S538 I. J. Radiation Oncology d Biology d Physics Volume 72, Number 1, Supplement, 2008

Materials/Methods: Twenty-five consecutive patients treated for esophageal cancer with daily IGRT using helical tomotherapywere included, resulting in alignment data from 625 fractions. After daily alignment based on skin tattoos, pretreatment IG wasperformed by obtaining a megavoltage CT (MVCT) and executing couch shifts based on MVCT registration with the planningCT. Seven IG strategies previously proposed in the literature were assessed: 0% (no IG), 12% and 20% (IG performed on first3 and 5 fractions, respectively, with mean shift used for all future fractions), 20% (weekly IG), 36% (IG on first 5 fractions, weeklythereafter, running mean shift used for non-IG fractions), 52% (IG every other day, with running mean), and 60% (IG on first 5fractions then every other day, with running mean). Residual positioning errors were calculated as the difference between the actualdaily shift and the shift according to the given IG protocol. Using TomoTherapy Planned Adaptive software, dose-volume histo-grams (DVHs) were calculated from daily MVCTs from the treatment of 5 representative patients. The CTV D95, heart V1.2, andtotal lung V0.8 were calculated for each fraction of 1.8 Gy prescribed to the CTV + 4 mm margin. DVH data based on daily IG werecompared to data from the less frequent IG protocols.

Results: With increasing IG frequency from 0% to 60%, residual patient positioning errors decreased in all axes, and the number offractions with shifts .5 mm in any axis was reduced from 66% to 20%. Dosimetric data showed that with increasing IG from 0% to60%, the number of fractions with .10% underdosing of the CTV D95 fell from 26% to 10%. With increasing IG frequency from0% to 60%, the number of fractions with a .10% increase in heart V1.2 and lung V0.8 fell from 30% to 13% and from 19% to 10%,respectively. With the use of daily IG, no dosimetric errors .10% to the CTV or normal structures were observed. All measures ofpositional and dosimetric error were described by a linear dependence on IG frequency.

Conclusions: Patient positioning errors, underdosing of the CTV, and excess dose to normal structures are reduced with increasingIG frequency. Even with 60% IG, significant errors still occur at an unacceptable rate of $10% of fractions. Daily IG eliminates theseerrors. To achieve optimal treatment quality, daily IGRT is required, and the cost and effort of daily image guidance appear justified.

Author Disclosure: D.C. Schiffner, None; T.E. Schultheiss, None; Y.J. Chen, None; C. Han, None; A. Liu, None; J.Y.C. Wong,Honoraria from TomoTherapy received for presentations, D. Speakers Bureau/Honoraria.

2826 Deformable Registration and Dose Accumulation to Investigate Marginal Liver Cancer Recurrences

K. K. Brock, M. Lee, C. L. Eccles, M. Velec, J. L. Moseley, L. A. Dawson

Princess Margaret Hospital, Toronto, ON, Canada

Purpose/Objective(s): Image guided radiotherapy (IGRT) combined with deformable registration enables calculation of the ac-cumulated treatment dose to normal tissues and targets. Correlating the delivered dose with follow-up images of patients who de-veloped local recurrences can help to establish if the recurrences were marginal (within dose fall off region). The purpose of thisresearch is to quantify the accumulated dose to the liver and gross tumor volume (GTV) and correlate this with location of recur-rences in liver cancer patients treated with SBRT.

Materials/Methods: Six liver cancer patients who developed local recurrences following six-fraction, image guided SBRT wereinvestigated. The median time between IGRT and the most recent follow-up image was 17 months (range, 5 - 31 months). The liverwas delineated on the planning CT image, CBCT images, and follow-up CT images and the GTV on the planning and follow-up CTimages. Deformable registration and dose accumulation was performed using MORFEUS, a biomechanical model-based deform-able registration algorithm. The accumulated dose was calculated based of deformable registration of the planning images and theCBCT images obtained immediately before each fraction. Deformable registration was then performed between the planning im-age, with the accumulated dose over treatment delivery, and the follow-up images in which local recurrences were delineated. Themean and minimum accumulated dose to the treated GTV was compared to the planned doses. The mean and minimum dose de-livered to the region of progressive and/or new local recurrences were quantified.

Results: The accumulated dose to the liver and GTVs were compared to the planned doses (37 to 57 Gy in 6 fractions). The min-imum dose to the GTV was more than 5% less than the planned minimum dose for 1 patient (1 Gy). This patient developed localprogression and 4 additional lesions, 2 correlated to a minimum dose of less than 4 Gy, indicating ‘out of field’ recurrences. Com-plete response of the GTV was seen in 1 patient, who developed apparent new disease correlated with a minimum dose of 14% ofthe minimum accumulated dose in the GTV, indicating a possible marginal recurrence. Two patients had no change in accumulatedminimum dose correlated with the recurrence on the follow-up image, but additional hepatic recurrences occurred in regions of theliver that received doses less than 12 Gy, indicating out of field or ‘de novo’ recurrences.

Conclusions: Deformable dose accumulation and correlation with follow-up images can aid in deciphering if local recurrences aremarginal (due to residual dosimetric or geometric uncertainties, inadequate CTV margin or insufficient dose) or occur ‘de novo’(unrelated to physical factors).

Author Disclosure: K.K. Brock, Elekta Oncology Systems, Philips Medical Systems, RaySearch Laboratories, Varian MedicalSystems, B. Research Grant; M. Lee, None; C.L. Eccles, None; M. Velec, None; J.L. Moseley, None; L.A. Dawson, Elekta On-cology Systems, B. Research Grant.

2827 Acute Small Bowel Toxicity for Image Guided Pelvic Irradiation of Rectal and Anal Cancer Patients using

3D-CRT in the Prone Position vs. Hypofractionated IMRT in the Supine Position

O. Gayou1,2, B. Karlovits1, M. Miften1,2, M. Wong2, A. Kirichenko1,2, B. Leicher1, D. Medich3, D. Parda1,2

1Department of Radiation Oncology, Allegheny General Hospital, Pittsburgh, PA, 2Drexel University College of Medicine,Allegheny Campus, Pittsburgh, PA, 3Department of Colorectal Surgery, Allegheny General Hospital, Pittsburgh, PA

Purpose/Objective(s): During conformal pelvic irradiation for colorectal and anal canal cancer, patients are usually setup in theprone position on a belly board in order to reduce dose to the small bowel (SB). Recently, hypofractionated IMRT has been usedas a treatment modality for patients with these malignancies. The objective of this study was to compare radiation-inducedSB acute toxicity in supine patients treated with hypofractionated IMRT vs. prone patients treated with standard fractionation3D-CRT.