to 1.5 at 70 keV. Finally, the relationship between RBE and a/b was evaluated. Although it has been assumed that RBE increaseswith decreasing a/b, the data analysed in this study can not conclusively support the assumption.

Conclusions: Institutions that treat patients with carbon ion beams currently assume an RBE of about 3.0 in the target volume.Based on our data analysis, this assumed RBE value appears reasonable. However, large variations with dose, LET and a/b canbe expected. As generally undisputed and predicted by all biophysical models, RBE increases with LET to a maximal value andthen decreases. Also, as can be expected due to the shouldered survival curve after 60Co irradiation, RBE increases withdecreasing radiation dose. Some RBE models used in clinical applications of carbon ions are based on the assumption that thereis an increase in RBE when a/b decreases. The data support a trend but not a one-to-one correlation.

Author Disclosure: A.J. Chang, None; H.D. Suit, None; H. Paganetti, None.

2884 Treatment of Canine Bladder Cancer Using Laparoscopically Implanted Tissue Expander Radiotherapy(LITE-RT)

A. N. Gutierrez, S. M. Murphy, L. J. Forrest, T. R. Mackie

University of Wisconsin, Madison, WI

Purpose/Objective(s): The aim of this study was to use LITE-RT to isolate and target the urinary bladder in dogs withtransitional cell carcinoma and reduce acute and chronic side effects to critical structures by decreasing their dose. LITE-RTis a technique which geometrically displaces the PTV through the use of a laparoscopically-placed, custom shaped tissueexpander (TE).

Materials/Methods: Two dogs with confirmed and fully staged TCC of the bladder underwent biopsy and tumor evaluationvia cystoscopy. Custom shaped TE, 440ml and 240ml, (Spec. Surg. Products, Victor MT) were developed and placed betweenthe bladder and colon via sutures to the prepubic tendon. Laparoscopic placement was performed in one dog and laparotomyplacement in the other. A planning CT was acquired with the TE inflated. The TE was inflated with a saline/contrast mix priorto each treatment to the same volume as the planning CT. The dogs underwent fractionated radiotherapy (2.5 Gy x 18 � 45Gy). Using tomotherapy, a MVCT was done prior to each treatment and fused to the planning kVCT to ensure accurate organpositioning. The TE was removed one week after completion of therapy. Colonoscopy and cystoscopy were performed priorto therapy in 2 dogs and 1 week after therapy in 1 dog. Clinical signs of bowel and urinary habits were monitored.

Results: Pretreatment cystoscopy and ultrasonography revealed a 2.5x2.4 cm tumor surrounding the right ureter in one dog anda 5.5x5.5 cm tumor involving the majority of the ventral bladder wall and neck in the second dog. Both dogs received all 18fractions with minimal radiation side effects. Visual organ displacement and bladder isolation was adequate on the planning CT.Sutures anchoring the TE pulled through the prepubic tendon in both dogs requiring an additional surgery. One dog had aforeign body reaction to the TE forming a fibrous capsule surrounding the TE. No signs of colitis were observed in either dog.Clinical signs of pollakiuria and incontinence worsened during therapy but resolved after treatment. One week follow upcystoscopy was available in one dog, and tumor volume was significantly decreased–ureteral papilla was now visual.Colonoscopy and biopsy obtained prior to and after therapy in this dog showed no change. Three months post therapy dog iscontinent with no hematuria.

Conclusions: Use of LITE-RT is feasible in the treatment of spontaneous TCC of the canine bladder. Early results of thistechnique show promise. Lack of acute colitis in this study represents a large decrease in previously reported patient morbidities.Future studies are planned to measure the bladder motion with TE inflation and quantize the daily repositioning accuracy.

Author Disclosure: A.N. Gutierrez, None; S.M. Murphy, None; L.J. Forrest, None; T.R. Mackie, Mackie has a financial interestin TomoTherapy Inc., E. Ownership Interest.

2885 Three-Dimensional Dose Verification for Radiation Therapy

P. Guo1, J. Adamovics2, M. Oldham1

1Duke University Medical Center, Durham, NC, 2Rider University, Lawrenceville, NJ

Purpose/Objective(s): Sophisticated dose delivery techniques like intensity-modulated radiation therapy (IMRT) requiredetailed dose verification in 3D for comprehensive validation of correct implementation. Previously we presented extensive‘small volume’ studies on a new material PRESAGETM, with promise as a breakthrough material for accurate and convenient3D dosimetery. Here this work is extended, and the first comprehensive 3D dosimetric verification of both conformal and IMRTdistributions, using PRESAGETM, is presented.

S710 I. J. Radiation Oncology ● Biology ● Physics Volume 66, Number 3, Supplement, 2006