TH‐C‐M100E‐04: Cerenkov Light From Phantom Cassettes in Absolute Dose Measurements Using Radiographic Film

Perrin, D; Hogstrom, K; Cheek, D

doi:10.1118/1.2761672

Cited by 2 publications

(2 citation statements)

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“…In order to determine the relative contribution of Cerenkov radiation to the bare film darkening, 14 a method similar to Perrin et al's work was utilized. 15 EDR2 film was inserted into the sagittal film cassette, which was placed on the flat treatment couch of a Varian Clinac 21EX linac, on top of 10 cm of Plastic Water® backscatter material. One film ͑No.…”

Section: Iid2 Correction For Cerenkov Radiationmentioning

confidence: 99%

Accuracy of TomoTherapy treatments for superficial target volumes

et al. 2008

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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 machine's 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.

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Section: Iid2 Correction For Cerenkov Radiationmentioning

confidence: 99%

Accuracy of TomoTherapy treatments for superficial target volumes

et al. 2008

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“…As some Cerenkov light is produced in the white paper, keeping the readypack sleeve over the film also maintained the same irradiation conditions as the calibration film, ensuring proper calibration. 10,11 Each calibration curve ͑dose versus intensity͒ consisted of ten points, values at the center of each of the eight squares, a point at the center of the calibration film ͑scatter dose contribution͒, and a zero-dose point from the background film. A piecewise polynomial curve in RIT software was fit to the data points.…”

Section: Iie Film Dosimetry Systemmentioning

confidence: 99%

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

2008

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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 maj...

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TH‐C‐M100E‐04: Cerenkov Light From Phantom Cassettes in Absolute Dose Measurements Using Radiographic Film

Cited by 2 publications

References 0 publications

Accuracy of TomoTherapy treatments for superficial target volumes

Accuracy of TomoTherapy treatments for superficial target volumes

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

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