Materials/Methods: Delivered dose-distributions were measured by a 3D dosimetry system consisting of the PRESAGETMdosimeter and an optical CT scanner (OCTOPUSTM). The PRESAGETM dosimeter (16cm in diameter and 11cm in height,see Fig.1a) was taken through the entire patient treatment procedure including CT simulation, treatment planning, and treatment.Treatment plans were generated in the ECLIPSE planning system, with doses of 10–15Gy prescribed to the isocenter. Thedosimeter was scanned 1 day after irradiation by acquiring 150 projections per slice, over 180 degrees, with a slice interval of5mm, and an in-plane pixel size of 0.5mm. The scan was converted to dose using a linear calibration curve. The planned andPRESAGETM dose distributions were compared with independent EBT film measurement to provide cross validation.

Results: Full 3D dose distributions (Fig.1b) were successfully acquired by the PRESAGETM/optical-CT dosimetry system. Forsimple conformal deliveries the PRESAGETM dose distribution (Fig.1c) shows good agreement with both the treatment plan(Fig.1d) and the film measurement (Fig.1e) (within 4% dose difference and 4mm distance to agreement). PRESAGETM showsbetter agreement with the film than with the plan in high dose gradient regions, consistent with penumbral blurring of theplanning system. For complex IMRT deliveries greater discrepancies have been observed the cause of which is underinvestigation.

Conclusions: This work supports the conclusion that PRESAGETM/optical-CT is a practical and effective dosimetry systemfor 3D dose verification, and as such has unique applicability to the dosimetric challenges of advanced radiation treatments likeIMRT, IGRT, and 4D treatments. The major advantages of PRESAGETM include high accuracy (by virtue of radiochromicdose response) and convenience (dose response is robust to lab environment).

Author Disclosure: P. Guo, None; J. Adamovics, None; M. Oldham, None.

2886 Dynamic Delivery Quality Assurance for IMRT to Account for Target Motion

H. A. Jaradat1, M. Mehta1, K. Nelson2, D. Schmidt2

1University of Wisconsin, Madison, WI, 2Standard Imaging Inc, Middleton, WI

Purpose/Objective(s): To develop a quality assurance (QA) technique that addresses the accuracy of radiotherapy deliveredto targets in motion. Helical tomotherapy has been used to deliver conformal IMRT to lung tumors where the target is incontinuous motion. QA techniques using static phantoms do not account for such target motion. We introduce a dynamicphantom that can be used to verify IMRT dose delivery to moving targets.

Materials/Methods: Respiratory Gating Platform (RGP) (Standard Imaging, Middleton, WI.) supports a cylindrical solid waterphantom. The phantom allows for both film and ionization chamber measurements in both coronal and sagittal planes. The RGPprovides oscillatory motion in one direction with amplitudes ranging from 5 to 40 mm in 5 mm steps, and cycle intervalsranging from 2 to 6 seconds with increments of 0.5 seconds. Based on our experience with 4D CT imaging for lung cancer theseranges are adequate to simulate the most significant thoracic motion scenarios, i.e. the cranio-caudal displacement caused bydiaphragmatic motion.To simulate different respiratory motion scenarios, six CT scans were acquired of the phantom with a cycle interval of 4 secondsand motion ranges of 0, 5, 10, 15, 20 and 40 mm. Previously delivered tomotherapy plans for NSCLC were recalculated on thesephantom CT image sets. Tomotherapy treatments were then delivered to the dynamic phantom. Sagittal and coronal films wereobtained to compute delivered dose. The dose distribution on each film was compared with the corresponding planneddistribution.

Results: Fig 1 shows coronal gamma distributions, profiles, planned (dashed) and delivered (solid) isodose lines for 5 rangesof motion, from 0 to 40 mm. The direction of phantom motion is also coronal. The static and the 5 mm range films reveal goodagreement between the planned and delivered dose distribution (white color). With 10 mm motion, the high dose regionagreement may be considered acceptable for delivery on a case-by-case basis. Motion of 20 mm or greater results inconsiderable disagreement between the planned and delivered dose distribution.

Conclusions: Because tomotherapy delivers dose in a continuous rotational fashion, the impact of motion on any single beamletis minimized, and consequently, minimization of thoracic motion is sufficient to produce good agreement between planned anddelivered IMRT dose distributions for motion ranges of 5 mm or less. With greater ranges of motion, caution needs to be

S711Proceedings of the 48th Annual ASTRO Meeting

exercised, and with motion of 20 mm or more, considerable disagreement results. The dynamic QA phantom introduced hereprovides suitable verification for IMRT for targets in motion.

Fig 1: Gamma distribution, profiles and Isodose lines for IMRT dynamic delivery verification.

Author Disclosure: H.A. Jaradat, None; M. Mehta, None; K. Nelson, Standard Imaging Inc., A. Employment; D. Schmidt,Standard Imaging Inc, A. Employment.

2887 Optimization of IMRT QA With EBT Gafchromic Film

F. Schneider1, M. Polednik1, D. Wolff1, A. Delana2, F. Lohr1, F. Wenz1, L. Menegotti2

1Department of Radiotherapy and Oncology, Mannheim University Hospital, Mannheim, Germany, 2Department ofRadiotherapy and Oncology, Ospedale Sta. Chiara, Trento, Italy

Purpose/Objective(s): Individual patient plan QA for IMRT is usually performed with dosimetry films to provide spatialinformation. Due to their range of application these films must have several characteristics, among them being constancy withinone batch and dose rate independency. Their absorption characteristics should be similar to water. Correlation of optical density(OD) and dose has to be unequivocal. An alternative to the widely used EDR2 films (Kodak) are the self developing GafchromicEBT films (ISP). In this project Gafchromic EBT films were evaluated with respect to several characteristics essential for IMRTQA.

Materials/Methods: To investigate the dependence of OD on dose rate, a linac with 6MV photons was used. Four films wereirradiated with the same dose but with different dose rates in the range from 55 to 450 MU/min and were compared with eachother. Two calibration films, taken from the same batch, were covering the dose range between 1 and 9 Gy to find out if equalOD is shown under equal irradiation conditions. Depth dose curves (DDC) were measured with films which were positionedin a water phantom. These were compared with the accelerator base data to assess the water equivalence of the films. Finally,several calibration curves were recorded to assign OD to dose. All films were scanned with the Epson Expression 1680 Pro flatbed scanner and were evaluated with VeriSoft from PTW.

Results: Dose rate dependence of EBT films is low with maximal OD difference between dose rates of 1% in the range of doserates studied. Homogeneity among one batch is excellent with a maximum deviation between two films of 0.9%. Thecomparison of the film DDC with the base data acquired with a water phantom shows a deviation of 1.2% � 0.8%. Thecalibration curves show a logarithmic course. Due to the monotony of the curve, an unequivocal translation of OD to dose ispossible.Larger errors (up to 30%) can be caused by the scanning process. When, however, a minimum interval of two minutes isintroduced between two scans, if the part of interest of a film is placed in the center of scanner field (20cm x 10cm), the scannedarea is small and the automatic colour correction is deactivated, the compound error could be reduced to 3%. Generatingcorrection matrices for individual scanners can help loosen these constraints. It is also important to scan the calibration and theverification film after the same developing time (�1h) as the OD changes even in opaque covers (after a month 3.5%).

Conclusions: Gafchromic EBT films have excellent characteristics as a dosimetry film with low dose rate dependency, easyhandling and self-developing characteristics. It is very important, however, to pay close attention to the choice of scanner andthe scanning process itself to reduce variation in the scanning results.

Author Disclosure: F. Schneider, None; M. Polednik, None; D. Wolff, None; A. Delana, None; F. Lohr, None; F. Wenz, None;L. Menegotti, None.

2888 Radiation Safety Issues With PET/CT Simulation for Stereotactic Body Radiotherapy

W. T. Kearns, W. H. Hinson, C. J. Hampton, J. M. Butler, J. J. Urbanic, D. Starnes, A. F. deGuzman, V. W. Stieber

Wake Forest University School of Medicine, Winston-Salem, NC

Purpose/Objective(s): To describe the process of PET/CT simulation for stereotactic body radiotherapy (SBRT) and addressradiation safety issues.

Materials/Methods: Our department performs PET/CT simulations with a dedicated radiation oncology PET/CT scanner(General Electric). Prior to the acquisition and implementation of the dedicated scanner in our department, patients wouldundergo a diagnostic PET scan. Because the patient was not in the treatment position, the subsequent fusion of the images witha treatment planning CT set was difficult. We have performed the first two PET/CT simulations of SBRT patients in acommercially available stereotactic bodyframe (Elekta). As with all of our PET/CT simulated patients, sophisticated immo-

S712 I. J. Radiation Oncology ● Biology ● Physics Volume 66, Number 3, Supplement, 2006