Proceedings of the 50th Annual ASTRO Meeting S593

Materials/Methods: H&N cancer patients, each of whom had weekly isocenter checks (isocheck), were identified by retrospectivechart review. So far a total of 30 CT scans have been analyzed. To quantify the inter-isocheck setup error, patients’ linear and ro-tational positions were measured. The left/right (L/R), inferior/superior (I/S), and posterior/anterior (P/A) isocenter displacementswere measured. For rotational displacement, the angle between the axes, each of which connects the center of the vertebral bodyand the spinous process at the isocenter level, in the isocheck CT and the planning CT was measured. Also, tumor and lymph nodevolume changes were calculated for isocheck and planning CT data sets. To evaluate the effects of patient setup error and tissuevolume changes, the original plan and region of interests were copied to a phantom of each isocheck CT data set. To assess if IGRTcan mitigate these effects, each isocheck data set was evaluated by either shifting the isocheck images to correct for any linear dis-placements or left unaltered. Dose volume histograms (DVHs) were calculated and compared for each treatment plan.

Results: Mean L/R, I/S, and P/A isocenter displacements are 3.6 mm, 2.5 mm, and 3.8 mm, respectively. Average rotational dis-placement was 3.6�. Tumor volume decreased to �70% and �25% of original size by week 3 and 6, respectively. Lymph nodevolumes decreased to �40% of original size by week 3. Without altering patient setup, DVH analysis showed an increase indose of 3%, 12%, and 16% to the tumor, cord, and parotids, respectively. With patient shifts to correct for setup errors, accuratedose delivery to the tumor was achieved. However, even with shifts, the cord and parotids were still overdosed by .10%. Detailedanalysis of this data will be presented.

Conclusions: Our findings suggest small linear and rotational displacements can manifest as significant effects on dose delivery tocritical structures. Tissue volume changes in H&N cancers may require at least one replanning and as early as week 3 of treatment.IGRT using superior quality images of diagnostic CT can accurately detect when replanning becomes necessary.

Author Disclosure: C.P. Chen, None; J. Wong, None; C.W. Chang, None; M. El-Gabry, None; S. Merrick, None; Z. Gao, None.

2946 Accuracy of MRI/CT for Immobilized Head and Neck Cancer Patients

J. E. Bayouth, M. Muruganandham, C. E. Duncan, A. K. Gupta

University of Iowa Hospitals & Clinics, Iowa City, IA

Purpose/Objective(s): MRI of head and neck (ENT) cancer has been limited by the inability to accurately position the patient intreatment position using radiation therapy immobilization devices. We propose this can be done at high field strength MRI usinga flexible coil and parallel imaging without significant distortion for immobilized ENT patients in treatment position improvingtarget delineation.

Materials/Methods: All MRI scans were performed on 3T (Siemens Trio Tim) scanner using flexible 6 element body matrix coilwith a 3D VIBE (Volumetric interpolated breath-hold exam) sequence with contrast. Six patients were immobilized with the cus-tomized thermoplastic head-and-shoulder device (MEDTEC S-frame) and head holder for both CT and MRI, with only the headportion of the mask snapped into the base-plate for MRI. The body matrix coil was placed over the patient within the immobili-zation system. The images were acquired using TR/TE = 9.0/4.27 ms, 0.9 x 0.8 x 2.0 mm resolution in parallel imaging mode withan acceleration factor of 2 providing a coverage of �34 cm from the skull cap to T1 in 5 min. Images were registered and fused ina commercial treatment planning system. The center-of-mass within 6 regions in the head (right/left eye, right/left internal auditorycanal, pituitary, and tentorium) and 15 regions in the neck (spinal canal and right/left vertebral artery, from C2 to C6) were iden-tified on both CT and MRI and vector displacement was computed. To assess the value of MRI in tumor segmentation, the relativecontrast of the tumor on MRI was compared to CT by computing the mean pixel value in the tumor and associated normal tissue.Both images were contrast enhanced.

Results: The MRI scanner was able to image all patients with their immobilization systems. Relative to the CT images, MRI reg-istration errors caused by either spatial distortion and/or non-rigid body deformation of the patient (mean 2.4 mm, RMS error 1.0mm) were within the spatial resolution of the imaging techniques (CT slice thickness 3.0 mm). In general, no difference was seenfor registration of regions in the head (2.1 ± 0.9 mm, range, 0-3.6 mm) and the neck (2.3 ± 1.0 mm, range, 0.5-5.0 mm). Cervicalspine rotation errors about C2 in two patients caused deviations up to 5 mm when extended down to C6, causing a subtle increase inregistration error at a rate of 0.1 mm / cm (r2 = 0.988) away as one extends inferior from the point of rotation. The contrast of thetumor on MRI was superior (21.6% increase relative pixel value) to that seen on CT (1.1%) and found to be significant (p = 0.002).

Conclusions: MRI acquisition of ENT patients in radiation therapy immobilization systems is feasible, imaging techniques andrepositioning of the patient introduced registration errors within the uncertainty inherent within the imaging voxel size, and pro-vided superior contrast for tumor delineation.

Author Disclosure: J.E. Bayouth, None; M. Muruganandham, None; C.E. Duncan, None; A.K. Gupta, None.

2947 Using Patient as Control to Identify the Threshold SUV for FDG-PET Tumor Delineation for Radiation

Therapy in Head and Neck Cancers

S. M. McGuire, K. C. Bylund, J. E. Bayouth

University of Iowa Hospitals and Clinics, Iowa City, IA

Purpose/Objective(s): One standard methodology for clinical target volume (CTV) delineation in head and neck cancers usingFDG-PET has been based on critical stand uptake value (SUV) thresholds from population studies. However, this thresholddoes not account for individual patient variations in normal glucose uptake. To address this issue, we have developed a methodusing SUVs of normal tissue equivalent to the diseased region for a patient specific SUV threshold value.

Materials/Methods: Head and neck cancer patients with FDG-PET scans obtained under sedation 90 minutes after tracer injectionwere selected based on having primarily unilateral disease sites for retrospective study. These patients were treated by identifyingnested clinical target volumes to treat the primary and boost regions simultaneously. CTV1 represented the tightest margin to theGTV and was used to identify the characteristic SUV values for the tumor region and the equivalent normal tissue. Typically, thevolume with an SUV greater than 2.5 was identified in the planning stages, but the CTV1 was not identified based solely upon this

S594 I. J. Radiation Oncology d Biology d Physics Volume 72, Number 1, Supplement, 2008

information. The CTV1 was mirrored on the opposite side of the patient to identify equivalent normal tissue to calculate the normalSUV. A new CTV1 was then created based on the mean of the normal SUV plus two standard deviations.

Results: Mirror CTV1s were created for 10 patients, and any overlap was eliminated. Nine of the cases had less than 15% overlapbetween the CTV1 and mirror CTV1s. One patient had �29% overlap and a mirror CTV1 mean SUV of 2.66, indicating that thedisease was too bilateral to use this method effectively. Another patient had high PET activity in the tonsils which extended into themirror region and artificially increased the normal mean SUV. Of the remaining eight patients, the CTV1 SUV population meanwas 3.44 ± 1.59, and the normal SUV population mean was 1.74 ± 0.62. The new SUV cutoff values ranged from 2.36 to 3.64, butthe new CTV1s created using only the patient specific SUV cutoffs were smaller than the treated CTV1s for all cases by an averageof 61.9 ± 16.5%. New CTV1s were largely contained within original CTV1s, however an average of 6.3 ± 9.4% of the new CTV1volume was identified outside the original CTV1. The large standard deviation is due to one patient with an SUV threshold of 2.36and 27.9% of the new CTV1 volume outside the original CTV1. This may be because new regions were identified using a thresholdlower than 2.5.

Conclusions: The range of SUV thresholds obtained by using a value two standard deviations above the equivalent normal tissuemean and the subsequent volume change in CTV1 indicates that patient specific information is important in tumor delineation.However, this method is limited to cases where there is primarily unilateral disease.

Author Disclosure: S.M. McGuire, None; K.C. Bylund, None; J.E. Bayouth, None.

2948 RapidArc - Commissioning and QA Procedures

C. Ling1,2, P. Zhang1, Y. Archambault2, J. Bocanek2, G. Tang3, T. LoSasso1

1Memorial Sloan-Kettering Cancer Center, New York, NY, 2Varian Medical Systems, Palo Alto, CA, 3University of Maryland,Baltimore, MD

Purpose/Objective(s): To develop a program of commissioning and quality assurance for the Varian RapidArc radiation deliverysystem. RapidArc performs IMRT using one gantry revolution to deliver a 2 Gy fraction in 2 minutes or less. The key advancedtechnical features are simultaneous dynamic MLC and dose-rate variation during variable-speed gantry rotation.

Materials/Methods: We designed a step-by-step program to evaluate the performance of a RapidArc-enabled Clinac. (1) Toassess the dynamic MLC accuracy, we performed RapidArc plans of picket-fence patterns, for comparison with the same patternat fixed gantry. (2) Using a custom-designed RapidArc plan we irradiated different parts of a film with different dose-rates, toascertain the accuracy of variable dose-rate. (3) We studied the combined use of dynamic MLC and variable dose-rate to achievea designed dose pattern. All tests were performed with radiographic films mounted at isocenter on a fixture that rotated with thegantry (Sun Nuclear Isocenter Mounting Fixture - IMF).

Results: (1) The mechanical stability of the IMF, as measured using a slit light field and a slit X-ray field at different gantry angles,was better than�0.5 mm. Comparison of picket-fence patterns, acquired at stationary gantry angle and during RapidArc delivery,showed shifts of 1 mm or less. ‘‘Intentional errors’’ inserted into the picket fence pattern showed that the sensitivity of this test fordetecting 0.5 mm errors in MLC position. (2) During RapidArc delivery, different parts of a film were exposed to the same dosewith variable dose rates (equivalent to 111, 222, 332, 443, 554, 665 and 776 MU/min or 0.33, 0.66, 0.99, 1.33, 1.67, 2 and 2.33MU/deg) by combining variable dose-rate (up to 600 MU/min) and variable gantry speed (up to 5.54 deg/sec). The observed meanvariation in the delivered dose was 1.3% (range, 0.1- 2.1%). (3) Different parts of a film were exposed to the same dose usingDMLC sliding window technique, combining different leaf speeds (0.5, 1, 2 and 3 cm/sec) with different dose rates (150 - 600MU/min). The mean deviation from the intended dose was 0.7% (range, 0.3 - 1.3%). (4) Log files of machine performance (Dy-nalog files) during RapidArc delivery, comparing intended and achieved parameter values, indicated mean standard deviations of0.04 MU and 0.26 deg at the control points of the RapidArc plans.

Conclusions: We have developed a prototypical program of commissioning and quality assurance for the Varian RapidArc radi-ation delivery system. The program tests the critical technical features of simultaneous dynamic MLC and dose-rate variation dur-ing variable-speed gantry rotation. The test results indicate that the Clinac control system accurately implements the designedRapidArc plans, with excellent agreement between the planned and delivered dose patterns.

Author Disclosure: C. Ling, Varian, A. Employment; P. Zhang, None; Y. Archambault, None; J. Bocanek, None; G. Tang, None;T. LoSasso, None.

2949 Dynamic Contrast Enhanced Magnetic Resonance Imaging (DCE-MRI) Assessment of Tumor Physiology

in Head and Neck Cancer: A Comparison of Intra Patient and Inter Patient Variability

O. I. Craciunescu1, D. Yoo1, E. W. Cleland1, D. P. Barboriak1, M. Dolguikh2, N. Muradyan2, M. Kasibhatla1, M. Carroll1,D. M. Brizel1

1Duke University Medical Center, Durham, NC, 2CAD Sciences, Inc., White Plains, NY

Purpose/Objective(s): DCE-MRI quantitatively measures tumor physiology and treatment induced changes including vasculartransfer (PERM, Ktrans), extracellular volume fraction (EVF, Ve), and initial area under the curve (iAUC), calculated from an intra-tumor region of interest (ROI). Optimal ROI delineation is not established. The valid use of DCE-MRI requires that the variationmeasured within a tumor be less than that observed between tumors in different patients. This work evaluated the impact of ROIselection on assessment of intra and inter patient variability in a prospective clinical trial.

Materials/Methods: Patients received hyperfractionated RT + concurrent cisplatin with synchronous Avastin (A) and Tarceva(T). Patients were randomized to receive 2 weeks (lead-in) of A, T, or A+T prior to targeted chemoradiation with A+T. DCE-MRI images acquired on a 1.5T GE Signa Exite scanner at baseline and at the end of lead-in were analyzed. A dynamic 3D spoiledgradient echo sequence was used before and after bolus injection of Gd DTPA (Magnevist): TR = 6.4 ms, field of view (FOV) = 24cm, matrix size: 256 x 256, single scan duration = 10 sec, number of scans = 31-33, 10 mm thickness, flip angle = 60o. Full time