E Chao

High-dose MVCT image guidance for stereotactic body radiation therapy

PURPOSE 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. METHODS 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. RESULTS 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. CONCLUSIONS 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.

Quality assurance of an image guided intracranial stereotactic positioning system.

This work reports on the development and testing of an intracranial stereotactic patient positioning system (ISPPS) for Tomotherapy. The ISPPS consists of the combination of a head frame, head frame couch interface (HCI), megavoltage CT (MVCT), and optical tracking camera system. Three quality assurance tests were designed to quantify the positioning systems ability to localize an intracranial target. The first two of these tests were designed to determine (a) the ability of the MVCT to detect a known shift applied to an anthropomorphic phantom and (b) the precision of fixing the phantom to the treatment couch via a head frame and specially designed head frame couch interface. A system verification test, using a phantom and EDR2 film, was used to determine the overall delivery precision through comparison of a measured dose distribution on film to calculated dose. The average net translational difference between a known shift applied to a phantom and that detected by MVCT image fusion was 0.62 mm. Setup reproducibility of the head frame was measured with both MVCT and optical tracking. The frame setup precision was found to be well within 1 mm for translations as well as rotations. A system delivery verification test in phantom using film showed spatial agreement between planned and delivered dose distributions to within 1 mm.

A slit method to determine the focal spot size and shape of TomoTherapy system

PURPOSE To obtain accurate x-ray source profile measurements using a slit-collimator, the slit-collimator should have a narrow width, large height, and be positioned near the source. However, these conditions may not always be met. In this paper, the authors provide a detailed analysis of the slit measurement geometry and the relationship between the slit parameters and the measured x-ray source profile. The slit model allows the use of a shorter and more easily available slit-collimator, while accurate source profile measurements can still be obtained. METHODS Measurements were performed with a variety of slit widths and/or slit to source distances. The relationship derived between the slit parameters and the measured profile was used to determine the true focal spot profile through a least square fit of the profile data. The model was verified by comparing the predicted profiles at a variety of slit-collimator parameters with the measured results on the TomoTherapy Hi-Art system. RESULTS Both the treatment beam and the imaging beam were measured. For treatment mode, it was found that a source consisting of one Gaussian with a 0.75 mm full-width-half-maximum (FWHM) and 72% peak amplitude and a second Gaussian with a 2.27 mm FWHM and 18% peak amplitude matched measurement profiles. The overall source profile has a FWHM of 0.93 mm, but with a higher amplitude in the tail region than a single Gaussian. For imaging mode, the source consists of one Gaussian with a 0.68 mm FWHM and 82% peak amplitude and a second Gaussian with a 1.83 mm FWHM and 18% peak amplitude. The overall source profile has a FWHM of 0.77 mm. CONCLUSIONS Our study of the focal spot measurement using slit-collimators showed that accurate source profile measurements can be achieved through fitting of measurement results at different slit widths and source-to-slit distances (SSD). Quantitative measurements of the TomoTherapy linac focal spot showed that the source distribution could be better described with a model consisting of two Gaussian components rather than a single Gaussian model as assumed in previous studies.

SU‐GG‐J‐120: Longitudinal Resolution of the TomoTherapy® MVCT Image and Potential Improvements

Purpose: To quantify the longitudinal resolution of the TomoTherapy® MVCT image and evaluate potential improvements to the resolution. Method and Materials: The image slice sensitivity profile is measured using a thin‐disc method to approximate a delta impulse function in the longitudinal direction. The sensitivity profile is measured for several different helical pitches and beam widths. In addition, the clinical image longitudinal resolution is limited not only by the image slice sensitivity profile, but also by the image spacing. For a constant image slice profile (constant helical pitch and beam width), the image spacing is varied to investigate the impact of image spacing. Results: With the current TomoTherapy® factory settings for the jaw and the default image pitches (Fine, Normal, and Coarse corresponding to 4, 8, and 12 mm of couch translation per gantry rotation), the slice sensitivity profiles are measured to have a full‐width at half‐maximum of 6, 7, and 8 mm for the Fine, Normal, and Coarse pitches, respectively. Reducing the jaw‐width by an amount corresponding to a 3 mm decrease in the field‐width at isocenter, and keeping the same couch speeds of 4, 8, and 12 mm/rot, results in slice sensitivity profiles with a full‐width at half‐maximum of 4, 5, and 7 mm. Finally, using the thin jaw setting and “Normal” pitch (8 mm/rot), images are reconstructed at the default 4 mm interval as well as at a smaller interval of 2 mm. The sagittal and coronal images are observed to have a significantly improved resolution using the smaller image interval. Conclusion: The TomoTherapy® system currently offers users a balance between imaging speed and image quality. However, a thinner jaw setting and reduced image spacing result in significantly improved longitudinal resolution for TomoTherapy® MVCT images, yielding improved image quality at the same scanning speed.

Reconstruction of the treatment area by use of sinogram in helical tomotherapy

BackgroundTomoTherapy (Accuray, USA) has an image-guided radiotherapy system with a megavoltage (MV) X-ray source and an on-board imaging device. This system allows one to acquire the delivery sinogram during the actual treatment, which partly includes information from the irradiated object. In this study, we try to develop image reconstruction during treatment with helical tomotherapy.FindingsSinogram data were acquired during helical tomotherapy delivery using an arc-shaped detector array that consists of 576 xenon-gas filled detector cells. In preprocessing, these were normalized with full air-scan data. A software program was developed that reconstructs 3D images during treatment with corrections as; (1) the regions outside the field were masked not to be added in the backprojection (a masking correction), and (2) each voxel of the reconstructed image was divided by the number of the beamlets passing through its voxel (a ray-passing correction).The masking correction produced a reconstructed image, however, it contained streak artifacts. The ray-passing correction reduced this artifact. Although the SNR (the ratio of mean to standard deviation in a homogeneous region) and the contrast of the reconstructed image were slightly improved with the ray-passing correction, use of only the masking correction was sufficient for the visualization purpose.ConclusionsThe visualization of the treatment area was feasible by using the sinogram in helical tomotherapy. This proposed method would be useful in the treatment verification.

Quantitative characterization of tomotherapy MVCT dosimetry.

Megavoltage computed tomography (MVCT) is used as image guidance for patient setup in almost every tomotherapy treatment. Frequent use of ionizing radiation for image guidance has raised concern of imaging dose. The purpose of this work is to quantify and characterize tomotherapy MVCT dosimetry. Our dose calculation was based on a commissioned dose engine, and the calculation result was compared with film measurement. We studied dose profiles, center dose, maximal dose, surface dose, and mean dose on homogeneous cylindrical water phantoms of various diameters for various scanning parameters, including 3 different jaw openings (of nominal value J4, J1, and J0.1) and couch speeds (fine, normal, and coarse). The comparison between calculation and film measurement showed good agreement. In particular, the thread pattern on the film of the helical delivery matched very well with calculation. For the J1 jaw and coarse imaging mode, the maximum difference between calculation and measurement was about 6% of the center dose. Calculation on various sizes of synthesized phantoms showed that the center dose decreases almost linearly as the phantom diameter increases, and that the fine mode (couch speed of 4mm/rotation) received twice the dose of the normal mode (couch speed of 8mm/rotation) and 3 times that of the coarse mode (couch speed of 12mm/rotation) as expected. The maximal dose ranged from 100% to ∼200% of the center dose, with increasing ratios for larger phantoms, smaller jaws, and faster couch speed. For all jaw settings and couch speeds, the mean dose and average surface dose vary from 95% to 125% of the center dose with increasing ratios for larger phantoms. We present a quantitative dosimetric characterization of the tomotherapy MVCT in terms of scanning parameters, phantom size, center dose, maximal dose, surface dose, and mean dose. The results can provide an overall picture of dose distribution and a reference data set that enables estimation of CT dose index for the tomotherapy MVCT.

Evaluation of TomoTherapy dose calculations with intrafractional motion and motion compensation

PURPOSE Anatomical motion, both cyclical and aperiodic, can impact the dose delivered during external beam radiation. In this work, we evaluate the use of a research version of the clinical TomoTherapy® dose calculator to calculate dose with intrafraction rigid motion. We also evaluate the feasibility of a method of motion compensation for helical tomotherapy using the jaws and MLC. METHODS Treatment plans were created using the TomoTherapy treatment planning system. Dose was recalculated for several simple rigid motion traces including a 4 mm step motion applied either longitudinally or transversely, and a sinusoidal motion. The calculated dose volumes were compared to dose measurements that were performed by translating the phantom with the same motion traces used in the calculations. Measurements were made using film and ion chambers. Finally, the delivery plans were modified to compensate for the motion by sweeping the jaws for longitudinal motion and shifting the MLC leaves for transverse motion, and the calculations and measurements were repeated. RESULTS A transverse step motion shifted the dose that was delivered after the step occurred, but otherwise did not impact the dose distribution. Film measurements agreed with dose calculations to within 2%/2 mm for 99% of dose points within the 50% isodose line. A shift in the MLC leaf delivery pattern successfully compensated for the step motion to within the 3 mm accuracy allowed by the finite leaf widths. A longitudinal step motion impacted the dose in the interior of the target volume to a degree that was dependent on the planning field width and step size. Film measurements agreed with dose calculations to within 2%/2 mm for 98% of dose points within the 50% isodose line. Shifts in the jaw position successfully compensated for the longitudinal step motion. Sinusoidal (breathing-like) motion was also studied, with similar results. CONCLUSIONS A research version of the clinical TomoTherapy dose calculator has been shown to accurately calculate the dose from treatment plans delivered in the presence of arbitrary rigid motion. Modifications to the delivery plan using jaw and MLC leaf shifts that follow the motion can successfully compensate for the target motion.

SU‐FF‐T‐639: IVDT Variations and Their Impact On Dose Calculations

Purpose: To investigate the impact of variations in the image value to density table on TomoTherapy® dose calculations. Method and Materials: An image value to density table (IVDT) was generated using the CTimages of a solid water phantom with various tissue equivalent inserts. Additional density points were obtained by imaging a cylinder containing water, as well as the surrounding air. One IVDT was generated which included mappings for air and water, but only included tissue‐inserts with HU values outside of the −100 to +100HU range, to avoid undue influence from the tissue‐inserts in the critical density around water. For comparison, two other IVDTs were generated that a) did not include a mapping for air, or b) included the tissue‐inserts in the −100 to +100HU range. All image values were mapped to the physical density of each material. An IMRT plan was generated with a cylindrical target volume near the center of the solid water phantom, and DQA doses were calculated using each of the IVDTs. Results: Without including an explicit mapping for air, the default IVDT only included a point mapping −1024HU to 0 g/cc density. Since the mean air CT value was near −1000HU, the IVDT without an air mapping resulted in air being mapped to a density of approximately 0.024 g/cm3, versus a correct value of 0.001 g/cm3. The artificially high density and corresponding increased attenuation resulted in a reduced dose to the target volume of only 0.7%. However, the increased scatter through air elevated the surface dose by 3 – 4%. Including all tissue‐inserts in the IVDT increased the resulting mapped density of the solid‐water phantom by 4%, and reduced the calculated dose delivered to the target by 2.2%. Conclusion: Small variations to the IVDT generation process can have a measurable impact on the resulting calculated dose.

Feasibility of real‐time motion management with helical tomotherapy

PURPOSE This study investigates the potential application of image-based motion tracking and real-time motion correction to a helical tomotherapy system. METHODS A kV x-ray imaging system was added to a helical tomotherapy system, mounted 90 degrees offset from the MV treatment beam, and an optical camera system was mounted above the foot of the couch. This experimental system tracks target motion by acquiring an x-ray image every few seconds during gantry rotation. For respiratory (periodic) motion, software correlates internal target positions visible in the x-ray images with marker positions detected continuously by the camera, and generates an internal-external correlation model to continuously determine the target position in three-dimensions (3D). Motion correction is performed by continuously updating jaw positions and MLC leaf patterns to reshape (effectively re-pointing) the treatment beam to follow the 3D target motion. For motion due to processes other than respiration (e.g., digestion), no correlation model is used - instead, target tracking is achieved with the periodically acquired x-ray images, without correlating with a continuous camera signal. RESULTS The systems ability to correct for respiratory motion was demonstrated using a helical treatment plan delivered to a small (10 mm diameter) target. The phantom was moved following a breathing trace with an amplitude of 15 mm. Film measurements of delivered dose without motion, with motion, and with motion correction were acquired. Without motion correction, dose differences within the target of up to 30% were observed. With motion correction enabled, dose differences in the moving target were less than 2%. Nonrespiratory system performance was demonstrated using a helical treatment plan for a 55 mm diameter target following a prostate motion trace with up to 14 mm of motion. Without motion correction, dose differences up to 16% and shifts of greater than 5 mm were observed. Motion correction reduced these to less than a 6% dose difference and shifts of less than 2 mm. CONCLUSIONS Real-time motion tracking and correction is technically feasible on a helical tomotherapy system. In one experiment, dose differences due to respiratory motion were greatly reduced. Dose differences due to nonrespiratory motion were also reduced, although not as much as in the respiratory case due to less frequent tracking updates. In both cases, beam-on time was not increased by motion correction, since the system tracks and corrects for motion simultaneously with treatment delivery.

SU-F-BRB-06: Validation of Dose Calculation for Helical Tomotherapy with a Rigidly Moving Object

Purpose: To validate dose calculations in a rigidly moving object. Methods: An off-line version of the TomoTherapy™ dose calculation software was extended to allow dose calculation for rigidly moving objects. The interface allows users to extract the necessary data from TomoTherapy patient archives to perform an off-line dose calculation, and to specify arbitrary translational motion by creating a text file with a column for time, and columns for x, y, and z translations. Film measurements were performed using a phantom that was rigidly moved with the same motion profile as used in the calculation. A variety of motion profiles were studied and the motion profiles were applied to delivery plans with varying field sizes (1, 2.5, and 5 cm) and with dynamic jaws and static jaws. Results: Calculations matched measurements to within 2% or 1 mm for the variety of motion profiles studied. A 4-mm step motion in the longitudinal (IEC-Y) direction timed to occur halfway through a 1 cm field-width delivery resulted in a 29% decrease in both the calculated dose and measured dose in a central region of the dose profile. For a 5 cm field-width dynamic jaw delivery, the 4 mm step motion resulted in a more broadly distributed reduction in dose of 8%. A 6 mm sinusoidal motion in the longitudinal direction with a 6 second period had a negligible impact on delivered dose for one particular delivery plan. Similarly, a step motion in the transverse direction (IEC-X) had a negligible impact on delivered dose other than to shift the dose volume. Conclusion: A research version of the TomoTherapy™ dose calculator can accurately calculate dose for a rigidly moving object. Further studies with more clinical motion traces and with clinical treatment delivery plans will yield valuable insight into the impact of motion on the delivered dose.