H Guan

Dose delivered from Varian's CBCT to patients receiving IMRT for prostate cancer

With the increased use of cone beam CT (CBCT) for daily patient setup, the accumulated dose from CBCT may be significantly higher than that from simulation CT or portal imaging. The objective of this work is to measure the dose from daily pelvic scans with fixed technical settings and collimations. CBCT scans were acquired in half-fan mode using a half bowtie and x-rays were delivered in pulsed-fluoro mode. The skin doses for seven prostate patients were measured on an IRB-approved protocol. TLD capsules were placed on the patients skin at the central axis of three beams: AP, left lateral (Lt Lat) and right lateral (Rt Lat). To avoid the ring artefacts centred in the prostate, the treatment couch was dropped 3 cm from the patients tattoo (central axis). The measured AP skin doses ranged 3-6 cGy for 20-33 cm separation. The larger the patient size the less the AP skin dose. Lateral doses did not change much with patient size. The Lt Lat dose was approximately 4.0 cGy, which was approximately 40% higher than the Rt Lat dose of approximately 2.6 cGy. To verify this dose asymmetry, surface doses on an IMRT QA phantom (oval shaped, 30 cm x 20 cm) were measured at the same three sites using TLD capsules with 3 cm table-drop. The dose asymmetry was due to: (1) kV source rotation which always starts from the patients Lt Lat and ends at Lt Lat. Gantry rotation gets much slower near the end of rotation but dose rate stays constant and (2) 370 degrees scan rotation (10 degrees scan overlap on the Lt Lat side). In vivo doses were measured inside a Rando pelvic heterogeneous phantom using TLDs. The left hip (femoral head and neck) received the highest doses of approximately 10-11 cGy while the right hip received approximately 6-7 cGy. The surface and in vivo doses were also measured for phantoms at the central-axis setup. The difference was less than approximately 12% to the table-drop setup.

A quality assurance program for the on-board imager ®

To develop a quality assurance (QA) program for the On-Board Imager (OBI) system and to summarize the results of these QA tests over extended periods from multiple institutions. Both the radiographic and cone-beam computed tomography (CBCT) mode of operation have been evaluated. The QA programs from four institutions have been combined to generate a series of tests for evaluating the performance of the On-Board Imager. The combined QA program consists of three parts: (1) safety and functionality, (2) geometry, and (3) image quality. Safety and functionality tests evaluate the functionality of safety features and the clinical operation of the entire system during the tube warm-up. Geometry QA verifies the geometric accuracy and stability of the OBI/CBCT hardware/software. Image quality QA monitors spatial resolution and contrast sensitivity of the radiographic images. Image quality QA for CBCT includes tests for Hounsfield Unit (HU) linearity, HU uniformity, spatial linearity, and scan slice geometry, in addition. All safety and functionality tests passed on a daily basis. The average accuracy of the OBI isocenter was better than 1.5mm with a range of variation of less than 1mm over 8 months. The average accuracy of arm positions in the mechanical geometry QA was better than 1mm, with a range of variation of less than 1mm over 8 months. Measurements of other geometry QA tests showed stable results within tolerance throughout the test periods. Radiographic contrast sensitivity ranged between 2.2% and 3.2% and spatial resolution ranged between 1.25 and 1.6lp∕mm. Over four months the CBCT images showed stable spatial linearity, scan slice geometry, contrast resolution (1%; <7mm disk) and spatial resolution (>6lp∕cm). The HU linearity was within ±40HU for all measurements. By combining test methods from multiple institutions, we have developed a comprehensive, yet practical, set of QA tests for the OBI system. Use of the tests over extended periods show that the OBI system has reliable mechanical accuracy and stable image quality. Nevertheless, the tests have been useful in detecting performance deficits in the OBI system that needed recalibration. It is important that all tests are performed on a regular basis.

Combining scatter reduction and correction to improve image quality in cone‐beam computed tomography (CBCT)

PURPOSE The authors propose a combined scatter reduction and correction method to improve image quality in cone-beam computed tomography (CBCT). Although using a beam-block approach similar to previous techniques to measure the scatter, this method differs in that the authors utilize partially blocked projection data obtained during scatter measurement for CBCT image reconstruction. This study aims to evaluate the feasibility of the proposed approach. METHODS A 1D grid, composed of lead septa, was placed between the radiation source and the imaging object for scatter measurement. Image data were collected from the grid interspace regions while the scatter distribution was measured in the blocked regions under the grid. Scatter correction was performed by subtracting the measured scatter from the imaging data. Image information in the penumbral regions of the grid was derived. Three imaging modes were developed to reconstruct full CBCT images from partial projection data. The single-rotation half-fan mode uses interpolation to fill the missing data. The dual-rotation half-fan mode uses two rotations, with the grid offset by half a grid cycle, to acquire two complementary sets of projections, which are then merged to form complete projections for reconstruction. The single-rotation full-fan mode was designed for imaging a small object or a region of interest. Full-fan projection images were acquired over a 360 degrees scan angle with the grid shifting a distance during the scan. An enlarged Catphan phantom was used to evaluate potential improvement in image quality with the proposed technique. An anthropomorphic pelvis phantom was used to validate the feasibility of reconstructing a complete set of CBCT images from the partially blocked projections using three imaging modes. Rigid-body image registration was performed between the CBCT images from the single-rotation half-fan mode and the simulation CT and the results were compared to that for the CBCT images from dual-rotation mode and conventional CBCT images. RESULTS The proposed technique reduced the streak artifact index from 58% to 1% in comparison with the conventional CBCT. It also improved CT number linearity from 0.880 to 0.998 and the contrast-to-noise ratio (CNR) from 4.29 to 6.42. Complete sets of CBCT images with overall improved image quality were achieved for all three image modes. The longitudinal resolution was slightly compromised for the single-rotation half-fan mode. High resolution was retained for the dual-rotation half-fan and single-rotation full-fan modes in the longitudinal direction. The registration error for the CBCT images from the single-rotation half-fan mode was 0.8 +/- 0.3 mm in the longitudinal direction and negligible in the other directions. CONCLUSIONS The proposed method provides combined scatter correction and direct scatter reduction. Scatter correction may eliminate scatter artifacts, while direct scatter reduction may improve the CNR to compensate the CNR degradation due to scatter correction. Complete sets of CBCT images are reconstructed in all three imaging modes. The single-rotation mode can be used for rigid-body patient alignment despite degradation in longitudinal resolution. The dual-rotation mode may be used to improve CBCT image quality for soft tissue delineation in adaptive radiation therapy.

A technique for on-board CT reconstruction using both kilovoltage and megavoltage beam projections for 3D treatment verification.

The technologies with kilovoltage (kV) and megavoltage (MV) imaging in the treatment room are now available for image-guided radiation therapy to improve patient setup and target localization accuracy. However, development of strategies to efficiently and effectively implement these technologies for patient treatment remains challenging. This study proposed an aggregated technique for on-board CT reconstruction using combination of kV and MV beam projections to improve the data acquisition efficiency and image quality. These projections were acquired in the treatment room at the patient treatment position with a new kV imaging device installed on the accelerator gantry, orthogonal to the existing MV portal imaging device. The projection images for a head phantom and a contrast phantom were acquired using both the On-Board Imager kV imaging device and the MV portal imager mounted orthogonally on the gantry of a Varian Clinac 21EX linear accelerator. MV projections were converted into kV information prior to the aggregated CT reconstruction. The multilevel scheme algebraic-reconstruction technique was used to reconstruct CT images involving either full, truncated, or a combination of both full and truncated projections. An adaptive reconstruction method was also applied, based on the limited numbers of kV projections and truncated MV projections, to enhance the anatomical information around the treatment volume and to minimize the radiation dose. The effects of the total number of projections, the combination of kV and MV projections, and the beam truncation of MV projections on the details of reconstructed kV/MV CT images were also investigated.

Accuracy of inhomogeneity correction in photon radiotherapy from CT scans with different settings

We report an investigation on the accuracy of inhomogeneity correction in photon radiotherapy from CT scans with different settings. Specifically, the dosimetric differences from different CT scan parameters (kV, mAs) to phantoms and from different Hounsfield unit versus electron density (HU-ED) curves to patients are investigated. The absolute dose per monitor units (dose/MU) is used to quantify the results. We found that only for high-density bones (cranium, femoral tube, etc) using small field 18 MV beams, the dose/MU is up to 2% higher for CT scans using 80 kV than for 130 kV at a depth just beyond the bone and is up to 1-1.5% higher for CT scans using 80 mAs than for 300 mAs. For low-density bones (such as femoral head) and lung, the difference is 1% or less with different kV or mAs settings. The dose/MU varies with different HU-ED curves by up to 2%. The HU-ED curve from the stochiometric calibration was found to be more accurate based on a real measurement. A simplified 4-point curve provides nearly the same accuracy as the stochiometric calibration and may be used as an alternative for routine clinical application.

Adaptive portal CT reconstruction: A simulation study

In radiotherapy, radiation treatment beams contain valuable information for patient setup verification. These beams may be used for portal CT reconstruction. However, direct use of the beam data for reconstruction may yield inadequate CT images simply because these beams cover only a part of the patient body. In this study, we use the treatment beams in addition to a set of regular CT projection beams to reconstruct a locally enhanced portal CT image. This approach is called adaptive portal CT reconstruction. A computer simulation demonstrated the advantages of the approach. The image reconstruction was carried out by the multilevel scheme algebraic reconstruction technique. Results indicated that the image quality of adaptive portal CT reconstruction is equivalent to that obtained from a full set of projections. This proposed technique should be not only valuable for three-dimensional radiotherapy verification, but also applicable to diagnostic CT imaging.

Clinical assessment and characterization of a dual tube kilovoltage X-ray localization system in the radiotherapy treatment room.

Although flat‐panel kilovoltage X‐ray imaging devices have been well tested for clinical use in diagnostic radiology, their use as a part of an image‐guided radiation therapy (IGRT) system in a treatment room is new and requires systematic assessment. We used the Novalis IGRT system (BrainLAB, Feldkirchen, Germany) for the present study. The system consists of two floor‐mounted kilovoltage X‐ray tubes projecting obliquely onto two flat‐panel detectors mounted on the ceiling. The system can automatically fuse two‐dimensional localization images with three‐dimensional simulation computed tomography images to provide positioning guidance. We evaluated these system parameters: Overall performance of the IGRT system, including isocenter correlation between the IGRT system and the linear accelerator (LINAC) Image quality of the system Exposure received by patients for a pair of images Linearity, uniformity, and repeatability of the system The Novalis system uses a daily isocenter calibration procedure to ensure consistency of isocenters between the IGRT and the LINAC systems. The localization accuracy was about 1 mm. We measured the relative modulation transfer function (RMTF) to quantify the spatial resolution of the imaging device, with f50 being 0.7 – 0.9 line pairs per millimeter. The maximal exposure of an image was 95 mR. We derived an empirical relationship between the exposure and the X‐ray technical settings. The other parameters of the system were quantitatively measured and generally met the requirements. The IGRT system is safe and reliable for clinical use as a target localization device. The measured data can be used as a benchmark for a quality assurance procedure. PACS number: 87.56‐v

A positioning QA procedure for 2D/2D (kV/MV) and 3D/3D (CT/CBCT) image matching for radiotherapy patient setup

A positioning QA procedure for Varians 2D/2D (kV/MV) and 3D/3D (planCT/CBCT) matching was developed. The procedure was to check: (1) the coincidence of on‐board imager (OBI), portal imager (PI), and cone beam CT (CBCT)s isocenters (digital graticules) to a linacs isocenter (to a pre‐specified accuracy); (2) that the positioning difference detected by 2D/2D (kV/MV) and 3D/3D(planCT/CBCT) matching can be reliably transferred to couch motion. A cube phantom with a 2 mm metal ball (bb) at the center was used. The bb was used to define the isocenter. Two additional bbs were placed on two phantom surfaces in order to define a spatial location of 1.5 cm anterior, 1.5 cm inferior, and 1.5 cm right from the isocenter. An axial scan of the phantom was acquired from a multislice CT simulator. The phantom was set at the linacs isocenter (lasers); either AP MV/R Lat kV images or CBCT images were taken for 2D/2D or 3D/3D matching, respectively. For 2D/2D, the accuracy of each devices isocenter was obtained by checking the distance between the central bb and the digital graticule. Then the central bb in orthogonal DRRs was manually moved to overlay to the off‐axis bbs in kV/MV images. For 3D/3D, CBCT was first matched to planCT to check the isocenter difference between the two CTs. Manual shifts were then made by moving CBCT such that the point defined by the two off‐axis bbs overlay to the central bb in planCT. (PlanCT can not be moved in the current version of OBI1.4.) The manual shifts were then applied to remotely move the couch. The room laser was used to check the accuracy of the couch movement. For Trilogy (or Ix‐21) linacs, the coincidence of imager and linacs isocenter was better than 1 mm (or 1.5 mm). The couch shift accuracy was better than 2 mm. PACS number: 87.55.Qr, 87.57.Q‐

AI-guided parameter optimization in inverse treatment planning.

An artificial intelligence (AI)-guided inverse planning system was developed to optimize the combination of parameters in the objective function for intensity-modulated radiation therapy (IMRT). In this system, the empirical knowledge of inverse planning was formulated with fuzzy if-then rules, which then guide the parameter modification based on the on-line calculated dose. Three kinds of parameters (weighting factor, dose specification, and dose prescription) were automatically modified using the fuzzy inference system (FIS). The performance of the AI-guided inverse planning system (AIGIPS) was examined using the simulated and clinical examples. Preliminary results indicate that the expected dose distribution was automatically achieved using the AI-guided inverse planning system, with the complicated compromising between different parameters accomplished by the fuzzy inference technique. The AIGIPS provides a highly promising method to replace the current trial-and-error approach.

SU‐FF‐T‐60: A Simplified Frame Work Using Deep Inspiration Breath‐Hold (DIBH) for the Treatment of Left Breast Cancer with Improved Heart Sparing

Purpose: To develop a simplified frame work using deep inspiration breath‐hold (DIBH) for left breast treatment. Materials and Methods: The current version of Varians RPM system was rarely used in amplitude gating mode, especially with breath hold. The major reason is that the breathing amplitude is much less reproducible than breathing phase. Further, the same signal captured by the infrared camera in simulation room and that in treatment room could be different in amplitude. In this study, we presented a simplified frame work to improve the reproducibility of patients breathing amplitude. First, an aqua‐plastic body mask of 1.0–1.5 in wide was made right before patients simulation while the patient is in DIBH. The body mask was set at umbilicus right superior to the marker box. It will then be used to guide the patient herself for DIBH. The DIBH signal is also displayed on a computer monitor set close to patient, which is a duplicate display of the DIBH signal in the RPM computer. The patient can see her own signal and can therefore guide her breath such that relatively constant amplitude can be achieved. Results: The frame work was tested by a few volunteers and all agree that the system is feasible for left breast treatment. The DIBH can last 15–35s with good constant amplitude. In case the captured amplitude is different in treatment room, the two gating threshold lines set in simulation can be adjusted overlay to the DIBH signal before treatment. Conclusion: The system is feasible for the treatment of left breast cancer with DIBH. Further improvement can be made by wiring the gating cable through patient using two electrodes; one on patients body and the other on the guiding mask so that the amplitude‐gated CT scans and treatment can be actively controlled by patient herself.