S680 I. J. Radiation Oncology d Biology d Physics Volume 78, Number 3, Supplement, 2010

Conclusions: By using an offline correction algorithm calculated from images taken during the first 4 fractions of treatment, thesystematic error was accurately predicted in all 3 dimensions. The shifts calculated offline would have placed the prostate within thePTV in 95% of cases. This study confirms in a large group of patients that an offline MVCT protocol can yield accurate estimates ofsystematic error in patients with prostate cancer, potentially increasing throughput and minimizing additional patient time-in-vault.

Author Disclosure: D.G. Baseman, None; C.D. Fuller, None; N. Papanikolaou, None.

3064 Single Photon Emission Computed Tomography (SPECT) and Computed Tomography (CT) Registration

for Liver Stereotactic Body Radiotherapy (SBRT)

E. E. Day1,2, S. Mohammadi1, O. Gayou1,2, A. Kirichenko1,2

1Allegheny General Hospital, Pittsburgh, PA, 2Drexel University College of Medicine, Pittsburgh, PA

Purpose/Objective(s): Describe a method to register 99mTc sulfur colloid SPECT with SBRT treatment planning CT imaging toidentify normal functioning liver and splenic parenchyma with increased precision for liver SBRT.

Materials/Methods: Eleven liver SBRT patients underwent SPECT and respiratory gated 4D-CT using a custom VacLoc bag forimmobilization. To reduce uncertainty in registration, up to nine radio-opaque markers, 1 mm in diameter, were placed on the pa-tient’s surface prior to the 4D-CT. To minimize potential anatomic changes due to stomach, bowel or bladder filling, patients werepositioned in the VacLoc bag for the SPECT scan within 2 hours of completing the CT scan. The markers on the patient’s surfacewere replaced with 99mTc labeled markers showing as 1.5 cm diameter circles on the SPECT image. To enable contouring of thephotopenic normal liver parenchyma, the SPECT 3D liver image was converted to slices using MatLab (Natick, MA). Once theslices were obtained, the free breathing CT and SPECT images were registered by two methods using the LEONARDO multi mo-dality workplace software (Siemens Inc., Malvern, PA). First, the markers were ignored and registration was based on identifyingthe liver on the SPECT and CT images. In the second registration method the markers were first used to obtain a global registration,which was then fine-tuned using the liver information. The average distance between the centers of the SPECT and CT markers wasrecorded after each registration. The physician then contoured volumes of photopenic SPECT normal liver parenchyma (SPECT-NLV) on the registered images and the contours were incorporated into 3D-conformal treatment planning.

Results: The mean SPECT-to-CT markers distance over all patients was 8.6 mm (range, 28.05/2.20) for the liver-only registrationmethod and 6.3 mm (range, 20.14/1.10) for the markers and liver combination method. While the data appears to show that bothregistration methods yield similar results, there are inherent difficulties in fusing a non-anatomic image such as SPECT to a CT scanwithout the use of external markers. In some cases the lesions in the liver can be identified on both images and can be used as ideallandmarks. However in most cases, using the markers gives a greater degree of confidence in the fusion process. Additionally, theuse of the markers as a starting point for the registration results in a significant decrease in the registration time.

Conclusions: Due to the inherent problems encountered when trying to fuse a non-anatomic image to a CT scan, the use of 99mTcmarkers allowed for a more robust registration in a shorter amount of time.

Author Disclosure: E.E. Day, None; S. Mohammadi, None; O. Gayou, None; A. Kirichenko, None.

3065 Cone-Beam CT (CBCT) May be Necessary to Ensure Planned Spinal Cord Doses are not Exceeded in Head

and Neck (H&N) Patients Treated with Intensity Modulated Radiotherapy (IMRT)

K. Farrey, M. Sadinski, D. W. Golden, G. Redler, K. M. Yenice, D. J. Haraf, C. A. Pelizzari, J. K. Salama, H. A. Al-Hallaq

University of Chicago, Chicago, IL

Purpose/Objective(s): To utilize CBCT to assess the effects of setup errors on delivered dose for H&N IMRT patients.

Materials/Methods: Five locoregionally advanced base-of-tongue (n = 4) and tonsil (n = 1) patients treated on a concurrent che-motherapy and IMRT phase II study were included. Per institutional practice, patients were immobilized in a light cast with alpha-cradle head and shoulder support. Treatment setup was performed using skin marks and standard MV imaging, while intrafractionmotion was corrected using real-time video positioning. For this analysis, patients were also imaged biweekly with CBCT with thetreated isocenter marked. Positioning using CBCT was optimized offline by expert physicians using translational shifts. Setup er-rors were calculated by comparing the ‘‘optimized’’ position to the treated position. CBCT images were spliced into the treatmentplanning CT to allow for dose calculation in both the treated and ‘‘optimized’’ positions. The accuracy and reproducibility of CBCTcalculated dose were assessed in an anthropomorphic H&N phantom. Dose parameters collected include: PTV V95% and V105%,and maximum dose to 0.5 cc of the spinal cord. Statistical tests and correlations of paired samples were considered significant at p\0.01.

Results: Setup errors calculated from 54 images indicate a mean three-dimensional (3D) deviation of 0.37 ± 0.17 cm (0.07-0.76 cm) of the treated isocenter. Compared to the treatment plan, PTV coverage decreased significantly from 99.9% to 97.1%while cord dose increased significantly by 4.5 ± 9.8%. Repositioning with CBCT would have significantly improved PTV cov-erage to 98.1%, eliminated differences in cord dose, and significantly reduced V105% by 3.1%. Increased cord dose in thetreated position correlated with setup errors in the anterior-posterior direction (r = 0.52) and with the minimum 3D distancefrom spinal cord to PTV (r = 0.77). Maximum cord doses were located at the level of the cervical spine (C2-C3), wherethe PTV and spinal cord separation was smallest. Despite the use of custom immobilization for the H&N, deformation wasvisible on CBCT.

Conclusions: We found that daily video positioning and frequent MV imaging leads to high and consistent PTV coverage for H&Npatients without CBCT. However, CBCT may be necessary to ensure planned spinal cord dose is not exceeded due to the low re-producibility tolerance (0.9 ± 1.7 mm) and high dose gradients located near this critical organ. CBCT allows for accurate assess-ment of the 3D separation between the spinal cord and PTV despite neck deformation.

Author Disclosure: K. Farrey, None; M. Sadinski, None; D.W. Golden, None; G. Redler, None; K.M. Yenice, None; D.J. Haraf,None; C.A. Pelizzari, Varian Corp, C. Other Research Support; J.K. Salama, None; H.A. Al-Hallaq, None.