Thomas H. Wagner

Performance characterization of megavoltage computed tomography imaging on a helical tomotherapy unit

Helical tomotherapy is an innovative means of delivering IGRT and IMRT using a device that combines features of a linear accelerator and a helical computed tomography (CT) scanner. The HI-ART II can generate CT images from the same megavoltage x-ray beam it uses for treatment. These megavoltage CT (MVCT) images offer verification of the patient position prior to and potentially during radiation therapy. Since the unit uses the actual treatment beam as the x-ray source for image acquisition, no surrogate telemetry systems are required to register image space to treatment space. The disadvantage to using the treatment beam for imaging, however, is that the physics of radiation interactions in the megavoltage energy range may force compromises between the dose delivered and the image quality in comparison to diagnostic CT scanners. The performance of the system is therefore characterized in terms of objective measures of noise, uniformity, contrast, and spatial resolution as a function of the dose delivered by the MVCT beam. The uniformity and spatial resolutions of MVCT images generated by the HI-ART II are comparable to that of diagnostic CT images. Furthermore, the MVCT scan contrast is linear with respect to the electron density of material imaged. MVCT images do not have the same performance characteristics as state-of-the art diagnostic CT scanners when one objectively examines noise and low-contrast resolution. These inferior results may be explained, at least partially, by the low doses delivered by our unit; the dose is 1.1 cGy in a 20 cm diameter cylindrical phantom. In spite of the poorer low-contrast resolution, these relatively low-dose MVCT scans provide sufficient contrast to delineate many soft-tissue structures. Hence, these images are useful not only for verifying the patients position at the time of therapy, but they are also sufficient for delineating many anatomic structures. In conjunction with the ability to recalculate radiotherapy doses on these images, this enables dose guidance as well as image guidance of radiotherapy treatments.

The use of megavoltage CT (MVCT) images for dose recomputations.

Megavoltage CT (MVCT) images of patients are acquired daily on a helical tomotherapy unit (TomoTherapy, Inc., Madison, WI). While these images are used primarily for patient alignment, they can also be used to recalculate the treatment plan for the patient anatomy of the day. The use of MVCT images for dose computations requires a reliable CT number to electron density calibration curve. In this work, we tested the stability of the MVCT numbers by determining the variation of this calibration with spatial arrangement of the phantom, time and MVCT acquisition parameters. The two calibration curves that represent the largest variations were applied to six clinical MVCT images for recalculations to test for dosimetric uncertainties. Among the six cases tested, the largest difference in any of the dosimetric endpoints was 3.1% but more typically the dosimetric endpoints varied by less than 2%. Using an average CT to electron density calibration and a thorax phantom, a series of end-to-end tests were run. Using a rigid phantom, recalculated dose volume histograms (DVHs) were compared with plan DVHs. Using a deformed phantom, recalculated point dose variations were compared with measurements. The MVCT field of view is limited and the image space outside this field of view can be filled in with information from the planning kVCT. This merging technique was tested for a rigid phantom. Finally, the influence of the MVCT slice thickness on the dose recalculation was investigated. The dosimetric differences observed in all phantom tests were within the range of dosimetric uncertainties observed due to variations in the calibration curve. The use of MVCT images allows the assessment of daily dose distributions with an accuracy that is similar to that of the initial kVCT dose calculation.

Characterization and use of EBT radiochromic film for IMRT dose verification

We present an evaluation of a new and improved radiochromic film, type EBT, for its implementation to IMRT dose verification. Using a characterized flat bed color CCD scanner, the films dose sensitivity, uniformity, and speed of development post exposure were shown to be superior to previous types of radiochromic films. The films dose response was found to be very similar to ion chamber scans in water through comparisons of depth dose and lateral dose profiles. The effect of EBT film polarization with delivered dose and film scan orientation was shown to have a significant effect on the scanners OD readout. In addition, the films large size, flexibility, and the ability to submerge it in water for relatively short periods of time allowed for its use in both water and solid water phantoms to verify TomoTherapy IMRT dose distributions in flat and curved dose planes. Dose verification in 2D was performed on ten IMRT plans (five head and neck and five prostate) by comparing measured EBT dose distributions to TomoTherapy treatment planning system calculated dose. The quality of agreement was quantified by the gamma index for four sets of dose difference and distance to agreement criteria. Based on this study, we show that EBT film has several favorable features that allow for its use in routine IMRT patient-specific QA.

Evaluation of a diode array for QA measurements on a helical tomotherapy unit

A helical tomotherapy system is used in our clinic to deliver intensity-modulated radiation therapy (IMRT) treatments. Since this machine is designed to deliver IMRT treatments, the traditional field flatness requirements are no longer applicable. This allows the unit to operate without a field flatness filter and consequently the 400 mm wide fan beam is highly inhomogeneous in intensity. The shape of this beam profile is mapped during machine commissioning and for quality assurance purposes the shape of the beam profile needs to be monitored. The use of a commercial diode array for quality assurance measurements is investigated. Central axis beam profiles were acquired at different depths using solid water built-up material. These profiles were compared with ion chamber scans taken in a water tank to test the accuracy of the diode array measurements. The sensitivity of the diode array to variations in the beam profile was checked. Over a seven week period, beam profiles were repeatedly measured. The observed variations are compared with those observed with an on-board beam profile monitor. The diode measurements were in agreement with the ion chamber scans. In the high dose, low gradient region the average ratio between the diode and ion chamber readings was 1.000 +/- 0.005 (+/- 1 standard deviation). In the penumbra region the agreement was poorer but all diodes passed the distance to agreement (DTA) requirement of 2 mm. The trend in the beam profile variations that was measured with the diode array device was in agreement with the on-board monitor. While the calculated amount of variation differs between the devices, both were sensitive to subtle variations in the beam profile. The diode array is a valuable tool to quickly and accurately monitor the beam profile on a helical tomotherapy unit.

TU‐FF‐A2‐06: The Use of MVCT Images for Dose Calculation in the Presence of Metallic Objects

Purpose: Metallic objects such as dental fillings and prostheses cause artifacts in kilovoltage CT (kVCT) studies. The problem with these artifacts is twofold. Firstly, they obscure soft tissue structures. Secondly, the artifacts create artificial CT numbers that compromise the accurate calculation of absorbed dose. Megavoltage CT (MVCT) imaging reduces these artifacts, and this study investigates the impact of such artifacts. Method and Materials: The MVCT to electron density curve for a Hi-ART II helical tomotherapy unit was extended to include high electron density materials using aluminum, titanium, and copper targets. MVCT images of a prostate patient with a hip prosthesis that is treated on a helical tomotherapy unit are then used to evaluate the impact of kVCT artifacts on the calculation of absorbed dose. At the time of treatment planning the kVCT image was used for planning. For actual patient treatment the artificial hip was generously contoured and no beam entrance though this region was allowed for treatment planning. Daily MVCT image of the patient were acquired for patient alignment. Retrospectively, a treatment plan was generated that allowed beam entrance through the hip prosthesis. This second plan was recalculated on the MVCT image to determine the dosimetric effect of the artifacts. Results: The recalculated dose in the MVCT image shows that the absorbed dose in the hip prostheses and immediately next to the prosthesis is lower in the MVCT image than in the kVCT image. This dose differential can be as large as 15%. Soft tissue areas that are affected by the kVCT image artifacts have higher absorbed dose in the MVCT based recalculation. Conclusion: In the presence of metallic artifacts, significant differences in the absorbed dose computation exist between dose calculations based KVCT images and that calculated in megavoltage images.

Megavoltage Computed Tomography Image-based Low-dose Rate Intracavitary Brachytherapy Planning for Cervical Carcinoma

Initial results of megavoltage computed tomography (MVCT) brachytherapy treatment planning are presented, using a commercially available helical tomotherapy treatment unit and standard low dose rate (LDR) brachytherapy applicators used for treatment of cervical carcinoma. The accuracy of MVCT imaging techniques, and dosimetric accuracy of the CT based plans were tested with in-house and commercially-available phantoms. Three dimensional (3D) dose distributions were computed and compared to the two dimensional (2D) dosimetry results. Minimal doses received by the 2 cm3 of bladder and rectum receiving the highest doses (DB2cc and DR2cc, respectively) were computed from dose-volume histograms and compared to the doses computed for the standard ICRU bladder and rectal reference dose points. Phantom test objects in MVCT image sets were localized with sub-millimetric accuracy, and the accuracy of the MVCT-based dose calculation was verified. Fifteen brachytherapy insertions were also analyzed. The ICRU rectal point dose did not differ significantly from DR2cc (p=0.749, mean difference was 24 cGy ± 283 cGy). The ICRU bladder point dose was significantly lower than the DB2cc (p=0.024, mean difference was 291 cGy ± 444 cGy). The median volumes of bladder and rectum receiving at least the corresponding ICRU reference point dose were 6.1 cm3 and 2.0 cm3, respectively. Our initial experience in using MVCT imaging for clinical LDR gynecological brachytherapy indicates that the MVCT images are of sufficient quality for use in 3D, MVCT-based dose planning.

SU‐FF‐T‐122: Characterization and Use of EBT Radiochromic Film for IMRT Dose Verification

We present an evaluation of a new and improved radiochromic film, type EBT, for its implementation to IMRT dose verification. Using a characterized flat bed color CCD scanner, the films dose sensitivity, uniformity, and speed of development post exposure were shown to be superior to previous types of radiochromic films. The films dose response was found to be very similar to ion chamber scans in water through comparisons of depth dose and lateral dose profiles. The effect of EBT film polarization with delivered dose and film scan orientation was shown to have a significant effect on the scanners OD readout. In addition, the films large size, flexibility, and the ability to submerge it in water for relatively short periods of time allowed for its use in both water and solid water phantoms to verify TomoTherapy IMRT dose distributions in flat and curved dose planes. Dose verification in 2D was performed on ten IMRT plans (five head and neck and five prostate) by comparing measured EBT dose distributions to TomoTherapy treatment planning system calculated dose. The quality of agreement was quantified by the gamma index for four sets of dose difference and distance to agreement criteria. Based on this study, we show that EBT film has several favorable features that allow for its use in routine IMRT patient-specific QA.

SU‐DD‐A3‐03: Evaluation of An Infrared Camera and X‐Ray System for Gated Radiation Therapy

Purpose: To determine the clinical feasibility of a gated treatmentdelivery system from BrainLab™. Method and Materials: The Exactrac from BrainLab™ is a localization system that uses both infrared cameras and x‐rays. In gated mode, target location is determined by implanted fiducials. Breathing patterns are determined by infrared reflectors attached to the patients surface. The User selects an x‐ray trigger point and radiotherapy beam‐on window relative to the breathing cycle. Multiple trigger levels may be selected to simulate a fluoroscopic mode to measure organ motion. Prior to clinical use feasibility tests including localization accuracy, gating window accuracy, and beam‐on accuracy were performed. Patients with small lung lesions were selected for treatment and implanted with a 20 by 0.7 mm gold fiducial. Treatment planning CT scans were taken at expiration breath hold with internal and external fiducials present. Results:_Localization accuracy was within 3mm when using 20% of the breathing cycle for beam on. To date, five patients with lung lesions were treated.Treatment times were approximately twenty minutes (standard dose fractionations). Implanted fiducials were well localizable in all patients. Target motion was on the order of 5mm average. Repeat CT scans showed implants did not migrate. The primary limitations with the system were related to breathing signal due to placement of external fiducials. Conclusion: Gating treatment technique from Exactrac™ has been used to treatlung lesions. This initial evaluation of the system verified the accuracy of the localization system under Gated mode. Implanted fiducials are localizable in patients, and gating is possible. The benefit of this system is the potential to decrease treatment margins and improve targeting. Continued evaluation of this system would help to define patient specific dose margins and beam‐on windows for treatment.

SU‐FF‐T‐06: Megavoltage CT Imaging Enables CT‐Based Low‐Doserate Brachytherapy Planning Without CT‐Compatible Applicators

Purpose: To determine feasibility of megavoltage computed tomography (MVCT) imaging for low dose rate (LDR) brachytherapy for cervical carcinoma.Method and Materials: A helical tomotherapy treatment unit (Tomotherapy HiArt ,Tomotherapy Inc., Madison, WI) using a nominal 3.5 MV x‐ray beam was used to image a dosimetry phantom containing standard, non‐CT compatible Fletcher‐Suit applicators and dummy Cs‐137 tube sources (3M Model 6500). The phantom contained several small steel BB markers which were used as dose points. The MVCT images were transferred to a commercially available treatment planning system (CMS XiO 4.2, Computerized Medical Systems, St. Louis, MO). Digitally reconstructed radiographs (DRRs) were generated from the MVCT data set, and doses(dose rates) were calculated at the phantom dose points using standard 2D, film‐based dosimetry methods. Doses were also calculated using a CT‐based, 3D dosimetry technique in which the brachytherapy source tips and ends were localized directly on MVCT images. Separate experiments were performed to verify the spatial accuracy of the MVCT image reconstruction in‐phantom. Results: In‐phantom localization accuracy was 0.6 ± 0.3 mm, slightly less than the size of an axial MVCT image pixel and less than half of the MVCT slice thickness. Point doses in‐phantom calculated by 2D and by 3D methods agreed within an average of 1.5%. The techniques developed for successful MVCT imaging in‐phantom can be used to calculate 3D dose distributions in a human patient undergoing a traditional (non‐CT compatible) Fletcher‐Suit implant. Conclusion: MVCT imaging allows CT‐based dosimetry planning without the use of special, “CT‐compatible” applicators.

Megavoltage Computed Tomography Imaging

Publisher Summary The most successful integration of megavoltage computed tomography (MVCT) imaging with radiation therapy devices is the TomoTherapy™ Hi-ART. It is currently the only type of patient treatment in which MVCT imaging is integrated as part of the treatment device itself. The Hi-ART II system can generate CT images from the same megavoltage X-ray beam it uses for treatment. These MVCT images offer verification of the patient position prior to and potentially during radiation therapy. The amount of integral dose (total dose in tissue) that the patient receives during MVCT images is dependent on the MVCT slice thickness and total length of the imaged region. The MVCT images need to be registered with the kVCT images in order to establish proper patient positioning. Once the MVCT images are acquired and reconstructed, the radiation therapist (RTT) has the option to use either automatic or manual registration, with the kVCT data set as the primary data set. Axial, sagittal, and coronal views are available on the treatment console to assist the operator during the manual registration process. There are currently three different available MVCT scanning modes—coarse, normal, and fine—which correspond to MVCT image slice thicknesses of 6.0, 4.0, and 2.0 mm, respectively.