Proceedings of the 50th Annual ASTRO Meeting S617

Materials/Methods: Sixteen intrathoracic (15 Lung, 1 esophagus) cancer patients received repeat 4D-CT imaging within thesame day, as a part of an institutional review board approved protocol. Specifically, the patient was scanned once, then allowedto get up from the table for several minutes, and a repeat 4D-CT was performed. As reference, the GTV and reference bony struc-ture (vertebral bodies) were delineated for the end-expiration (EE) phase of the first 4D-CT. To remove setup error in the second4D-CT set, all CT images were first aligned to the bony reference. Location of the GTV relative to bony structures was then de-tected for each phase using an in-house 3D rigid registration algorithm. GTV locations were compared in pairs phase-by-phasebetween the first and second 4D-CT sets and then across patients. In order to assess GTV volume variations as results of changes inGTV location and GTV shape, we propagated the original GTV from the reference phase CT to the rest of CT data sets usinga previously developed deformable image registration method. The shape and misalignment for a GTV pair was analyzed accord-ing to each phase using a volume overlap index (VOI), which is defined as the overlap of two volumes divided by the average ofthe two GTVs.

Results: The detected GTV positional variations (mean shifts ± 1 standard deviation [1SD]) were 2.4 ± 1.6 mm across all phasesand all patients (range, 0 to 9.4 mm). The smallest mean positional shift (2.1 mm) occurred in the EE phase and the largest positionalshift (2.9 mm) occurred in the mid-inspiration (MI) phase. Similarly, the variations across patients (1SD) were the smallest for theEE phase (2.4 mm) and the largest for the MI phase (3.5 mm). The VOI for GTV ranged from 0.45 to 0.95 (mean: 0.84) for the EEphase, and 0.48 to 0.95 (mean: 0.87) for the end-inspiration phase. A smaller VOI indicates worse volume alignment of the GTV forthe corresponding phase. The misalignment of smaller GTVs has the most significant impact. The ITV determined using two 4D-CT sets increased by 12% on average (range, 4% to 39%) over the ITV determined using only one 4D-CT set.

Conclusions: We found phase-by-phase positional uncertainties of GTV in repeat 4D-CT imaging of thoracic patients. It is rec-ommended that an additional margin be assigned if the ITV is derived from only one 4D-CT scan.

Author Disclosure: H. Wang, None; S. Vedam, None; P. Balter, None; G. Starkschall, None; L. Zhang, None; J. Cox, None; R.Mohan, None; L. Dong, None.

2998 Impact of Individualized Target Volumes on Stereotactic Body Radiotherapy Treatment Planning (SBRT):

Volumetric Analysis

C. J. Hampton, W. T. Kearns, J. J. Urbanic, K. P. McMullen, A. W. Blackstock, W. H. Hinson

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

Purpose/Objective(s): This work assesses the impact of individualized target volumes derived from helical and 4D-CT on treat-ment plans for lung SBRT patients. Individualized target volumes are contrasted with patient population-based target volumes rec-ommended by the RTOG 0236 phase II clinical trial.

Materials/Methods: Six patients were simulated using the Elekta Stereotactic Body Frame (Elekta, Inc., Norcross, GA), three ofwhich met the criteria for the use of abdominal compression to force shallow breathing by the patient. Another three patients wereunable to tolerate the abdominal compression or, limited by anatomy, unable to avoid collisions with the device. 4D-CT was usedfor all patients to create patient-specific internal target volumes (ITV) either as a complement to or in the absence of the compres-sion device. Our SBRT simulation/treatment planning protocol introduces a hybrid internal target volume (ITVInd) defined by thecombination of 4D-CT-derived maximum intensity projections (MIP) capturing intrafractional changes in tumor position due torespiration, with gross tumor volumes (GTV1 and GTV2) contoured from 2 sequential helical CT acquisitions interspaced by pa-tient repositioning. Non-gated PET imaging was also included at our physicians’ discretion. Isotropic margins of 5mm were addedto ITVInd as a ‘‘safety margin" creating PTVInd. For comparison, a second PTV (PTV0236) was created based on the expansion ofGTV2 with population-based margins of 5mm (10mm S/I) as recommended by RTOG 0236.

Results: The maximum 3D vector difference in the center-of-mass position of GTV1 and GTV2 was 6.3 mm. The inclusion of setuperror and 4D breathing motion resulted in an ITVInd that was larger than GTV2 for all patients (average ratio ITVInd/GTV2 = 2.2).When analyzed as subgroups, the ratio of ITVInd/GTV2 was 1.6 and 2.8 for patients with and without abdominal compression,respectively. Expanding the ITVInd to PTVInd resulted in a larger volume than the expansion of the GTV2 to PTV0236 (averageratio PTVInd/PTV0236 = 1.2) for all patients except one for which PTV0236 slightly overestimated the expansion needed to coversetup/motion error. PTV0236 can underestimate the expansion needed to cover setup/motion error by as much as 60% if no abdom-inal compression is used.

Conclusions: The use of abdominal compression in combination with individualized setup and motion imaging for PTV determi-nation correlated well with the population-based margins recommended by RTOG 0236. Individualized target margins are espe-cially advisable when abdominal compression is not available since patient population-based margins may fail to completelyencompass the extent of inter- and intrafraction motion.

Author Disclosure: C.J. Hampton, None; W.T. Kearns, None; J.J. Urbanic, None; K.P. McMullen, None; A.W. Blackstock, None;W.H. Hinson, None.

2999 Validation and Comparison of Dosimetry for Small Lung Targets in Stereotactic Body Radiation Therapy

(SBRT)

T. Boike, K. Park, P. Lee, M. Edwards, S. Shakouri, S. Shaddock, P. DeRose, R. Abdulrahman, R. Timmerman, T. Solberg

University of Texas Southwestern Medical Center, Dallas, TX

Purpose/Objective(s): SBRT has yielded excellent clinical results for early stage lung cancer and now challenges surgery in ratesof local control. With broader implementation of this technique, we are treating an increasing number of small lesions for whichdosimetry is not well known. This study aims to compare measured film doses with clinically predicted doses in small lung targetsfrom superposition/convolution, pencil beam, and Monte Carlo algorithms using patient and phantom data.

Materials/Methods: From July 2007 to March 2008, 9 patients with 12 thoracic lesions\9cc in volume treated with SBRT wereselected. All patients were simulated in a stereotactic body frame with abdominal compression to limit tumor motion to #1 cm.