represented as a set of nonlinear elliptic partial differential equations (PDEs). A Gauss-Seidel finite difference scheme is used to iteratively solve the PDE. The registration is refined by a multi-resolution approach. The whole process is fully automatic. It takes less than 3 min to register two three-dimensional (3D) image sets of size 256 x 256 x 61 using a single 933 MHz personal computer. Extensive experiments are presented. These experiments include simulations, phantom studies and clinical image studies. Experimental results show that our model and algorithm are suited for registration of temporal images of a deformable body. The registration of inspiration and expiration phases of the lung images shows that the method is able to deal with large deformations. When applied to the daily CT images of a prostate patient, the results show that registration based on iterative refinement of displacement field is appropriate to describe the local deformations in the prostate and the rectum. Similarity measures improved significantly after the registration. The target application of this paper is for radiotherapy treatment planning and evaluation that incorporates internal organ deformation throughout the course of radiation therapy. The registration method could also be equally applied in diagnostic radiology.

Megavoltage computed tomography imaging: a potential tool to guide and improve the delivery of thoracic radiation therapy.

Helical tomotherapy is an innovative means of delivering intensity-modulated radiation therapy (IMRT) using a device that merges features of a linear accelerator and a helical computed tomography (CT) scanner. The tomotherapy unit can generate CT images from the megavoltage radiation it uses for treatment as often as needed during a course of radiation therapy. These megavoltage CT (MVCT) images offer verification of patient position prior to and potentially during radiation therapy, and provide considerably more anatomical detail than the conventional radiation therapy port films used for patient set-up verification. Also, MVCT imaging may enable reconstruction of the radiation dose delivered, thereby providing unprecedented verification of the actual treatment. These key features of helical tomotherapy distinguish it from other IMRT approaches. We report results from a pilot feasibility trial of 10 patients with non-small-cell lung cancer (NSCLC) on whom we obtained MVCT images using a prototype helical tomotherapy system. All patients underwent conventional CT imaging for radiation therapy treatment planning. Specific aims were to subjectively compare MVCT and conventional CT images and then to objectively compare the 2 modalities by contouring tumors and performing a volumetric comparison. Seven patients had disease located primarily in the lung parenchyma, 2 primarily in the mediastinum, and 1 in both. When evaluated by location, all 7 patients with lesions primarily in the lung parenchyma had subjectively high-quality MVCT images. Objectively, the volumetric agreement between conventional and MVCT for parenchymal lesions was excellent in 5 of the 7 patients. Megavoltage CT imaging via the helical tomotherapy prototype provided adequate information for use in verification of patient position and dose reconstruction for lesions within the pulmonary parenchyma, but presently appears suboptimal for primarily mediastinal disease. Further studies are ongoing to optimize MVCT imaging and better define its utility in patients with NSCLC.

Methods for improving limited field-of-view radiotherapy reconstructions using imperfect a priori images.

There are many benefits to having an online CT imaging system for radiotherapy, as it helps identify changes in the patient's position and anatomy between the time of planning and treatment. However, many current online CT systems suffer from a limited field-of-view (LFOV) in that collected data do not encompass the patient's complete cross section. Reconstruction of these data sets can quantitatively distort the image values and introduce artifacts. This work explores the use of planning CT data as a priori information for improving these reconstructions. Methods are presented to incorporate this data by aligning the LFOV with the planning images and then merging the data sets in sinogram space. One alignment option is explicit fusion, producing fusion-aligned reprojection (FAR) images. For cases where explicit fusion is not viable, FAR can be implemented using the implicit fusion of normal setup error, referred to as normal-error-aligned reprojection (NEAR). These methods are evaluated for multiday patient images showing both internal and skin-surface anatomical variation. The iterative use of NEAR and FAR is also investigated, as are applications of NEAR and FAR to dose calculations and the compensation of LFOV online MVCT images with kVCT planning images. Results indicate that NEAR and FAR can utilize planning CT data as imperfect a priori information to reduce artifacts and quantitatively improve images. These benefits can also increase the accuracy of dose calculations and be used for augmenting CT images (e.g., MVCT) acquired at different energies than the planning CT.

Limited-data image registration for radiotherapy positioning and verification.

PURPOSE: One benefit to having on-line CT imaging integrated into a radiotherapy system is that images can be collected with the patient in the treatment position. These images can then be automatically registered to planning images for improved positioning and verification. However, many such on-line imaging systems have a limited field of view (LFOV) that could potentially impair registration. Thus, the viability of automatic registration was investigated in the context of collecting on-line LFOV and also limited-slice CT images for radiotherapy. METHODS AND MATERIALS: Mutual information and two new voxel-based registration algorithms were tested to align LFOV and limited-slice prostate and breast images given known displacements. Success rates were tallied for different field-of-view sizes, slice distributions, and initial displacements. RESULTS: Most of the automatic registration algorithms tested were useful for solving these LFOV and limited-slice problems. Registration of LFOV images was generally successful, especially for fields of view of at least half the patient's size. For limited-slice images, success was more closely correlated to the slice spacing than to the number of slices used, with sparse slice spacing being preferable. CONCLUSIONS: Mutual information and other automatic registration algorithms have been identified as useful methods for registering LFOV and limited-slice radiotherapy images with planning CT images.