Daniel W. Neck

Evaluation of a commercial flatbed document scanner and radiographic film scanner for radiochromic EBT film dosimetry

The purpose of this study was to quantify the performance and assess the utility of two different types of scanners for radiochromic EBT film dosimetry: a commercial flatbed document scanner and a widely used radiographic film scanner. We evaluated the Epson Perfection V700 Photo flatbed scanner and the Vidar VXR Dosimetry Pro Advantage scanner as measurement devices for radiochromic EBT film. Measurements were made of scan orientation effects, response uniformity, and scanner noise. Scanners were tested using films irradiated with eight separate 3×3 cm2 fields to doses ranging from 0.115–5.119 Gy. ImageJ and RIT software was used for analyzing the Epson and Vidar scans, respectively. For repeated scans of a single film, the measurements in each dose region were reproducible to within ±0.3% standard deviation (SD) with both scanners. Film‐to‐film variations for corresponding doses were measured to be within ±0.4% SD for both Epson scanner and Vidar scanners. Overall, the Epson scanner showed a 10% smaller range of pixel value compared to the Vidar scanner. Scanner noise was small: ±0.3% SD for the Epson and ±0.2% for the Vidar. Overall measurement uniformity for blank film in both systems was better than ±0.2%, provided that the leading and trailing 2 cm film edges were neglected in the Vidar system. In this region artifacts are attributed to the film rollers. Neither system demonstrated a clear measurement advantage. The Epson scanner is a relatively inexpensive method for analyzing radiochromic film, but there is a lack of commercially available software. For a clinic already using a Vidar scanner, applying it to radiochromic film is attractive because commercial software is available. However, care must be taken to avoid using the leading and trailing film edges. PACS number: 87.55.Qr

Accuracy of cranial coplanar beam therapy using an oblique, stereoscopic x-ray image guidance system

A system for measuring two-dimensional (2D) dose distributions in orthogonal anatomical planes in the cranium was developed and used to evaluate the accuracy of coplanar conformal therapy using ExacTrac image guidance. Dose distributions were measured in the axial, sagittal, and coronal planes using a CIRS (Computerized Imaging Reference Systems, Inc.) anthropomorphic head phantom with a custom internal film cassette. Sections of radiographic Kodak EDR2 film were cut, processed, and digitized using custom templates. Spatial and dosimetric accuracy and precision of the film system were assessed. BrainScan planned a coplanar-beam treatment to conformally irradiate a 2-cm-diameter x 2-cm-long cylindrical planning target volume. Prior to delivery, phantom misalignments were imposed in combinations of +/- 8 mm offsets in each of the principal directions. ExacTrac x-ray correction was applied until the phantom was within an acceptance criteria of 1 mm/1 degrees (first two measurement sets) or 0.4 mm/0.4 degrees (last two measurement sets). Measured dose distributions from film were registered to the treatment plan dose calculations and compared. Alignment errors, displacement between midpoints of planned and measured 70% isodose contours (Deltac), and positional errors of the 80% isodose line were evaluated using 49 2D film measurements (98 profiles). Comparison of common, but independent measurements of Deltac showed that systematic errors in the measurement technique were 0.2 mm or less along all three anatomical axes and that random error averaged [formula: see text] 0.29+/-0.06 mm for the acceptance criteria of 1 mm/1 degrees and 0.15 +/- 0.02 mm for the acceptance criteria of 0.4 mm/0.4 degrees. The latter was consistent with independent estimates that showed the precision of the measurement system was 0.3 mm (2sigma). Values of Deltac were as great as 0.9, 0.3, and 1.0 mm along the P-A, R-L, and I-S axes, respectively. Variations in Deltac along the P-A axis were correlated to misalignments between laser isocenter and radiation isocenter as documented by daily clinical Lutz tests. Based on results of comparisons of measured with calculated positions of the 80% dose lines along the major anatomical axes, a 1.25, 1.0, and 1.0 mm (0.75, 0.5, and 0.25 mm) gross tumor volume (GTV)-planning target volume (PTV) margin to account for delivery error would be appropriate for the P-A, R-L, and I-S axes, respectively, for an acceptance criteria of 1 mm/1 degrees (0.4 mm/0.4 degrees). It typically took 2 (3) ExacTrac x-ray image sets to achieve and verify acceptance criteria of 1 mm/1 degrees (0.4 mm/0.4 degrees). Our results demonstrated a measurement technique using a CIRS anthropomorphic head phantom with a modified film cassette, radiographic film (Kodak EDR2) with a custom film cutting template, and film dosimetry software has been developed and successfully applied to our clinic. It is recommended that a third party offer this service. Our goal of achieving accuracy of delivery of 1 mm or better in each of the three major anatomical axes was almost, but not quite achieved, not because of the accuracy of the image guidance system, but likely due to inaccuracy of laser isocenter and other systematic errors.

Target localization accuracy in a respiratory phantom using BrainLab ExacTrac and 4DCT imaging

This study evaluated the accuracy of measuring the motion of an internal target using four‐dimensional computed tomography (4DCT) scanning and the BrainLAB ExacTrac X‐ray imaging system. Displacements of a metal coil implanted in a commercial respiratory phantom were measured in each system and compared to the known motion. A commercial respiratory motion phantom containing a metal coil as a surrogate target was used. Phantom longitudinal motions were sinusoidal with a 4.0 second period and amplitudes ranging from 5–25 mm. We acquired 4DCT and ExacTrac images of the coil at specified respiratory phases and recorded the coordinates of the coil ends. Coil displacement relative to the 0% phase (full‐inhale) position were computed for the ExacTrac and 4DCT imaging systems. Coil displacements were compared to known displacements based on the phantoms sinusoidal motion. Coil length distortion due to 4DCT phase binning was compared to the known physical length of the coil (31 mm). The maximum localization error for both coil endpoints for all motion settings was 3.5 mm for the 4DCT and 0.8 mm for the ExacTrac gating system. Coil length errors measured on the 4DCT were less than 0.8 mm at end inhale/exhale phases, but up to 8.3 mm at mid‐inhalation phases at the largest motion amplitude (25 mm). Due to the fast image acquisition time (100 ms), no coil distortion was observable in the ExacTrac system. 4DCT showed problems imaging the coil during mid‐respiratory phases of higher velocity (phases 20%–30% and 70%–80%) due to distortion caused by residual motion within the 4DCT phase bin. The ExacTrac imaging system was able to accurately localize the coil in the respiratory phantom over all phases of respiration. For our clinic, where end‐respiration phases from 4DCT may be used for treatment planning calculations, the ExacTrac system is used to measure internal target motion. With the ExacTrac system, planning target size and motion uncertainties are minimized, potentially reducing internal target volume margins in gated radiotherapy. PACS number: 87.56.‐v

Intra‐ and intervariability in beam data commissioning among water phantom scanning systems

Accurate beam data acquisition during commissioning is essential for modeling the treatment planning system and dose calculation in radiotherapy. Although currently several commercial scanning systems are available, there is no report that compared the differences among the systems because most institutions do not acquire several scanning systems due to the high cost, storage space, and infrequent usage. In this report, we demonstrate the intra‐ and intervariability of beam profiles measured with four commercial scanning systems. During a recent educational and training workshop, four different vendors of beam scanning water phantoms were invited to demonstrate the operation and data collection of their systems. Systems were set up utilizing vendor‐recommended protocols and were operated with a senior physicist, who was assigned as an instructor along with vendor. During the training sessions, each group was asked to measure beam parameters, and the intravariability in percent depth dose (PDD). At the end of the day, the profile of one linear accelerator was measured with each system to evaluate intervariability. Relatively very small (SD < 0.12%) intervariability in PDD was observed among four systems at a region deeper than peak (1.5 cm). All systems showed almost identical profiles. At the area within 80% of radiation field, the average, and maximum differences were within ± 0.35% and 0.80%, respectively, compared to arbitrarily chosen IBA system as reference. In the penumbrae region, the distance to agreement (DTA) of the region where dose difference exceed ± 1% was less than 1 mm. Repeated PDD measurement showed small intravariability with SD < 0.5%, although large SD was observed in the buildup region. All four water phantom scanning systems demonstrated adequate accuracy for beam data collection (i.e., within 1% of dose difference or 1 mm of DTA among each other). It is concluded that every system is capable of acquiring accurate beam. Thus the selection of a water scanning system should be based on institutional comfort, personal preference of software and hardware, and financial consideration. PACS number: 87.53.Bn

SU‐FF‐J‐08: Accuracy of Cranial Co‐Planar Beam Therapy with BrainLab ExacTrac Image Guidance

Purpose: To evaluate the positional accuracy (displacement between planned and delivereddose distributions) of cranial co‐planar beam treatments for image guided stereotactic radiation therapy with Novalis. Methods and Materials: Positional accuracy was investigated using a CIRS anthropomorphic head phantom loaded with 6.35cm × 6.35cm sections of EDR2 film oriented to measure dose in the three principal planes. BrainScan was used to develop a treatment plan consisting of seven equally spaced coplanar mMLC beams that conformed to irradiate a 2‐cm diameter by 2‐cm long PTV. Prior to delivery phantom misalignments were imposed in combinations of ±8‐mm offsets in one or more of the principal directions. ExacTrac X‐ray corrections were applied 1–3 times until the reported alignment was within 0.4mm/0.4° in all 6 degrees of freedom based on X‐ray to DRRimage fusion prior to treatmentdelivery. Phantom positions were tracked by the Novalis IR system. The delivereddose distribution was measured with a precision of ±0.3mm. Measured and calculated dose distributions were registered using 4 fiducial rods in the phantom. Results: Based on the 70% dose contour, the displacements of the delivered from the planned dose distributions ranged from 0.7 to 1.3mm, −0.4 to 0mm, and −0.6mm to 0.5mm in Posterior‐Anterior, Right‐Left and Inferior‐Superior directions respectively. For the 80% dose contour, the displacements of the delivered from the planned dose distributions ranged from 0.8 to 1.8mm, 0.1 to 0.8mm, and 0.1 to 1.1mm, respectively. Final displacements were independent of initial misalignments. Conclusions: Using recommended Novalis calibration procedures, the ExacTrac X‐ray image‐guidance system used in our clinic can deliver cranial dose distributions within 1.3mm of the planned dose distributions when initial misalignments are within ±8mm in the three principal directions. 80% isodose contours can have up to 1.8‐mm errors. This work is part of a research agreement with BrainLab, Inc.

Comparison of measured electron energy spectra for six matched, radiotherapy accelerators

Abstract This study compares energy spectra of the multiple electron beams of individual radiotherapy machines, as well as the sets of spectra across multiple matched machines. Also, energy spectrum metrics are compared with central‐axis percent depth‐dose (PDD) metrics. Methods A lightweight, permanent magnet spectrometer was used to measure energy spectra for seven electron beams (7–20 MeV) on six matched Elekta Infinity accelerators with the MLCi2 treatment head. PDD measurements in the distal falloff region provided R 50 and R 80–20 metrics in Plastic Water®, which correlated with energy spectrum metrics, peak mean energy (PME) and full‐width at half maximum (FWHM). Results Visual inspection of energy spectra and their metrics showed whether beams on single machines were properly tuned, i.e., FWHM is expected to increase and peak height decrease monotonically with increased PME. Also, PME spacings are expected to be approximately equal for 7–13 MeV beams (0.5‐cm R90 spacing) and for 13–16 MeV beams (1.0‐cm R90 spacing). Most machines failed these expectations, presumably due to tolerances for initial beam matching (0.05 cm in R 90; 0.10 cm in R 80–20) and ongoing quality assurance (0.2 cm in R 50). Also, comparison of energy spectra or metrics for a single beam energy (six machines) showed outlying spectra. These variations in energy spectra provided ample data spread for correlating PME and FWHM with PDD metrics. Least‐squares fits showed that R 50 and R 80–20 varied linearly and supralinearly with PME, respectively; however, both suggested a secondary dependence on FWHM. Hence, PME and FWHM could serve as surrogates for R 50 and R 80–20 for beam tuning by the accelerator engineer, possibly being more sensitive (e.g., 0.1 cm in R 80–20 corresponded to 2.0 MeV in FWHM). Conclusions Results of this study suggest a lightweight, permanent magnet spectrometer could be a useful beam‐tuning instrument for the accelerator engineer to (a) match electron beams prior to beam commissioning, (b) tune electron beams for the duration of their clinical use, and (c) provide estimates of PDD metrics following machine maintenance. However, a real‐time version of the spectrometer is needed to be practical.

SU‐E‐T‐97: Intra and Inter Variability in Beam Data Commissioning among Water Phantom Scanning Systems

PURPOSE There are many water-phantom scanning systems with advanced features to collect accurate commissioning data. However the intra- and inter-variability of commissioning data has not been reported which is attempted in this study. METHODS Four vendors with modern water-phantom scanning systems; PTW, Sun Nuclear (SN), Standard Imaging (SI) and IBA were invited to an institution to demonstrate beam data collection. Each system was used to collect percent depth dose (PDD) and profiles several times in a day with their choice of detector for four different machines for photon and electron beam commissioning. This provided information on intra-variability. At the end, each vendor was allowed to setup and collect data on a single unit for inter-variability. All data were sent to a central location for analysis and evaluation. RESULTS The depth dose and profiles for 2×2cm2 and 10×10cm2 fields were analyzed for intra- and inter-variability. With repeated measurements, the intra-variability provided a detailed degree of fidelity of data collection. This was shown to be with (± 0.1%) among all vendors. Ignoring data in buildup region and comparing with one system (PTW), the PDDs variability were slightly larger 0.02±0.17%, 0.14±0.21%, 0.17±0.2%, for SI, SN and IBA, respectively. The profiles inter-variability in central region were <0.1 %, however in penumbra up to ± 4.8%were observed. The polarity effect was also noted up to 3% which was depth and detector dependent. CONCLUSIONS Intra- and inter-variability among various scanning system are very small indicting that all modern systems if used properly could collect data within±0.2% accuracy. The selection of device should be based on institutional comfort and personal preference of software and hardware. This study provides unique opportunity to compare data among systems which is otherwise not possible.

SU‐E‐T‐415: Investigation of VMAT Patient Specific Quality Assurance Action Levels

Purpose: To examine the appropriateness of patient specific IMRTquality assurance (QA) action levels for use in VMAT QA. Methods: QA measurements were evaluated for the test geometries provided in AAPM Task Group Report 119. The structure sets were copied onto a cylindrical water‐equivalent phantom. Using the Philips Pinnacle treatment planning system, IMRT and VMAT treatments were planned. The plans were delivered and the resulting dose distributions were measured (1) in 2 planes and at 3–4 points in the cylindrical phantom using radiochromic film and ion chamber, respectively, and (2) using a commercial 2D diode array. Ion chamber and diode array measurements were taken five times each, and film measurements were taken three times. Measured planar doses were analyzed using gamma analysis with criteria of 3%/3mm. Measured point doses were analyzed using percent difference. Differences between IMRT QA and VMAT QA results were tested for significance using a Students t‐test. Results: The radiochromic film results showed averages of 98.9%±1.0% and 99.1%±0.9% (p=0.47) of measured doses within 3%/3mm of calculated doses for IMRT and VMAT plans, respectively. Ion chamber results showed average differences between measured and calculated point doses of − 0.95%±1.5% and −1.8%±1.8% (p<0.01) for IMRT and VMAT plans, respectively. The diode array results showed averages of 98.7%±0.5% and 98.6%±0.7% (p=0.70) of measured doses within 3%/3mm of calculated doses for IMRT and VMAT plans, respectively. Conclusion: Differences between IMRT QA and VMAT QA results were not statistically significant for planar doses measured with radiochromic film and 2D diode array. The average difference between measured and calculated point doses was smaller in IMRT plans (p<0.01); however, the average point dose differences for both IMRT and VMAT were less than 2%. These results suggest that current action levels used in our clinic for evaluating IMRT QA may be applicable to VMAT QA. This work is supported in part by a sponsored research agreement with Elekta, Inc.

SU‐GG‐J‐198: Phantom Evaluation of Implanted Coil Localization Accuracy of the BrainLab ExacTrac Gating System and 4DCT

Purpose: The purpose of this study was to evaluate the accuracy of measured target motion using 4DCT scanning and BrainLab ExacTrac (ET) Gating system for gated therapy. Motions of an implantable coil as a function of respiratory phase were compared from the two systems to the known motion for a commercial gating phantom. Method and Materials: A Quasar respiratory motion phantom containing an implantable coil as a surrogate target was used. Phantom motion was sinusoidal with a 4‐second period and amplitudes of 5–25 mm. 4DCT datasets were sorted in 10 distinct respiratory phases, reconstructed, and imported into the Pinnacle TPS. Coil endpoints were identified on each phase‐sorted CT datasets to measure coil distortion. To compare coil localization accuracy, both systems imaged the implanted coil at the same respiratory phases and measured overall relative coil displacement as a function of respiratory phase. Results: Coil length errors measured on the 4DCT were <0.8 mm at end inhale/exhale phases, but 8.1 mm at mid‐inhalation. Maximum localization error from the expected position for all motion profiles was 5.5 mm for both coil tips in 4DCT. but only 0.8 mm for the ET Gating system. Even at the greatest coil velocity, ExacTrac coil localization agrees with calculated coil motion within 1 mm. 4DCT showed problems resolving a coil during large respiratory‐induced velocities, but accurately resolved the coil length within 1 mm of actual coil length at end expiration/inhalation. Conclusion: 4DCT can provide accurate representation of the phantom at end‐respiration for treatment planning. ExacTrac can accurately localize the coil to determine target motion over all phases. Good agreement of the techniques will allow minimization of internal motion margins in gated radiotherapy.Conflict of Interest: This work was supported in part by a research agreement with BrainLab, Inc.

SU‐GG‐T‐276: Comparison of the Epson Perfection V700 Photo Flatbed and the Vidar VXR‐16 Dosimetry PRO AdvantageTM Film Scanners for Use with Radiochromic Film

Purpose: The purpose of this study was to compare the performance and utility of a commercially available flatbed scanner with that of a widely‐used medical film scanner for use with radiochromic film. Method and Materials: This project compares the Vidar VXR Dosimetry Pro and the Epson Perfection V700 Photo flatbed scanner. Using radiochromic EBT film irradiated to a known range of doses 0–512 cGy using eight 3×3 cm fields on a single film, both scanners were characterized for scan repeatability, orientation effects, uniformity, and scanner noise. RIT V5.0 was used to determine pixel value (PV) of Vidar scans, while ImageJ software was used for the Epson scans. Results: For the range of doses measured, average pixel values of a central 1.5×1.5 cm region of interest (ROI) were reproducible within 0.1% standard deviation with Vidar and approximately 0.2% with Epson. Epson mean PV differed by up to 18% for all dosed regions from landscape to portrait orientation, while only up to 5% for Vidar. From 0 to 512 cGy, identical films showed a 10% wider PV range for Vidar than Epson. Scanner noise was minimal; Epson showed a maximum SD of 1.00% from the mean of the central ROI, Vidar showed 1.14%. In both systems, cross profiles for an unirradiated film showed deviations in PV no more than 2% of the mean. Conclusion: No clear significant accuracy advantage was noted in either system, provided the leading and trailing 1 cm film edges were neglected with the Vidar system. Although the Epson scanner is a relatively inexpensive method for analyzing radiochromic film, the lack of commercially available software for it could be a major disadvantage. Hence, an institution already having a Vidar system may find its use with radiochromic film attractive.