Prior studies have suggested that females and patients with low body mass index (BMI) are at higher risk of hematologic toxicity during pelvic chemoradiation, but the reasons remain unknown. We conducted a multi-institutional retrospective analysis to explore this association. Fifty-six patients with stage I-III anal cancer were treated at 2 academic centers with concurrent 5-FU and mitomycin C (days 1, 28) and RT. Median RT dose was 54 Gy to the primary tumor, 45 Gy to the true pelvis, and 40 Gy to the whole pelvis, using IMRT (n = 49) or 3D planning (n = 7). Patient demographics, pelvic bone marrow (BM) volume, and BM dose metrics were correlated with blood counts at week 3 of treatment. This endpoint was chosen because it correlated with initial blood count nadirs and would be less subject to confounding from treatment breaks, dose reductions, and GM-CSF that occur after the initial nadir. To find what metric best predicted grade 3 toxicity, receiver-operator characteristics (ROC) were calculated for total BM volume receiving < 5-40 Gy, and volumetric sparing goals were tested by 50 cc intervals at each dose level. Females had smaller pelvic BM volumes than males (mean 1224 vs 1508 cc, P < 0.01). On univariate analysis, several factors correlated with low white blood cell (WBC) and neutrophil (ANC) counts: smaller total BM volume (and each subsite of lower pelvic, lumbosacral, and iliac BM), greater relative V40 Gy to lower pelvic BM, and smaller absolute volume of BM receiving <30 or 40 Gy (all P < 0.05). Female gender correlated with lower ANC (P = 0.03). Patients in the top quartile of lumbosacral BM volume were spared from grade 3 nadirs in WBC (0% vs 22.5%, P = 0.043) and ANC (0% vs 27.8%, P = 0.031) compared to others. On multivariate analysis, smaller pelvic BM volume correlated with the severity of WBC (P = 0.04) and ANC nadirs (P = 0.04), but gender and dose metrics did not (P > 0.1). If at least 750 cc of total BM received < 30 Gy (i.e. ∼10 Gy by week 3), patients were spared from grade 3 leukopenia (0% vs. 24%, P = 0.02) and neutropenia (0% vs 30%, P = 0.01) compared to those who did not meet this goal. If this goal was not met, a previously proposed parameter (lower pelvic BM V40 Gy) remained associated with grade 3 leukopenia, with the top quartile having a rate of 46.2%, vs 12.5% for others (P = 0.03).Abstract 2528; Table 1.BM receiving < "X" dose (Gy)Area Under ROC Curve for WBCArea Under ROC Curve for ANCBest Fit Volume-Sparing Goal (cc) for WBC (P-value)Best Fit Volume-Sparing Goal (cc) for ANC (P-value)50.590.52100 (0.25)100 (0.40)100.620.57200 (0.19)200 (0.11)150.620.56300 (0.14)250 (0.29)200.670.62400 (0.04)400 (0.03)300.770.77750 (0.02)750 (0.01)400.790.781100 (0.01)1200 (0.04) Open table in a new tab Smaller absolute pelvic BM volume correlates with WBC and ANC nadirs, which may explain prior associations with female gender and low BMI. We propose that BM be modeled as a parallel organ that can maintain proper function provided that a critical "reserve" volume, such as 750 cc, remains spared from moderate RT doses.

Purpose:The use of non‐circular scanning trajectories with optimization‐basedreconstruction algorithms can be used in conjunction with non‐planaracquisition geometries for axial field‐of‐view (FOV) extension incone‐beam CT (CBCT). To evaluate the utility of these trajectories,quantitative image quality metrics should be evaluated. Low‐contrastresolution (LCR) and CT number accuracy are significant challenges forCBCT. With unprecedented axial coverage provided by thesetrajectories, measuring such metrics throughout the axial range iscritical. There are currently no phantoms designed to measurelow‐contrast resolution over such an extended volume.Methods:The CATPHAN (The Phantom Laboratory, Salem NY) is the current standardfor image quality evaluation. While providing several useful modulesfor different evaluation metrics, each module was designed to beevaluated in a single slice and not for comparison across axialpositions. To characterize the LCR and HU accuracy over an extendedaxial length, we have designed and built a phantom with evaluationmodules at multiple and adjustable axial positions.Results:The modules were made from a cast polyurethane resin. Holes rangingfrom 1/8 to 5/8 inch were added at a constant radius from the modulecenter into which rods of two different plastic materials were pressedto provide two nominal levels of contrast (1.0% and 0.5%). Largerholes were bored to accept various RMI plugs with known electrondensities for HU accuracy evaluation. The modules can be inserted intoan acrylic tube long enough to cover the entire axial FOV and theirpositions adjusted to desired evaluation points.Conclusion:This phantom allows us to measure the LCR and HU accuracy across theaxial coverage within a single acquisition. These metrics can be usedto characterize the impact different trajectories and reconstructionparameters have on clinically relevant image quality performancemetrics.Funding was provided in part by Varian Medical Systems and NIH R01 Grants Nos. CA158446, CA182264, EB018102, and EB000225. The contents of this poster are solely the responsibility of the authors and do not necessarily represent the official view of any of the supporting organizations.

Dose-volume histogram (DVH)–toxicity relationships are not well defined in the treatment of postoperative prostate cancer. We evaluated 2 sets of normal tissue-sparing criteria for their ability to minimize late toxicity. We also evaluated dose to the bladder trigone and bladder outside of the prostate bed (bladder clinical target volume [CTV]) as normal tissue structures. One hundred sixty-four men were treated with radiation therapy (RT) between 2001 and 2012 after prostatectomy (RP). Median age was 61 years. Median time from RP to RT was 22 months, given as adjuvant (13%) or salvage (87%) therapy. Forty-six percent had concurrent hormonal therapy (androgen deprivation therapy [ADT]). Intensity modulated RT was given to the pelvic lymph nodes (in 46%) and prostate bed to a median dose of 66.6 Gy (interquartile range [IQR], 66-68.4 Gy). The trigone was contoured (n=89) as the posterior portion of the bladder at the ureters to the vesicourethral junction (median 16 mL). The median volumes of the bladder and bladder-CTV (n=89) were 180 and 154 mL, respectively. DVH relationships for the rectum, bladder, trigone, and bladder-CTV were analyzed against CTC late (>3 months after RT) grade 2+ (G2+) gastrointestinal (GI) and genitourinary (GU) toxicity by the log-rank test. Rectum and bladder constraints evaluated included Radiation Therapy Oncology Group (RTOG) 0534 (rectum V65,40Gy ≤35,55%, and bladder-CTV V65,40 ≤50,70%) and institutional guidelines for intact prostate cancer proposed in 2007 (rectum V70,65,40 ≤20,40,80% and bladder V70,65,40 ≤30,60,80%, respectively). Median follow-up was 33 months. Four year freedom from (FF) G2+ GI and GU toxicity were both 91%. GI and GU toxicity included 50% diarrhea, 50% hemorrhage, and 8% proctitis (n=12), and 46% frequency, 13% obstructive, 20% cystitis, and 33% incontinence (n=15), respectively. RTOG 0534 rectum and bladder goals were not achieved in 65% and 41% of cases, while institutional goals were not achieved in 21% and 25% of cases, respectively. However, none of these proposed goals were associated with late toxicity (P>.1). Median trigone V70 Gy was 23%. Dose to the bladder trigone was not associated with GU toxicity, although V70 Gy <25% trended toward an association (4 year FFG2+ GU toxicity, 94% vs 87%, P=.2). Meanwhile, dose to the bladder-CTV was not associated with GU toxicity at various dose levels. Univariate analysis for age, pelvic RT, RT dose, anticoagulation use, ADT, time from RP to RT, and tobacco history were not associated with toxicity. No tested tissue parameters were associated with post-RP RT late toxicity, perhaps influenced by the low rate of toxicity. In the absence of any established DVH relationships in the post-RP setting, it is reasonable to use normal tissue sparing guidelines for intact prostate cancer. Dose to the trigone requires further study and may be more meaningful than dose to the bladder or bladder-CTV.

Imaging plays an important role in the delivery of external beam radiation therapy. It is used to confirm the setup of the patient and to ensure accurate targeting and delivery of the therapeutic radiation dose. Most modern linear accelerators come equipped with a flat panel detector opposite the MV source as well as an independent kV imaging system, typically mounted perpendicular to the MV beam. kV imaging provides superior soft tissue contrast and is typically lower dose to the patient than MV imaging, however it can suffer from artifacts caused by metallic objects such as implants and immobilization devices. In addition to being less artifact prone, MV imaging also provides a direct measure of the attenuation for the MV beam which is useful for computing the therapeutic dose distributions. Furthermore either system requires a large angular coverage, which is slow for large linear accelerators. We present a method for reconstructing tomographic images from data acquired at multiple x-ray beam energies using a statistical model of inherent physical properties of the imaged object. This approach can produce image quality superior to traditional techniques in the case of limited measurement data (angular sampling range, sampling density or truncated projections) and/or conditions in which the lower energy image would typically suffer from corrupting artifacts such as the presence of metals in the object. Both simulation and real data results are shown.

Purpose: Cone-beam CT (CBCT) in image-guide radiation therapy (IGRT) typicallyacquires scan data via the circular trajectory of the linearaccelerator's (linac) gantry rotation. Though this lends itself toanalytic reconstruction algorithms like FDK, iterative reconstructionalgorithms allow for a broader range of scanning trajectories. Weimplemented a non-circular scanning trajectory with Varian's TrueBeamDeveloper Mode and performed some preliminary reconstructions toverify the geometry. Methods: We used TrueBeam Developer Mode to program a new scanning trajectorythat increases the field of view (FOV) along the gantry rotation axiswithout moving the patient. This trajectory consisted of moving thegantry in a circle, then translating the source and detector along theaxial direction before acquiring another circular scan 19 cm away fromthe first. The linear portion of the trajectory includes an additional4.5 cm above and below the axial planes of the source's circularrotation. We scanned a calibration phantom consisting of a lucite tubewith a spiral pattern of CT spots and used the maximum-likelihoodalgorithm to iteratively reconstruct the CBCT volume. Results: With the TrueBeam trajectory definition, we acquired projection dataof the calibration phantom using the previously described trajectory.We obtained a scan of the treatment couch for log normalization byscanning with the same trajectory but without the phantom present.Using the nominal geometric parameters reported in the projectionheaders with our iterative reconstruction algorithm, we obtained acorrect reconstruction of the calibration phantom. Conclusion: The ability to implement new scanning trajectories with the TrueBeamDeveloper Mode enables us access to a new parameter space for imagingwith CBCT for IGRT. Previous simulations and simple dual circle scanshave shown iterative reconstruction with non-circular trajectories canincrease the axial FOV with CBCT. Use of Developer Mode allowsexperimentally testing these and other new scanning trajectories. Support was provided in part by the University of Chicago Research Computing Center, Varian Medical Systems, and NIH Grants 1RO1CA120540, T32EB002103, S10 RR021039 and P30 CA14599. The contents of this work are solely the responsibility of the authors and do not necessarily represent the official views of the supporting organizations.

With the increased prevalence of complex radiation treatments utilizing noncoplanar beams, and CBCT in IGRT, collisions between patient, couch and gantry are of rising concern in radiation therapy. The actual patient anatomy is necessary for reliable collision predictions and the limited region scanned for treatment planning is often insufficient. Detection of potential collisions during treatment simulation and planning using a surface image of the patient is a novel technique. It can help ensure patient safety, reduce potential machine damage and streamline treatment delivery in the clinic. We created a fast and efficient collision detection package that incorporates existing CT planning data with captures from a surface imaging system to help resolve these issues. This package consists of two different components to assess collisions with the collimator and imaging arms using the actual patient space. A computer model of the gantry and imaging arms was used. The patient space encompasses patient anatomy, immobilization devices and the treatment couch. The patient anatomy was generated from a surface image captured with a commercial surface imaging system and registered to the skin surface from the planning CT. The 3D model of the immobilization device was created using a Kinect camera. The couch was modeled as a slab. All three components of the patient space were combined and placed at the appropriate treatment position. The first program evaluates a full gantry rotation with the couch at 0⁰, consistent with imaging and some VMAT procedures. It allows for the addition of a safety margin to account for possible patient position inconsistencies. This program outputs the gantry angles of collision, if any, and couch shifts necessary for clearance. This can be used to detect collisions during CBCT and determine the potential "treatment collision-free space" at the CT simulator for treatment planning. The second program takes a specific gantry-couch angle combination and produces a colormap of the patient geometry to indicate minimum clearance distance and highlight potential collisions. This is useful to assess valid treatment geometries for planning noncoplanar beams. We successfully calculated the collision-free space and gantry-couch angle combinations, as well as the necessary couch shifts to avoid collision using the software package presented. The results were validated in the treatment room with measurements on a phantom. This software allows for reliable and efficient collision detection using an accurate model of the full patient space. The implementation of this package in the clinic can lead to improved patient safety, elimination of machine collisions during treatment and reduction of treatment planning and delivery times.

Urinary incontinence is a common complication following prostatectomy for prostate cancer. There are limited data available on dosimetric parameters that may predict for poor continence recovery in men who receive post-operative intensity modulated radiation therapy (IMRT). The aim of this study was to analyze dosimetric or clinical factors that may correlate to patient-reported outcomes. Ninety-four men with non-metastatic prostate cancer who underwent prostatectomy followed by adjuvant (12%) or salvage (88%) IMRT were included in this study. The median age was 61, and median time from surgery to IMRT was 23.3 months. Androgen deprivation therapy (ADT) was given in 55%. The median dose to the prostate bed was 68 Gy; 56% of men also received a median dose of 50.4 Gy to the pelvic lymph nodes. Clinical characteristics and patient-reported outcomes using the modified expanded prostate cancer index composite (EPIC) questionnaire were collected at baseline (n = 87) and post-treatment at 6 wk (n = 81), 6 mo (n = 78), 12 mo (n = 74), 18 mo (n = 56), 24 mo (n = 53), and 36 mo (n = 41), and 48 mo (n = 27). Dose-volume metrics of relevant structures, including the bladder, genitourinary diaphragm (GUD), vesicourethral junction (VUJ), and penile bulb (PB) were retrospectively contoured and collected using a prospectively defined approach. The distance between the VUJ and GUD (VUJ-GUD) was also measured. The primary endpoint was urinary incontinence as measured by the global score derived from the EPIC questionnaire. Generalized estimating equation (GEE) models were used to test the association of clinical and dosimetric variables with urinary incontinence. The median EPIC urinary incontinence global scores for each time point were: 66.5 at baseline, 72.8 at 6 wk, 72.8 at 6 mo, 72.8 at 12 mo, 66.5 at 18 mo, 75 at 24 mo, 72.8 at 36 mo, and 79 at 48 mo (all p>0.05 compared to baseline by Wilcoxon test). In GEE models adjusted for by baseline continence, bladder V70Gy and penile bulb V70 Gy were associated with urinary incontinence (both p<0.05), while age, race, RT dose, pelvic nodal RT, ADT, time to RT, diabetes, BMI, VUJ-GUD, and lower dose metrics to bladder and penile bulb were not (all p>0.05). In a model which included body mass index (BMI), VUJ-GUD, time from surgery, age, diabetes, and bladder V70Gy, only the bladder V70Gy (p = 0.02) was associated with outcome. The median bladder V70Gy was 21.86 cc. Volumetrically, the median V70Gy was 11%. In this cohort of men, the bladder V70Gy was significantly associated with urinary incontinence after adjuvant or salvage IMRT. This planning metric may play an important role towards preserving continence after post-prostatectomy RT.

Purpose:For image guidance tasks full image quality is not required throughout the entire image. With dynamic filtration of the kV imaging beam the noise properties of the CT image can be locally controlled, providing a high quality image around the target volume with a lower quality surrounding region while providing substantial dose sparing to the patient as well as reduced scatter fluence on the detector.Methods:A dynamic collimation device with 3mm copper blades has been designed to mount in place of the bowtie filter on the On‐Board Imager (Varian Medical Systems). The beam intensity is reduced by 95% behind the copper filters and the aperture is controlled dynamically to conformally illuminate a given ROI during a standard cone‐beam CT scan. A data correction framework to account for the physical effects of the collimator prior to reconstruction was developed. Furthermore, to determine the dose savings and scatter reduction a monte carlo model was built in BEAMnrc with specifics from the Varian Monte Carlo Data Package. The MC model was validated with Gafchromic film.Results:The reconstructed image shows image quality comparable to a standard scan in the specified ROI, with higher noise and streaks in the outer region but still sufficient information for alignment to high contrast structures. The monte carlo modeling showed that the scatter‐to‐primary ratio was reduced from 1.26 for an unfiltered scan to 0.45 for an intensity weighted scan, suggesting that image quality may be improved in the inner ROI. Dose in the inner region was reduced 10–15% due to reduced scatter and by as much as 75% in the outer region.Conclusion:Dynamic intensity‐weighted ROI imaging allows reduction of imaging dose to sensitive organs away from the target region while providing images that retain their utility for patient setup and procedure guidance.Funding was provided in part by Varian Medical Systems and NIH Grants 1RO1CA120540, T32EB002103, S10 RR021039 and P30 CA14599.The contents of this work are solely the responsibility of the authors and do not necessarily represent the official views of any of the supporting organizations.

6 patients with standard and segmented DWI presented with visible distortion of the posterior prostate boundary abutting the endorectal coil relative to T2w on standard, but not on segmented DWI. In 2 of 4 patients with posterior tumors abutting the prostate boundary, tumors were erroneously displaced outside of the prostate boundary in standard DWI, which was corrected using segmented DWI. Conclusions: Readout-segmented DWI improves the geometric performance of clinical endorectal coil prostate DWI at 3T, and should reduce uncertainties in tumor delineation for dose escalation. Author Disclosure: W. Foltz: None. T. Stanescu: None. J. Lee: None. A. Simeonov: None. D. Jaffray: None. T. Craig: None. P. Chung: None. C. Menard: None.

Martyna Elas , Jessica M. Magwood , Brandi Butler , Chanel Li , Rona Wardak, Eugene D. Barth , Boris Epel , Samuel Rubinstein , Charles A. Pelizzari, Ralph R. Weichselbaum 1 and Howard J. Halpern Departments of: Radiation and Cellular Oncology and Radiology, University of Chicago Pritzker School of Medicine, Chicago, IL; Center for Electron Paramagnetic Resonance Imaging In Vivo Physiology, University of Chicago, Chicago, IL; Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, Krakow, Poland

Human Papillomavirus (HPV)-associated head and neck cancers have improved prognosis suggesting that de-intensified treatments may adequately control tumors while minimizing toxicity. However, at present, the animal models to study the radiosensitivity of HPV tumors are limited to xenotransplants in immunosuppressed models and to irradiation of tumors at non-orthotopic sites. Here, we developed a spontaneous HPV-associated oral tumor model that was treated with image guided radiation therapy. To generate an inducible HPV (iHPV) model, we placed a loxP-STOP-loxP cassette between a CMV promoter and an HPV E6E7 oncogene that was upstream of a luciferase gene in order to monitor E6E7 expression using bioluminescence. We generated triple transgenic mice expressing this iHPV construct as well as a K14-CreERtam transgene (K14) that expressed a tamoxifen regulated Cre recombinase in the basal epithelial layer and an inducible K-ras transgene (iRas) containing a G12D mutation (KHR mice). When KHR mice were treated with tamoxifen, the Cre-ERtam recombined the iHPV and iRas alleles to induce the E6E7 and mutant K-ras oncogenes. Mice bearing oral tumors were irradiated to 8 Gy or 20 Gy using an X-RAD 225Cx small animal image guided irradiator to avoid irradiating adjacent vital organs. Tamoxifen treatment resulted in recombination of the iHPV and iRas transgenes and expression of their respective mRNA transcripts. Within 12 days of tamoxifen treatment, KHR mice developed oral cavity tumors faster than double transgenic K14 x iRas (KR) mice (doubling times: KHR tumors 4.5 days vs KR tumors 8 days p < 0.001). Furthermore, KHR mice had a shorter median survival after tamoxifen induction compared to KR mice (median survival: 24 days for KHR mice vs 34 days for KR mice; p < 0.001). Using bioluminescence, tumor development in KHR mice correlated with increased luciferase activity within 6 days after tamoxifen treatment and reached a maximum of 74.8-fold higher bioluminescence than uninduced tumors (p = 0.0007). Histological examination of KHR tumors demonstrated more dysplastic lesions in KHR mice and increased MCM7 expression, a known downstream target of the E7 oncogene. KHR tumors irradiated to 20 Gy regressed within 7 days and demonstrated decreased bioluminescence and increased tumor cell apoptosis (p = 0.02). We developed a novel, inducible HPV oral tumor model where tumors arose in immunocompetent hosts and regressed after image guided radiation therapy. This model will enable us to study how HPV oncogenes impact tumor and host responses after irradiation in order to more rationally de-escalate radiation therapy regimens and to improve outcomes for head and neck cancer patients.

Purpose: Intra‐fraction organ motion can be problematic for highly conformal radiation therapy techniques. Fluoroscopy with the on‐board imager available on many linear accelerators can provide real time tumor position information however potentially high radiation dose can be a concern. The target may only account for a few percent of the image area, thus using a dynamic collimation device to restrict the field of view to an appropriate region‐of‐interest and follow its motions the imaging dose can be substantially reduced. Methods: A prototype collimator has been developed which can be mounted to the kV x‐ray source of a Varian Trilogy linear accelerator in place of the standard bowtie filter. The collimator tracking was tested using realistic motion profiles derived from patient data and a 3‐axis motion stage with a CT spot as the target fiducial. The clinical system does not currently provide real time access to the image data so the collimator was fed the fiducial position from the motion control system with controllable lag to simulate image‐processing time. Results: In the acquired fluoroscopic data the collimator blade edges were identified using a hough line transform on the thresholded gradient magnitude image and the fiducial position was identified with a template matching technique. Over four minutes of continuous tracking with intermittent burst of fluoroscopic imaging the fiducial was never more than 1.7mm from the aperture center and the root mean square distance was 0.8mm. Conclusion: The dynamic collimator exhibits accuracy that could enable highly conformal fluoroscopic imaging to provide real‐time target position information with greatly reduced imaging dose. This work was funded, in part, by Varian Medical Systems, Palo Alto CA. The contents of this work are solely the responsibility of the authors and do not necessarily represent the official views of any supporting organizations.

Purpose: Cone-beam CT (CBCT) is widely used for providing image guidance in radiotherapy and interventional procedures. Due to the patient safety concern involved in repetitive CBCT scans, significant effort has been devoted to possibly lowering imaging dose in CBCT. For a given total dose, it is also important to investigate how image quality can be affected by dose allocations over projection views and by image-reconstruction algorithms. In this work, we investigate quantitatively how image quality changes resulted from different combinations of dose-allocation parameters and some existing reconstruction algorithms. Methods: We performed simulation studies by using a numerical phantom containing structures ofdifferent contrast levels. We also acquired real data of a Catphan phantom (The Phantom Laboratory, Salem, NY) by using the on-board imaging system (Varian Medical Systems, Palo Alto, CA). At three different total dose levels, we allocated each of them to different numbers of views ranging from 120 to 650. For each allocation scheme we applied the FDK and ASD-POCS algorithms to obtain reconstructed images. A set of quantitative metrics were used to evaluate image quality based on relevant tasks. Results: Preliminary results showed that for total dose levels under study, both FDK and ASD-POCS algorithms yield images with comparable quality when a large number of views are considered and that images reconstructed by the ASD-POCS from smaller number of views generally exhibit higher quality. Overall, for each given total dose level the ASD-POCS algorithm yields images of comparable or higher quality than does the FDK algorithm. Conclusions: We demonstrated that CBCT image quality can be optimized for a fixed total dose by choosing an appropriate combination of dose- allocation parameters and reconstruction algorithm. This finding may potentially be used for improving current CBCT image quality and for designing innovative, low-dose CBCT imaging protocols.

Purpose: The purpose of this study was to quantify the effect of deformable registration on the values of lung texture features extracted from chest computed tomography (CT) scans. Methods: Two clinical chest CT scans (slice thickness=1mm, pixel spacing<0.9mm, time between scans: 1 week–3 years) were collected for each of nine patients with no evidence of abnormal lung pathology. Automated lung segmentation was performed, and a deformable registration algorithm (“demons”) was employed to register each patientˈs follow‐up scan to their original baseline scan. Deformation accuracy was evaluated through inspection of a difference image and measurement of the mean Euclidean distance between landmarks in the baseline scan and deformed follow‐up scan. Over 1,500 spatially‐matched region‐of‐interest (ROI) pairs were extracted from each scan pair. First‐order, fractal, Fourier, Lawsˈ filter, and gray‐level concurrence texture features (196 total features) were computed for each ROI, and the percent change of each feature between matched ROIs was calculated. Because scans contained normal pathology, we expected texture changes to be introduced primarily by deformation. Fourteen features with a percent change standard deviation less than 10% across patients were identified. Analysis of variance (ANOVA) was performed to determine whether percent changes in texture features between patients were statistically different. Results: ANOVA showed mean percent changes in texture values to be significantly different between patients for all fourteen features (p<0.001); nevertheless, percent changes in texture features between baseline and follow‐up deformed scans were consistently small across patients. Twelve of fourteen features selected demonstrated mean percent differences less than 5%. These features were distributed among first‐order, fractal, and Lawsˈ filter classes. Conclusions: For some texture features, deformable registration may be performed without altering the feature value by more than 5% on average. These “invariant” features have potential for use in texture‐based evaluation of progressive lung disease using serial scans requiring registration.

Purpose: Kilovoltage CBCT has become a useful clinical tool which provides in-room image guidance for radiation therapy. Frequent use of CBCT also raises patient safety concern, because tomographic images are obtained by reconstruction from projection data acquired at hundreds of angular views, which may accumulate to an un-negligible amount of radiation dose. It is therefore desired to develop a low-dose CBCT imaging technique that requires smaller amount of projection data while can still yield images sufficing clinical needs. In this work, we have applied a new TV-minimization algorithm to reconstruct CBCT images from a fraction of the amount of projection data currently used and have evaluated the image quality according to established quality-assurance procedures.Materials and methods: We have acquired real projection data of a CATPHAN phantom using a clinical on-board imaging system (Varian Medical Systems, Palo Alto, CA). We then applied the TV-minimization algorithm to reconstruct images from subsets of the full, 628-projection data set. The subsets contain from 360 down to 60 projections. We then examined the reconstructed images by calculating a number of metrics, including spatial resolution, contrast linearity, and low-constrast resolution, and compared the results to the established quality-assurance (QA) standards. Results: The images reconstructed by use of the TV-minimization algorithm from the reduced data sets appeared to be comparable to the images reconstructed from the full data set by the FDK algorithm. In addition, the tested image-quality metrics can satisfy the QA requirement for images reconstructed from as few as 100 projections. Conclusion: The TV-minimization algorithm can yield CBCT images from reduced projection data without significant sacrifice of image quality. The images reconstructed from about 100 projections may be potentially useful for clinical applications according to QA results.

Purpose: As a result of internal organ motion, the ITV used during treatment planning must be larger than the CTV, reducing the benefit of highly conformal RT. In prostate cancer cases, there is no strong correlation between internal organ motion and externally visible motion, so application of IGRT requires internal imaging. Previous work has shown the effectiveness of real‐time 3D fiducial tracking using reduced aperture combined MV‐kV imaging. The aim of this work further advances MV‐kV tracking by incorporation of delivered kV tracking dose at the treatment planning stage through combined MV‐kV beam modeling. Method and Materials: A Monte Carlo scheme was developed to model dose deposition kernels that were imported into Phillips' Pinnacle treatment planning system. The kV source of an on‐board‐imager from a Varian Trilogy LINAC was commissioned in the TPS. Using previously treated 9‐field prostate patients, the TPS was used to calculate dose that would be delivered by the kV beam during real‐time tracking. To provide stereoscopic imaging at each gantry angle during treatment, the kV and MV beam times were matched. Apertures studied included a small 3×3 cm field capable of tracking closely placed fiducials in prostate cases. Results: In the four patients studied, the average prostate dose from a 3×3 cm field was 5 cGy, increasing to 9 cGy per fraction for a CBCT aperture, 2–5% of the prescribed dose. The 3×3 cm aperture spared more tissue, delivering 5 cGy or more per fraction to less than 1% of a patient volume, compared to 80% for the full field. Conclusion: The use of conformal kV apertures shaped to the tracking region of interest can lead to significant reductions in total kV diagnostic radiation delivered. This allows for the kV beam to provide useful tracking information, while simultaneously delivering therapeutic dose to the prostate.

Purpose: Currently, real‐time 3D MV‐kV monitoring requires the use of continuous kV imaging throughout the treatment process leading to high diagnostic dose costs. For MV‐kV tracking purposes the only needed kV image information are the projected images of the metallic fiducial markers. Generally these markers are small (3mm in length and 0.8mm in diameter), and for a standard 40cm × 30cm kV image comprise less than 1% of the total area. The proposed technique here uses a dynamic kV aperture to confine the kV exposure to a small region of interest (ROI) encompassing only the markers. As the internal markers move, the aperture is dynamically updated using feedback information provided by the last known marker positions. Method: A Varian Trilogy equipped with both an EPID and a kV imaging system was used. The kV collimator was mounted over the kV source and consists of four lead blades placed orthogonally on low friction linear guide rails. The position of each blade was controlled independently using a servomotor. MV‐kV imaging was performed and software was used to calculate a suitable ROI that will be used as an input for the kV collimator. As the internal markers move, the aperture will be dynamically updated using the last known marker positions. Results: The combination of controller circuitry and chosen servomotors allows for blade travel speed of up to 11mm/s, depending on orientation, with an accuracy of 0.2mm in the collimator plane. This should be sufficient to keep the ROI properly centered on nearly any fiducial cluster. Conclusion: The technique proposed here would potentially lower the kV exposure by a factor of 50–500 depending on the speed, number, and spatial separation of the fiducials. The technology is directly applicable to any kV imaging system where only selective ROI information is required.