Megavoltage CT Images Research Articles

Effect of jaw size in megavoltage CT on image quality and dose

Recently, the jaw size for the TomoTherapy Hi-Art II(®) (TomoTherapy Inc., Madison, WI) was reduced from 4 mm (J4) to 1 mm (J1) to improve the longitudinal (IEC-Y) resolution in megavoltage computed tomography (MVCT) images. This study evaluated the effect of jaw size on the image quality and dose, as well as the dose delivered to the lens of the eye, which is a highly radiosensitive tissue. MVCT image quality (image noise, uniformity, contrast linearity, high-contrast resolution, and full width at half-maximum) and multiple scan average dose (MSAD) were measured at different jaw sizes. A head phantom and photoluminescence glass dosimeters (PLDs) were used to measure the exposed lens dose (cGy). Different MVCT scan modes (pitch = 1, 2, and 3) and scan lengths (108 mm, 156 mm, and 204 mm) were applied in the MSAD and PLDs measurements. The change in jaw size from J4 to J1 produced no change or only a slight improvement in image noise, uniformity, contrast linearity, and high-contrast resolution. However, the full-width at half-maximum reduced from approximately 7.2 at J4 to 4.5 mm at J1, which represents an enhancement in the longitudinal resolution. The MSAD at the center point changed from approximately 0.69-2.32 cGy (peripheral: 0.83-2.49 cGy) at J4 to 0.85-2.81 cGy (peripheral: 1.05-2.86 cGy) at J1. The measured lens dose increased from 0.92-3.36 cGy at J4 to 1.06-3.91 cGy at J1. The change in jaw size improved longitudinal resolution. The MVCT imaging dose of approximately 3.86 cGy, 1.92 cGy, and 1.22 cGy was delivered at a pitch of 1, 2, and 3, respectively, per fraction in the head and neck treatment plans. Therefore, allowance for an approximately 15% increase in lens dose over that with J4 should be provided with J1.

High‐dose MVCT image guidance for stereotactic body radiation therapy

Stereotactic body radiation therapy (SBRT) is a potent treatment for early stage primary and limited metastatic disease. Accurate tumor localization is essential to administer SBRT safely and effectively. Tomotherapy combines helical IMRT with onboard megavoltage CT (MVCT) imaging and is well suited for SBRT; however, MVCT results in reduced soft tissue contrast and increased image noise compared with kilovoltage CT. The goal of this work was to investigate the use of increased imaging doses on a clinical tomotherapy machine to improve image quality for SBRT image guidance. Two nonstandard, high-dose imaging modes were created on a tomotherapy machine by increasing the linear accelerator (LINAC) pulse rate from the nominal setting of 80 Hz, to 160 Hz and 300 Hz, respectively. Weighted CT dose indexes (wCTDIs) were measured for the standard, medium, and high-dose modes in a 30 cm solid water phantom using a calibrated A1SL ion chamber. Image quality was assessed from scans of a customized image quality phantom. Metrics evaluated include: contrast-to-noise ratios (CNRs), high-contrast spatial resolution, image uniformity, and percent image noise. In addition, two patients receiving SBRT were localized using high-dose MVCT scans. Raw detector data collected after each scan were used to reconstruct standard-dose images for comparison. MVCT scans acquired using a pitch of 1.0 resulted in wCTDI values of 2.2, 4.7, and 8.5 cGy for the standard, medium, and high-dose modes respectively. CNR values for both low and high-contrast materials were found to increase with the square root of dose. Axial high-contrast spatial resolution was comparable for all imaging modes at 0.5 lp∕mm. Image uniformity was improved and percent noise decreased as the imaging dose increased. Similar improvements in image quality were observed in patient images, with decreases in image noise being the most notable. High-dose imaging modes are made possible on a clinical tomotherapy machine by increasing the LINAC pulse rate. Increasing the imaging dose results in increased CNRs; making it easier to distinguish the boundaries of low contrast objects. The imaging dose levels observed in this work are considered acceptable at our institution for SBRT treatments delivered in 3-5 fractions.

TU-E-BRA-07: Post-Operative Eye Plaque Imaging Using Tomotherapy MVCT.

Intra-operative ultrasound is used to verify the positioning of episcleral eye plaques used to treat ocular melanoma. Ultrasound can be ambiguous because of image artifacts, and plaques may shift position after surgery. Ultrasound verification is particularly challenging for anterior tumors. Post-operative imaging could be used to trigger interventions that would prevent local treatment failure. We investigated if, and under what conditions, the Tomotherapy megavoltage computed tomography (MVCT) system could be used to perform post-implantation verification of eye plaque positioning. Plaques were placed on a preserved cow's eye, and imaged with the megavoltage CT of a Tomotherapy linear accelerator (Accuray, Sunnyvale, CA). The images were visually and quantitatively assessed to determine if they were of sufficient quality to verify tumor coverage and plaque tilt with respect to the sclera. We used the visibility of the lens as a proxy for visibility of a tumor. To test the utility of hypothetical higher beam current Tomotherapy images, we averaged sequential images of the same setup. The plaque, the lens of the eye, and the globe are visible in the images. The CNR of the lens with respect to the vitreous was 5.6 for a single image. For 10 images averaged, the CNR was 9.2. Estimated dose from a single image was 1.3 cGy (body CTDIvol); even 10 times this dose would be an acceptable image-guidance dose for radiotherapy patients. One limitation of the imaging procedure is the long scan time (up to 240 seconds), during which time any significant patient motion would lead to image artifacts. Human trials on eye plaque patients are planned. Tomotherapy MVCT imaging could be used to verify tumor coverage and plaque tilt after episcleral plaque implantation. Tumors should be visible in standard Tomotherapy images but higher beam current images would be preferred if available.

SU-D-217BCD-03: Impact of Jaw Width on the Megavoltage Computed Tomography Image Quality and Imaging Dose with Helical Tomotherapy.

On-treatment megavoltage computed tomography on Helical Tomotherapy (Accuray Inc., Sunnyvale, CA) is critical for image guided radiotherapy. A strategy was developed to assess the impact of various jaw widths on image quality and imaging dose with Tomotherapy. A cheese phantom (Gammex RMI, Middleton, WI) made of water equivalent materials was employed in this study. Three sets of measurements were independently carried out. Firstly, in the imaging dose measurement, the phantom was placed on the couch and aligned with a stationary green laser and beam isocenter. The measurement point was 10 mm up from the cente of the phantom. Three slices on either side of the middle slice were selected. Secondly, two inserts with different rows of holes of various sizes were placed inside the phantom for image contrast and resolution investigation. Lastly, twelve density inserts were placed into the outer holes in the phantom for measurement of the image value to density table (IVDT). A comparison of imaging dose, image resolution and contrast, IVDT table between different jaw configurations was performed to evaluate the imaging system. Imaging dose was 2.93 cGy with a jaw size of one mm as opposed to 1.62 cGy with a four mm jaw, both of which are below the vendor's requirement: 3 cGy. However, image quality is improved significantly with the smaller jaw. Four lines of holes can be readily identified on images using smaller jaw while only three lines visible with the larger jaw. Image contrast is similarly enhanced when reducing the jaw size. On average CT numbers are 6% higher with the smaller jaw than those obtained with the larger one. Significant improvement in image quality is achieved with the smaller jaw field in Tomotherapy while the imaging dose is kept at a clinically acceptable level.

Evaluation of the accuracy of different combinations of automatic registration selection factors in obtaining tomotherapy megavoltage computed tomography images

The purpose of our study is to find the potential best options for the registration selection parameters when performing automatic registration between kilovoltage computed tomography (KVCT) and megavoltage computed tomography (MVCT) images in helical tomotherapy. We compared nine different combinations of automatic registration parameters, including technique and resolution in automatic registration control and tomoimage filtering in scan image control. In the phantom study, we compared the translational and the rotational adjustments between the original position (lateral 0 cm; longitudinal 0 cm; vertical 0 cm) and the manually moved position (lateral 1 cm; longitudinal 1 cm; vertical 1 cm). In the patient study, we compared translational (lateral, longitudinal, vertical) and rotational (pitch, roll, yaw) adjustments between the KVCT and the MVCT images for different anatomic treatment sites. The ranges of translational and rotational displacements in the phantom study were 14.87 ± 0.12 ∼ 15.70 ± 0.04 mm and 0° ∼ 1.03 ± 0.58°, respectively. In cranial cases, the ranges of translational and rotational root-mean-square values were 4.3 ± 0.4 mm ∼ 5.3 ± 0.6 mm, and 0.3 ± 0.3° ∼ 0.7 ± 0.4°, respectively. In the extra cranial cases, the ranges of translational and rotational root-mean-square values were 14.1 ± 3.6 mm − 16.1 ± 2.5 mm, and 0.5 ± 0.3° ∼ 1.0 ± 0.5°, respectively. We found that the combination of “full image - super fine” could minimize the mechanical deviations in the cranial case, and that both the combinations of “bone tissue - super fine” and “full image - super fine” were the best options in extra cranial cases.

Megavoltage Computed Tomography Image Guidance With Helical Tomotherapy in Patients With Vertebral Tumors: Analysis of Factors Influencing Interobserver Variability

To evaluate megavoltage computed tomography (MVCT)-based image guidance with helical tomotherapy in patients with vertebral tumors by analyzing factors influencing interobserver variability, considered as quality criterion of image guidance. Five radiation oncologists retrospectively registered 103 MVCTs in 10 patients to planning kilovoltage CTs by rigid transformations in 4 df. Interobserver variabilities were quantified using the standard deviations (SDs) of the distributions of the correction vector components about the observers' fraction mean. To assess intraobserver variabilities, registrations were repeated after ≥4 weeks. Residual deviations after setup correction due to uncorrectable rotational errors and elastic deformations were determined at 3 craniocaudal target positions. To differentiate observer-related variations in minimizing these residual deviations across the 3-dimensional MVCT from image resolution effects, 2-dimensional registrations were performed in 30 single transverse and sagittal MVCT slices. Axial and longitudinal MVCT image resolutions were quantified. For comparison, image resolution of kilovoltage cone-beam CTs (CBCTs) and interobserver variability in registrations of 43 CBCTs were determined. Axial MVCT image resolution is 3.9 lp/cm. Longitudinal MVCT resolution amounts to 6.3 mm, assessed as full-width at half-maximum of thin objects in MVCTs with finest pitch. Longitudinal CBCT resolution is better (full-width at half-maximum, 2.5 mm for CBCTs with 1-mm slices). In MVCT registrations, interobserver variability in the craniocaudal direction (SD 1.23 mm) is significantly larger than in the lateral and ventrodorsal directions (SD 0.84 and 0.91 mm, respectively) and significantly larger compared with CBCT alignments (SD 1.04 mm). Intraobserver variabilities are significantly smaller than corresponding interobserver variabilities (variance ratio [VR] 1.8-3.1). Compared with 3-dimensional registrations, 2-dimensional registrations have significantly smaller interobserver variability in the lateral and ventrodorsal directions (VR 3.8 and 2.8, respectively) but not in the craniocaudal direction (VR 0.75). Tomotherapy image guidance precision is affected by image resolution and residual deviations after setup correction. Eliminating the effect of residual deviations yields small interobserver variabilities with submillimeter precision in the axial plane. In contrast, interobserver variability in the craniocaudal direction is dominated by the poorer longitudinal MVCT image resolution. Residual deviations after image guidance exist and need to be considered when dose gradients ultimately achievable with image guided radiation therapy techniques are analyzed.

Stereotactic body radiation therapy for early-stage non-small-cell lung cancer utilizing a helical intensity modulated image-guided approach.

e17524 Background: To report the techniques and results of a large cohort of patients treated with stereotactic body radiotherapy (SBRT) for early-stage non-small-cell lung cancer (NSCLC) using image-guided helical intensity modulated radiation therapy (IMRT) and FDG-PET assisted targeting. Methods: Between February 2005 and August 2010, 79 patients with stage 1 NSCLC were treated with SBRT. 59 patients had T1 tumors and 20 patients had T2 tumors. 62 patients had biopsy proven malignancies and 17 patients were treated empirically based on strong clinical indications. Central tumor location did not disqualify patients. All patients underwent FDG-PET or FDG-PET/CT scanning in the treatment position for planned fusion with a separate kilovoltage (KV) CT simulation. Rigid immobilization was achieved during CT simulation using a double vacuum cast device with abdominal compression. SBRT was delivered via 12 Gy x 4 or 12 Gy x 5 fractionation scheme (48-60 Gy total). Megavoltage CT images were obtained on the treatment unit prior to therapy and repeated mid-fraction with comparison to the KV CT simulation planning images to assure set-up accuracy. Serial follow-up with FDG-PET or FDG-PET/CT studies were performed at 3 months and every 6 months thereafter on all surviving patients. Results: Median patient age was 73 years. Mean tumor size was 2.3 cm (range 0.5 – 6 cm). Median follow-up was 24 months (range 4-78 months). Overall local control rate (LCR) was 94% (95% confidence interval (CI), 86-97%) based on serial FDG-PET. LCR for T1 tumors was 95% vs. 90% for T2 tumors (p = 0.60). Entire cohort overall survival rate (OSR) was 87% (CI, 78-93%) at 1 year and 58% (CI, 47-68%) at 3 years. T1 patients had a 3-year OSR of 71% vs. 43% OSR for T2 patients (p = 0.04). No patients developed life threatening radiation pneumonitis or pulmonary complications. Chest wall pain or rib fracture was reported in 8 patients (10%; CI, 5-19%). Conclusions: Image-guided SBRT with helical IMRT delivered in 4 or 5 fractions of 12 Gy with rigid immobilization and FDG-PET assisted targeting is an effective treatment for early NSCLC and is associated with excellent local control rates and acceptable treatment-related morbidity.

Feasibility of image-guided radiotherapy based on helical tomotherapy to reduce contralateral parotid dose in head and neck cancer

BackgroundTo evaluate the feasibility of image-guided radiotherapy based on helical Tomotherapy to spare the contralateral parotid gland in head and neck cancer patients with unilateral or no neck node metastases.MethodsA retrospective review of 52 patients undergoing radiotherapy for head and neck cancers with image guidance based on daily megavoltage CT imaging with helical tomotherapy was performed.ResultsMean contralateral parotid dose and the volume of the contralateral parotid receiving 40 Gy or more were compared between radiotherapy plans with significant constraint (SC) of less than 20 Gy on parotid dose (23 patients) and the conventional constraint (CC) of 26 Gy (29 patients). All patients had PTV coverage of at least 95% to the contralateral elective neck nodes. Mean contralateral parotid dose was, respectively, 14.1 Gy and 24.7 Gy for the SC and CC plans (p < 0.0001). The volume of contralateral parotid receiving 40 Gy or more was respectively 5.3% and 18.2% (p < 0.0001)ConclusionTomotherapy for head and neck cancer minimized radiotherapy dose to the contralateral parotid gland in patients undergoing elective node irradiation without sacrificing target coverage.

Three-dimensional patient setup errors at different treatment sites measured by the Tomotherapy megavoltage CT

Reduction of interfraction setup uncertainty is vital for assuring the accuracy of conformal radiotherapy. We report a systematic study of setup error to assess patients' three-dimensional (3D) localization at various treatment sites. Tomotherapy megavoltage CT (MVCT) images were scanned daily in 259 patients from 2005-2008. We analyzed 6,465 MVCT images to measure setup error for head and neck (H&N), chest/thorax, abdomen, prostate, legs, and total marrow irradiation (TMI). Statistical comparisons of the absolute displacements across sites and time were performed in rotation (R), lateral (x), craniocaudal (y), and vertical (z) directions. The global systematic errors were measured to be less than 3mm in each direction with increasing order of errors for different sites: H&N, prostate, chest, pelvis, spine, legs, and TMI. The differences in displacements in the x, y, and z directions, and 3D average displacement between treatment sites were significant (p < 0.01). Overall improvement in patient localization with time (after 3-4 treatment fractions) was observed. Large displacement (> 5mm) was observed in the 75(th) percentile of the patient groups for chest, pelvis, legs, and spine in the x and y direction in the second week of the treatment. MVCT imaging is essential for determining 3D setup error and to reduce uncertainty in localization at all anatomical locations. Setup error evaluation should be performed daily for all treatment regions, preferably for all treatment fractions.

Tomotherapy planning and image registration for planning CT image slice thickness for stereotactic radiosurgery

Tomotherapy (TomoTherapy, Inc., Madison, WI, USA) is a helical intensity modulated radiation therapy (IMRT) modality using rotational delivery of a fan beam like a CT scanner to deliver highly conformal radiation dose distributions in an intensity-modulated fashion. Tomotherapy can image guided radiation therapy (IGRT) using megavoltage computed tomography (MVCT). In this study, different CT slice thicknesses were acquired in order to evaluate the effect of CT slice thickness on tomotherapy planning and image registration for stereotactic body radio therapy (SBRT) or stereotactic radiosurgery (SRS) by using end-to-end (EtoE) phantom (Accuray, USA). EtoE phantom kilovoltage CT (KVCT) and MVCT images were acquired using CT (LightSpeed 16, GE, USA), and tomotherapy (Tomotherapy, USA), respectively. Reference planning target volumes (RPTV) in the EtoE phantom were contoured. The volume of the RPTV was 14.13 cc, and the CT image thicknesses were 2, 4, and 6 mm. CT images were planned based on the tomotherapy planning system (TPS), and all plans had the same constraints and iterations. The PTV volumes were 16.58 cc, 18.88 cc, and 20.28 cc in 2-, 4- and 6 mm planning KVCTs, respectively. The PTV volumes were 16.32, 18.00, and 22.04 cc in 2-, 4-, and 6 mm using MVCT, respectively. Heavy thickness planning of the CT slice overestimated the RPTV volume in TPS. Also, the heavy thickness MVCT image skipped target slice images. The target coverage of the treatment plans for different slice thicknesses was correlated with the tumor volume. At 10-Gy prescription dose of PTV, dose coverages of the reference planning target volume were 10.14, 10.25, and 10.27 Gy for 2-, 4-, and 6-mm slice thicknesses. Therefore, when radiosurgery is supplemented with tomotherapy, thin slice (<2 mm) planning should be done and MVCT images should be acquired

Effect of image value-to-density table (IVDT) on the accuracy of delivery quality assurance (DQA) process in helical tomotherapy

The effect of the accuracy in the application of the image value-to-density table (IVDT) from kilovoltage computed tomography (kVCT) and mega-voltage computed tomography (MVCT) images on the delivery quality assurance (DQA) in helical tomotherapy (HT) was analyzed. The DQA plans were prepared with a kVCT image of a cheese phantom for 10 HT patients, and the difference in absolute dose equivalence between the planned and real measurement was evaluated according to the accuracy of IVDT application. The difference between the calculated dose distribution and real dose distribution measured with MapCHECK (SunNuclear, Melbourne, FL) was analyzed through the DQA process with a kVCT MapCHECK image and the same analysis was performed with an MVCT MapCHECK image. The IVDT for kVCT was applied to MVCT and the variation in error between the planned and real measurement caused by improper application of IVDT was evaluated. The accuracy of the IVDT application in the homogeneous water-equivalent cheese phantom had only a minor influence on the dose calculation. Although the overall accuracy of the calculated dose was increased when the proper IVDT was applied, this result had no statistical significance. The MVCT image of MapCHECK contained less error between the calculated dose and delivered dose with a high pass rate. The proper IVDT application to the MVCT image of MapCHECK increased the accuracy of dose calculation, and this result had a statistical significance. Application of the correct IVDT is important in HT DQA and its significance is increased when using phantoms consisting of inhomogeneous density materials.

The effect of temporal HU variations on the uncertainty of dose recalculations performed on MVCT images

Over the course of radiation therapy, a patient's anatomy may change substantially. The relatively recent addition of frequent in-room imaging to assist with patient localization has provided a database of images that may be used to recalculate dose distributions for adaptive radiotherapy purposes. The TomoTherapy Hi-Art II unit (Accuray Inc., Sunnyvale, CA, USA) uses a helical scanning geometry and a megavoltage (MV) beam to acquire volumetric patient images. This study evaluated the uncertainty of dose calculations performed on megavoltage CT (MVCT) images as a function of temporal Hounsfield Unit (HU) variations observed in the imaging system over three years on two machines. A baseline error between dose calculations performed on kVCT and MVCT images was established using a series of phantoms. This baseline error ranged from −1.4% to 0.6%. Materials of differing densities were imaged and MVCT numbers were measured periodically. The MVCT number of solid water varied from 5 to 103 HU and consistently increased prior to target replacement. Finally, the dosimetric uncertainty of the temporal HU variation was assessed using MVCT images of typical head and neck, lung and prostate cancer patients. Worst-case MVCT recalculation errors could approach 5%, 7% and 10% for the head and neck, lung and prostate images, respectively. However, if a tolerance of ±30 HU were maintained for the MVCT number of solid water, dosimetric errors were limited to ±2.5%, ±3% and ±4%, respectively.

Validation of image registration and fusion of MV CBCT and planning CT for radiotherapy treatment planning

In areas like adaptive therapy, multi-phase radiotherapy, and single fraction palliative treatment or in the treatment of patients with metal implants where megavoltage(MV) CT could be considered as a treatment planning modality, the reduced contrast in the MV CT images could lead to limited accuracy in localization of the structures. This would affect the precision of the treatment. In this study, as an extension our previous work on bespoke MV cone beam CT (MV CBCT), we propose to register the MV CBCT with kilovoltage (kV) CT for treatment planning. The MV CBCT images registered with kV CT would be effective for treatment planning as it would account for the inadequate soft tissue information in the MV CBCT and would allow comparison of changes in patient dimensions and assist in localization of the structures. The intensity based registration algorithm of the BrainSCAN therapy planning software was used for image registration of the MV CBCT and kV CT images. The accuracy of the registration was validated using qualitative and quantitative measures. The effect of image quality on the level of agreement between the contouring done on both the MV CBCT and kV CT was assessed by comparing the volumes of six structures delineated. To assess the level of agreement between the plans after the registration, two independent plans were generated on the MV CBCT and the planning CT using the posterior fossa of the skull as the target. The dose volume histograms and conformity indices of the plans were compared. The results of this study show that treatment planning with MV CBCT images would be effective, using additional anatomical structure information derived from registering the MV CBCT image with a standard kVCT.

Magnitude of Interfractional Vaginal Cuff Movement: Implications for External Irradiation

To quantify the extent of interfractional vaginal cuff movement in patients receiving postoperative irradiation for cervical or endometrial cancer in the absence of bowel/bladder instruction. Eleven consecutive patients with cervical or endometrial cancer underwent placement of three gold seed fiducial markers in the vaginal cuff apex as part of standard of care before simulation. Patients subsequently underwent external irradiation and brachytherapy treatment based on institutional guidelines. Daily megavoltage CT imaging was performed during each external radiation treatment fraction. The daily positions of the vaginal apex fiducial markers were subsequently compared with the original position of the fiducial markers on the simulation CT. Composite dose-volume histograms were also created by summing daily target positions. The average (± standard deviation) vaginal cuff movement throughout daily pelvic external radiotherapy when referenced to the simulation position was 16.2 ± 8.3 mm. The maximum vaginal cuff movement for any patient during treatment was 34.5 mm. In the axial plane the mean vaginal cuff movement was 12.9 ± 6.7 mm. The maximum vaginal cuff axial movement was 30.7 mm. In the craniocaudal axis the mean movement was 10.3 ± 7.6 mm, with a maximum movement of 27.0 mm. Probability of cuff excursion outside of the clinical target volume steadily dropped as margin size increased (53%, 26%, 4.2%, and 1.4% for 1.0, 1.5, 2.0, and 2.5 cm, respectively.) However, rectal and bladder doses steadily increased with larger margin sizes. The magnitude of vaginal cuff movement is highly patient specific and can impact target coverage in patients without bowel/bladder instructions at simulation. The use of vaginal cuff fiducials can help identify patients at risk for target volume excursion.

SU‐E‐T‐802: Dosimetric Examination and Verification of Megavoltage Computed Tomography (MVCT) Based IMRT Treatment Planning with Helical TomoTherapy

Purpose: Intensity modulated radiation therapy (IMRT) planning using pre‐treatment megavoltage computed tomography (MVCT) provides a viable solution to adaptive radiotherapy. A strategy was developed to assess the impact of MVCT image quality on treatment plans using Helical TomoTherapy. Methods: The Octavius phantom (PTW) was scanned with a Philips CT‐scanner and the Helical TomoTherapy unit for kilovoltage CT (kVCT) and MVCT, respectively. MVCT images were acquired with three modes (slice thickness): coarse (6‐mm), normal (4‐mm) and fine (2‐mm). Contours from existing head‐and‐neck plans were transferred from the Pinnacle treatment planning system to phantom images and IMRT plans were generated using the TomoTherapy planning station. Comparison between the kVCT‐based plan (benchmark plan) and MVCT‐based plans were conducted to evaluate dosimetric variations. Dose volume histograms (DVH) were examined for plan quality. Plans using MVCTs acquired with three modes were also evaluated for dosimetric differences. Additional plan evaluations were performed with retrospective dose reconstruction based on the delivered leaf sequences. Results: Isodose distributions of four plans are comparable despite the intrinsic difference between MVCT and kVCT images. No significant variations on the target coverage were found from DVHs. However, plan using coarse MVCT images gave largest dose deviations (&gt; 50%) for critical structures from that of benchmark plan while the best agreement was observed between fine MVCT and kVCT. Plan parameters were found to be consistent among four plans. Comparisons between planning doses and reconstructed treatment doses revealed high fidelity delivery. Conclusions: IMRT planning using MVCTs proved to be equivalent with that using standard kVCT when fine imaging mode was selected. Target coverage on MVCT plans is comparable to that on the standard kVCT plan. MVCT thicknesses were shown to have impact on plan quality. Caution is needed when performing treatment planning with MVCTs.

SU‐E‐T‐199: Three‐Dimensional Dose Verification Using Normoxic Polymer Gel Dosimeters for Tomotherapy

The aim of this study is to evaluate the feasibility of using MAGAT as a near real‐time 3‐dimensional dose measurement device for tomotherapy. MAGAT is a new type of normoxic polymer gel dosimeter, which responses well to absorbed dose and can be easily made in the presence of normal oxygen surroundings. Its dose response was measured by irradiating MAGAT‐gel‐filled testing vials with tomotherapy and its linear relationship with dose was present from 0 to 6.5 Gy. One group of gel samples were measured in near real‐time, in which the gel phantom was read right after the irradiation. The other group was measured 12 hours after irradiation so the gel phantom can be exposed to oxygen. Several post‐imaging processing filters including Nagao, Guess, median, mean, min and max, were applied on megavoltage computed tomography (MVCT) images for better discrimination on dose responses. Our results show that dose responses for MVCT with real‐time and 12‐hour delayed measurement were 4.76 and 4.69 &amp;#916;SI&amp;#65294;cGy‐1, respectively, and show no significant difference (p‐value = 0.72). For study of the filtering effects, Gauss, median and mean filters offer better linear correction coefficients of dose response. In conclusion, the MAGAT polymer gel dosimeter read from MVCT imaging is a promising method for dose verification in clinical tomotherapy.

SU-E-T-558: Intrafractional Target Displacement during Stereotactic Body Radiation Therapy in Lung for Patients Immobilized Using BodyFix System

Puropose: To investigate intrafractional target displacement and assess the adequacy of margins used for early stage lungcarcinoma patients treated to 50Gy in five fractions and immobilized using BodyFix (Elekta, Stockholm, Sweden). Methods: Patients selected are medically inoperable or refuse surgery with T1 or T2 tumors <5cm. Patients are immobilized head first supine using BodyFix system. If target motion exceeds 1cm, as seen on fluoroscopic imaging, abdominal compression is used. The abdominal compression device consists of a rigid belt and pressure cuff. Treatments are performed on Tomotherapy (Tomotherapy Inc, Madison, WI) using megavoltage (MV) CTimage guidance. MVCT images are slow, thus, representing an average image of the moving target. Each fraction was divided into two parts to permit assessment of intrafraction movement and allow for an interruption if long treatment is too difficult to tolerate. In many cases, patients are willing to continue with the second part of the delivery without a break. Based on our clinical protocol, the second part is still preceded by the imaging. Comparison of the images prior to the first and second portions of radiation provided us with target shifts during treatment. 23 treatments of 5 patients have been studied. Image registration was performed using Eclipse (Varian Medical Systems, Palo Alto, CA). Results: Average of absolute target displacement during treatment was 2mm in lateral, 1mm in anterior‐posterior, and 2mm in craniocaudal directions. Maximum displacement was 4mm. Average target displacements were below 0.3mm for all directions. Per patient, average absolute shifts were similar to the population average, however, averages shifts were about 1mm in anterior posterior and 2mm in craniocaudal directions. Conclusions: Keeping in mind that hardware error is of the order of 1mm, and target delineation error in lung is about 1–2mm, 5mm margins, currently accepted in our institution, are sufficient for adequate coverage.

SU-E-T-488: Treatment of a Unique CNS Patient with Tomotherapy

Purpose: We describe a whole spinal cord, cauda equine, and brainstem radiation treatment using an in‐house developed tomotherapy approach for a unique patient diagnosed with an extramedullary spinal melanocytoma with leptomeningeal seeding, treated with 48.6 Gy in 28 fractions. Methods and Materials: Given that the prescribed dose is within the range of tolerance to the spinal cord, tomotherapy was chosen to take advantage of the superior dose uniformity achievable with this technology and ability to deliver modulated radiotherapy in a single treatment to a long volume. The patient was treated supine and immobilized with a thermoplastic mask for the head and shoulders and a long Vac‐Lok bag for the body. The CTV consisted of the entire spinal cord, thecal sac to the level of S2 and brainstem. A 1 cm margin was applied to create the PTV. Jaws, pitch and the modulation factor were set to 5 cm, 0.43 and 2.5, respectively. Before each treatment, the treated volume was imaged for setup verification using the integrated megavoltage CT (MVCT). Weekly the patient was also imaged post‐ treatment to confirm setup stability. Results: The patient is finishing treatment at the time of abstract submission. A highly conformal dose distribution was created with doses to the organs at risk within their tolerance limits. The beam on time was 17 minutes. Patient setup proved to be trouble‐free and reproducible. The total patient‐on‐the‐bed time was approximately one hour. The 1 cm PTV margin was adequate according to pre‐ and post‐treatment MVCT image analysis. Conclusions: Tomotherapy is a safe and effective tool for treating long CNS volumes to high dose. It allows avoiding junctions and sparing healthy CNStissue and other organs at risk. A relatively simple immobilization technique used for this patient proved to be stable and reproducible with a 1 cm PTV margin.

Inter- and intra-observer variation in gross tumor delineation on MVCT images in patients undergoing tomotherapy based IGRT for postoperative vault recurrences: Updated result.

e15587 Background: This study was designed to evaluate inter and intra-observer concordance in gross tumor volume (GTV) delineation on megavoltage CT (MVCT) images. Methods: Three observers delinea...

Evaluation of tomotherapy MVCT image enhancement program for tumor volume delineation

The aims of this study were to investigate the variability between physicians in delineation of head and neck tumors on original tomotherapy megavoltage CT (MVCT) studies and corresponding software enhanced MVCT images, and to establish an optimal approach for evaluation of image improvement. Five physicians contoured the gross tumor volume (GTV) for three head and neck cancer patients on 34 original and enhanced MVCT studies. Variation between original and enhanced MVCT studies was quantified by DICE coefficient and the coefficient of variance. Based on volume of agreement between physicians, higher correlation in terms of average DICE coefficients was observed in GTV delineation for enhanced MVCT for patients 1, 2, and 3 by 15%, 3%, and 7%, respectively, while delineation variance among physicians was reduced using enhanced MVCT for 12 of 17 weekly image studies. Enhanced MVCT provides advantages in reduction of variance among physicians in delineation of the GTV. Agreement on contouring by the same physician on both original and enhanced MVCT was equally high.PACS numbers: 87.57.N‐, 87.57.np, 87.57.nt