Proceedings of the 50th Annual ASTRO Meeting S537

2823 Comparison of Dose Distribution between 3D-CRT and IMRT Plans for LINAC Based Stereotactic

Radiosurgery (SRS): Tumor Dose Inhomogeneity and Normal Tissue Dose Fall Off

L. X. Hong, R. Yaparpalvi, M. Garg, C. Guha, S. Kalnicki, D. Mah

Montefiore Medical Center, Bronx, NY

Purpose/Objective(s): High tumor dose inhomogeneity and sharp dose fall off outside tumor are essential features for single frac-tion SRS plans. This study aims to explore the relationship between tumor dose inhomogeneity and dose fall off in normal tissuesfor SRS plans using LINAC based 3D-CRT and IMRT radiation beams.

Materials/Methods: On a representative patient’s brain CT scan, spherical volumes with diameters of 10, 15, and 20 mm were drawnand defined as PTV for different plans. Normal structures were drawn as concentric spherical shells with 2.5 mm thickness outside thePTV. Both 3D-CRT and IMRT plans with beam-target margins varying from 1 to 4 mm were produced with the same beam arrange-ment. All plans were generated using BrainLAB iPlan with m3 MLC and normalized to same minimum prescribed PTV dose of 20 Gy.

Results: For 3D-CRT plans, tumor dose inhomogeneity and dose fall off outside the tumor were inversely related to the beammargins around the PTV. For a 15 mm PTV, as the beam margin increased from 1 to 4 mm, the PTV mean dose decreasedfrom 118% to 106%; for normal tissue shells at 2.5 and 5 mm distances away from the PTV, V50 (volume receiving at least50% dose) increased from 71% to 100% and from 27% to 77%, respectively.

For the corresponding IMRT plan, penumbra sharpening and normal tissue dose constraints mainly influenced tumor dose in-homogeneity and dose fall off outside the tumor. If there were no specific dose constraints placed on the normal structures, for beammargins ranging from 1 to 4 mm and a 15 mm PTV, the mean PTV dose varied little (105% to 103%). For tissue shells at 2.5 and 5mm distances away from the PTV, V50 increased from 81% to 100% and from 28% to 90%, respectively. However, if one applieddose constraints for the shell structures outside the tumor using the DVH constraints from the corresponding 3D-CRT plan, for thesame 15 mm PTV, the PTV mean dose increased to 113%, independent of the beam margin. As the beam margin increased from 1to 4 mm, for tissue shells at 2.5 and 5 mm distances away from the PTV, V50 increased from 78% to 88%, and from 27% to 41%respectively. The same trends held for 10 mm and 20 mm PTVs.

Conclusions: For SRS, compared to 3D-CRT plan, IMRT plans can produce similar dose fall off but with decreased mean dose to thetumor. Dose fall off in IMRT plan, however, was strongly influenced by beam margin or normal tissue dose constraints. We recom-mend that a quick 3D-CRT plan be done to obtain initial DVH dose constraints for IMRT optimization. Whether reduction in meandose due to better homogeneity in IMRT would affect control rates remains to be seen. Should the existing IMRT optimization al-gorithms penalizing inhomogeneity in tumor dose in their cost functions be modified to increase mean tumor dose in SRS plans?

Author Disclosure: L.X. Hong, None; R. Yaparpalvi, None; M. Garg, None; C. Guha, None; S. Kalnicki, None; D. Mah, None.

2824 Pancreatic Tumor Delineation with FDG-PET/CT: Smaller Volumes and Regions not Identified with CT

Alone

E. C. Ford1, M. Brenner2, E. Fishman2, S. Goldstein1, R. Wahl2, J. M. Herman1

1Johns Hopkins University, Department of Radiation Oncology and Molecular Radiation Sciences, Baltimore, MD, 2JohnsHopkins University, Department of Radiology, Baltimore, MD

Purpose/Objective(s): Accurate delineation of pancreatic masses on CT alone is challenging and remains a major obstacle forprecision radiotherapy. Here we quantify the effects of including FDG-PET in tumor delineation for radiation treatment planning.

Materials/Methods: FDG-PET/CT scans were acquired on 11 patients with unresectable or borderline resectable pancreatic can-cer using the Discovery LS and ST systems (GE Medical Systems, Inc.). Scans were loaded into the Pinnacle treatment planningsystem (Philips, Inc.). Pancreatic tumor volumes were delineated by a single radiation oncologist using the CT which was obtainedduring the PET scan. A radiologist who specializes in pancreatic cancer contoured the pancreatic GTV with the assistance of a di-agnostic CT with IV/oral contrast. A nuclear medicine physician delineated the tumor volume using the FDG-PET/CT scan whileviewing both modalities. Each physician’s contouring was blinded to the others. To minimize the effect of window/level setting onthe PET scans, automatic contouring was performed at a threshold level of 40% of the maximum voxel value, and then edited asrequired by the physician. Volumes were calculated for the CT and PET/CT, and a region of interest was defined that representedthe PET/CT volume that was not included in the CT-only volumes.

Results: CT based volumes were 77.0 cm3 (36.0-118.7 cm3) from the radiation oncologist and 65.7 cm3 (36.9-94.2 cm3) from theradiologist (median and [range]). PET/CT-based volumes were significantly smaller than either of the CT-based volumes: 32.2 cm3

(10.4-105.8 cm3) with p\0.001 (paired t test). In general the PET/CT helped to better distinguish the inferior pancreatic/duodenalborder, with PET/CT delineation indicating a more cranial boundary in 91% of cases (20/22 contours). The median distance fromthe border of the PET/CT volume to the border of the CT volumes was 0.8 cm, with distances in excess of 3 cm in some cases.Though smaller on average, there were PET/CT-delineated regions that were not included on either of the CT analyses. The frac-tional volume of PET/CT region that was not contoured in either CT was 24.9% (1.0-76.0%).

Conclusions: The inclusion of FDG-PET scans significantly alters the delineated tumor volumes in patients with pancreatic cancer,generally resulting in a smaller overall volume but also indicating large regions that would not have been included in CT-baseddelineation. This can have an important impact on radiation treatment planning and warrants additional study.

Author Disclosure: E.C. Ford, None; M. Brenner, None; E. Fishman, None; S. Goldstein, None; R. Wahl, None; J.M. Herman, None.

2825 Can Less-than-daily Image Guided Radiation Therapy (IGRT) be used to Treat Esophageal Cancer? A

Study of Patient Positioning Errors and their Dosimetric Consequences

D. C. Schiffner1,2, T. E. Schultheiss1, Y. J. Chen1, C. Han1, A. Liu1, J. Y. C. Wong1

1City of Hope National Medical Center, Duarte, CA, 2University of California, Irvine, Orange, CA

Purpose/Objective(s): The cost and effort of IGRT could be reduced if less-than-daily image guidance (IG) was employed. Thepurpose of this study is to assess the performance of less-than-daily IG strategies in the treatment of esophageal cancer by retro-spectively measuring residual patient positioning errors and their dosimetric consequences.