D Cheek

Accuracy of TomoTherapy treatments for superficial target volumes

Helical tomotherapy is a technique for delivering intensity modulated radiation therapy treatments using a continuously rotating linac. In this approach, fan beams exiting the linac are dynamically modulated in synchrony with the motion of the gantry and couch. Helical IMRT deliveries have been applied to treating surface lesions, and the purpose of this study was to evaluate the accuracy of dose calculated by the TomoTherapy HiArt treatment planning system for superficial planning target volumes (PTVs). TomoTherapy treatment plans were developed for three superficial PTVs (2-, 4-, and 6-cm deep radially by 90 degrees azimuthally by 4-cm longitudinally) contoured on a 27-cm diameter cylindrical white opaque, high-impact polystyrene phantom. The phantom included removable transverse and sagittal film cassettes that contained bare Kodak EDR2 films cut such that their edges matched the phantom surface (+/-0.05 cm). The phantom was aligned to the machines isocenter (+/-0.05 cm) and was irradiated according to the treatment plans. Films were scanned with a Vidar film digitizer, and optical densities were converted to dose using a calibration determined from a 6 MV perpendicular film exposure. This perpendicular calibration required that axial film doses (parallel irradiation) be scaled by 1.02 so that mid-arc depth doses matched those measured in the sagittal plane (perpendicular irradiation). All film readings were scaled by 0.935 to correct for over-response due to phantom Cerenkov light. Measured dose distributions were registered to calculated ones and compared. Calculated doses overpredicted measured doses by as much as 9.5% of the prescribed dose at depths less than 1 cm. At depths greater than 1 cm, calculated dose distributions showed agreement to measurement within 5% in the high-dose region and within 0.2 cm distance-to-agreement in the dose falloff regions. In the low-dose region posterior to the PTVs (<10% of the prescribed dose), the dose algorithm underpredicted the dose by 1%-2% of the prescribed dose. Clinically, it is recommended that 1 cm of bolus be used on the surface to ensure that cancerous tissues less than 1 cm depth are not underdosed.

Independent calculation of dose from a helical TomoTherapy unit

A new calculation algorithm has been developed for independently verifying doses calculated by the TomoTherapy® Hi·Art® treatment planning system (TPS). The algorithm is designed to confi rm the dose to a point in a high dose, low dose‐gradient region. Patient data used by the algorithm include the radiological depth to the point for each projection angle and the treatment sinogram file controlling the leaf opening time for each projection. The algorithm uses common dosimetric functions [tissue phantom ratio (TPR) and output factor (Scp)] for the central axis combined with lateral and longitudinal beam profile data to quantify the off‐axis dose dependence. Machine data for the dosimetric functions were measured on the Hi·Art machine and simulated using the TPS. Point dose calculations were made for several test phantoms and for 97 patient treatment plans using the simulated machine data. Comparisons with TPS‐predicted point doses for the phantom treatment plans demonstrated agreement within 2% for both on‐axis and off‐axis planning target volumes (PTVs). Comparisons with TPS‐predicted point doses for the patient treatment plans also showed good agreement. For calculations at sites other than lung and superficial PTVs, agreement between the calculations was within 2% for 94% of the patient calculations (64 of 68). Calculations within lung and superficial PTVs overestimated the dose by an average of 3.1% (σ=2.4%) and 3.2% (σ=2.2%), respectively. Systematic errors within lung are probably due to the weakness of the algorithm in correcting for missing tissue and/or tissue density heterogeneities. Errors encountered within superficial PTVs probably result from the algorithm overestimating the scatter dose within the patient. Our results demonstrate that for the majority of cases, the algorithm could be used without further refinement to independently verify patient treatment plans. PACS number(s): 87.53.Bn, 87.53.Dq, 87.53.Xd

SU‐FF‐T‐213: Evaluation of Dose From TomoTherapy Irradiation of Superficial PTVs

Purpose: To evaluate the accuracy of dose calculated by the TomoTherapy HI⋅ART treatment planning system for superficial planning target volumes (PTVs). Methods and Materials: TomoTherapy treatment plans were developed for three superficial PTVs (2−, 4−, and 6−cm deep radially by 90° azimuthally by 4−cm longitudinally) contoured on a 27−cm diameter cylindrical white opaque, high‐impact polystyrene phantom. The phantom included removable transverse and sagittal film cassettes. Kodak EDR2 films were cut using templates to match the phantom surface (±0.5mm). The phantom was aligned to the machines isocenter (±0.5mm), and films were irradiated according to the treatment plans. Films were then scanned with a Vidar film digitizer and converted to dose using a calibration determined from a 6 MV perpendicular film exposure. Hence, axial film doses were scaled by 1.027 so that mid‐arc depth doses matched those measured in the sagittal plane. All film readings were scaled by 0.934 to correct for over response due to phantom Cerenkov light. Measured dose distributions were registered to calculated ones and compared. Results: For all PTVs, at depths less than 10mm calculated overestimated measured doses by as much as 8%. At depths greater than 10mm, the calculated and measured doses agreed to within 5%. In the dose falloff region, measured and calculated dose distributions agreed to within ±2mm. The calculated dose in the low dose region distal to the PTV was lower by as much as 2%. Each of these differences is related to an assumption in the dose model. Conclusion: Calculated dose distributions showed good agreement with measurement for depths greater than 10mm. For more superficial depths, the dose model should be carefully evaluated or one should use bolus of at least 10mm to ensure accurate calculated dose to superficial PTVs. Supported in part by a research agreement with TomoTherapy, Inc.

Evaluation of MVCT images with skin collimation for electron beam treatment planning.

This study assessed the potential of using megavoltage CT (MVCT) images taken with high density skin collimation in place for electron beam treatment planning. MVCT images were taken using the TomoTherapy Hi‐Art system (TomoTherapy Inc., Madison, WI), and the CT numbers were converted to density by calibrating the Hi‐Art system using an electron density phantom. Doses were computed using MVCT images and kVCT images and compared by calculating dose differences in the uniform dose region (>90%, excluding buildup region) and calculating distance‐to‐agreement (DTA) in high dose‐gradient regions (penumbra and distal falloff, 90%–10%). For 9 and 16 MeV electron beams of 10×10 cm calculated on a homogeneous CIRS Plastic Water (Computerized Imaging Research Systems Inc., Norfolk, VA) phantom without skin collimation, the maximum dose differences were 2.3% and the maximum DTAs were 2.0 mm for both beams. The same phantom was then MVCT scanned nine times with square skin collimators of Cerrobend on its surface ‐ field sizes of 3×3, 6×6, and 10×10 cm and thicknesses of 6, 8, and 10 mm. Using the Philips Pinnacle 3 treatment planning system (Philips Medical Systems, N.A., Bothwell, WA), a treatment plan was created for combinations of electron energies of 6, 9, 12, and 16 MeV and each field size. The same treatment plans were calculated using kVCT images of the phantom with regions‐of‐interest (ROI) manually drawn to duplicate the sizes, shapes, and density of the skin collimators. With few exceptions, the maximum dose differences exceeded ±5% and the DTAs exceeded 2 mm. We determined that the dose differences were due to small distortions in the MVCT images created by the high density material and manifested as errors in the phantom CT numbers and in the shape of the skin collimator edges. These results suggest that MVCT images without skin collimation have potential for use in patient electron beam treatment planning. However, the small distortion in images with skin collimation makes them unsuitable for clinical use. PACS: 87.53.Tf, 87.59.Fm, 87.53.Fs

SU‐GG‐T‐133: TomoTherapy for Post‐Mastectomy Radiotherapy (PMRT): TLD Chest Wall Dose Measurements

Purpose: To compare measured and calculated skindoses in post‐mastectomy radiation therapy (PMRT) of the chest wall (CW) treated with the TomoTherapy Hi‐Art system. Method and Materials: In‐vivodosimetry has been used to assess CW skindose at a single point for multiple fractions of 9 patients. On the first treatment day, a radiation therapist marked a point on the CW near the mastectomy scar and took pictures of the mark placement. A thin packet of TLD LiF powder was taped over the mark prior to megavoltage CT alignment and dose delivery. Following treatment,TLD readout was performed using a REXON TLD reader, with TL being converted to dose using a 6‐MV calibration curve. In the Pinnacle TPS, a Point of Interest (POI) was added at the location of the mark by comparing the pictures with 3D skin rendering. Calculated POI dose was obtained and compared to measured dose.Results: The number of daily TLD measurements acquired throughout the course of treatment ranged from 5 to 25 per patient. Overall for the 9 patients, the TLD measured (delivered) dose was less than the calculated (TPS) dose by 3.7±4.2%. This results in approximately 7% of the patients receiving less than 90% of the calculated dose. Reasons for the delivered dose being low are under investigation, but are likely due to inaccuracies in the TPS dose calculation, CW respiratory motion, and air gap between the bolus and patient. Conclusion: In‐vivoTLD measurements are useful in evaluating the dose delivered by TomoTherapy for PMRT of the CW. Understanding the reasons for the dose differences could allow improvement to our existing technique. Conflict of Interest: This work funded in part by a research grant with TomoTherapy, Inc.

SU‐FF‐J‐114: Dosimetric Effects of Image Quality in a TomoTherapy MVCT Dataset

Purpose: To evaluate the dosimetric effects of imagenoise and detector alignment artifacts in TomoTherapy MVCT datasets. Method and Materials: MVCT images of CTquality assurancewater phantoms were obtained on a TomoTherapy HI‐ART treatment system. Images were used as input to a commercial treatment planning system (Philips Pinnacle3) which allowed for adjustment of voxel densities within contoured structures. The entire phantom was contoured to override the densities determined from the MVCT to the known physical densities within the phantom. Heterogeneous treatment plans using static and rotational beams were performed on both the raw images and the images with density corrections. Dose differences between the two sets of plans were analyzed to determine the magnitude of discrepancies between the datasets. Results: Average MVCT densities of regions distant from the central region of the phantom showed densities with in 2% of densities of water. The central region which is aligned with the TomoTherapy central axis showed an average density as greater than water by 5% due to a detector alignment artifact. For both the static and rotational beams, the isodoses distributions for the two sets of plans were very similar. In general, rotational plans demonstrated less error than static relative to the prescription point placed at the phantom center. More detailed information was obtained with dose difference displays which demonstrate relative errors proportional to the delivered dose in phantom. The maximum error observed throughout the datasets was found to be small (<1%). Conclusion: Dosimetric differences resulting from imagenoise and artifacts were found to be insignificant. These results may be beneficial in the determination appropriate action limits for routine MVCT QA. Supported in part by a research agreement with TomoTherapy, Inc.

SU‐FF‐T‐217: Evaluation of the Dosimetric Accuracy of a Commercial Adaptive Radiotherapy Process

Purpose: To evaluate the dosimetric accuracy and reproducibility of the TomoTherapy Planned Adaptive system using MVCT datasets. Method and Materials: Comparisons were made between reference dose distributions computed with the TomoTherapy treatment planningsystem (TPS) and verification dose distributions computed on the TomoTherapy Planned Adaptive (PA) system. Both calculations were performed on TomoTherapy megavoltage CT (MVCT) datasets to eliminate differences associated with variations in CT‐density tables. Simulated PTVs drawn on an MVCT of an anthropomorphic radiosurgery head phantom (CIRS, Inc.) were used as input to calculate doses on the TomoTherapy TPS. After plan optimization, a post‐plan MVCT image was taken and used to calculate dose on the TomoTherapy PA system. Initially, calculations were made using the same MVCT dataset as input to both programs to investigate the agreement between the algorithms. Next, calculations were made using registered pre‐ and post‐plan MVCT images to investigate the variation with MVCT datasets. Comparisons were made using the tools available on the PA system and in‐house software to compare doses outside of contoured structures. Results: For calculations on identical datasets, isodose and DVH comparisons between the TomoTherapy TPS and PA systems showed insignificant dose differences (< 0.2%) to all contoured structures. Dose discrepancies within the phantom but outside of contoured structures showed slightly larger variations, although well within 0.5%. Similar results were found for dose calculations using different MVCT datasets. In both cases, the doses computed in air outside the phantom showed the largest differences (up to 21%). Conclusion: Comparisons between TomoTherapys TPS and PA system MVCT dose calculations showed clinically acceptable agreement everywhere within the phantom. Differences between doses calculated in air are due to differences in the way the algorithms mask low density voxels. Research sponsored by TomoTherapy, Inc.

TH‐D‐M100E‐04: Evaluation of MVCT Images Containing Lead Alloy Masks for Electron Beam Treatment Planning

Purpose: To evaluate the accuracy of electron beam dose calculations in MVCT images containing lead alloy masks. Method and Materials: A phantom consisting of two 30×30×5‐cm3 slabs of CIRS plastic water® was imaged using kVCT (GE Lightspeed‐RT) and MVCT (TomoTherapy Hi⋅Art). The MVCT scans were taken with nine square masks of Cerrobend® (density = 9.4gcm−3) on top of the phantom. The masks contained square openings of 3×3cm2, 6×6cm2 and 10×10cm2 and had thicknesses of 6mm, 8mm and 10mm. The same collimation was simulated in the kVCT images by creating regions‐of‐interest (ROI) duplicating the sizes, shapes, and density of the masks. Using the Philips Pinnacle3treatment planning system, twelve treatment plans were created using electron energies of 6, 9, 12, and 16 MeV for each opening size. For each plan, the mask thickness appropriate for the electron energy was used and the dose distributions calculated using the kVCT and MVCT images were compared. In uniform dose regions (doses above 90% of maximum) dose differences were calculated; in high‐dose gradient regions (doses below 90% of maximum) distances‐to‐agreement (DTA) were determined. Results: In the uniform dose region, the maximum difference between the doses in the MVCT images and the doses in the kVCT image was greater than or equal to ±5% for all opening and energy combinations. In the high‐dose gradient region, almost half of the maximum DTA values exceeded 2mm. Analysis of the MVCT images showed that DTA differences were largely due to distortions in the phantom CT numbers caused by the masks. Conclusion: Although Cerrobend® produces dramatically less distortion in MVCT images compared to kVCT images,image distortion is still too great for accurate electron beam dose calculations. Supported in part by a research agreement with TomoTherapy, Inc.

WE‐D‐AUD‐01: Clincal Evaluation of An Independent Dose Check Algorithm for Helical TomoTherapy

Purpose: To determine the clinical acceptability of an algorithm developed to independently verify doses calculated by the TomoTherapy treatment planning system. Method and Materials: Point doses from treatment plans for 97 patients treated on a TomoTherapy system were compared to an independent dose calculation algorithm developed at our facility. All patient plans for which treatmentsinograms were available were included in the comparison, which represented a variety of treatment sites and plan parameters. For each treatment plan, the calculation point was selected to be in the geometric center of the primary PTV, where beam modulation and high dose‐gradients were expected to be minimal. If this process put the point into a high dose‐gradient region or very near a tissue interface region, the point was manually repositioned. Results: Comparisons of our calculation to the TomoTherapy‐predicted point doses for the patient treatment plans showed good agreement. For sites other than lung and superficial PTVs, agreement between the calculations was within 2% for 94% of the patient calculations (64 of 68). Our independent calculations within the lung or superficial PTVs overestimated the dose by an average of 3.1% (σ =2.4%) and 3.2% (σ=2.2%), respectively. Conclusion: Systematic errors at points within lung are probably the result of the known weakness of the radiological path length method for correcting for missing tissue and/or tissue density heterogeneities. Errors encountered at points within superficial PTVs are probably the result of our algorithm overestimating the scatterdose at points near the surface of the patient. For the majority of cases, the algorithm demonstrates sufficient accuracy for clinical use and may be used to independently verify patient treatment plans. Supported in part by a research agreement with TomoTherapy, Inc.

SU-FF-T-261: Independent Point Dose Verification Using TomoTherapy Quality Assurance Phantom

Purpose: To detect TomoTherapy planning errors by relating a point dose in the patient treatment plan to the dose calculated at the corresponding point in the quality assurance phantom. Methods and Materials: Standard TomoTherapy patient‐specific quality assurance applies the patient treatment plan to a cylindrical QA phantom and then compares measured phantom dose to calculated phantom dose. However, there is no check of dose in the phantom to dose in the patient. We applied a ratio of TMRs to a point dose in the patient treatment plan in order to estimate the dose to the corresponding point in the phantom. Rotational delivery of TomoTherapy was approximated using either a 360° arc, 8 equally‐spaced angles, or 4 equally‐space angles. For each approximation, calculations were done using average physical depth and average radiological depth. The method was tested using data from 87 TomoTherapy patients. TMR ratio method doses were compared to treatment planning system doses. Agreement within ±5% was considered clinically acceptable. Results: Using average radiological depth, the pass rates were 70.6%, 69.4%, and 65.9% for the 360°, 8−, and 4‐;angle approximations, respectively. Using average physical depth, the pass rates were 63.5%, 56.5%, and 58.8% for the 360°, 8‐, and 4‐angle approximations, respectively. The best results were obtained for centrally‐located tumors such as prostate (83.9% pass rate for radiological depth and 360° arc approximation). The pass rates were worst for superficial tumors (50.0% pass rate for radiological depth and 360° arc approximation). Conclusions: The TMR ratio method was used to relate dose in the QA phantom to dose in the patient. For each beam approximation, radiological depths gave better results than physical depths. The site‐specific pass rates could be used to determine action levels for implementing this method in the clinic. Supported in part by a research agreement with TomoTherapy, Inc.