The doses in a 16cm diameter spherical QA phantom were measured with and without a 1.5mm lead cover to simulate the effectof the skull bone. The dose measurement in the QA phantom was also simulated using Monte Carlo.

Results: For the QA phantom, the dose ratios with and without the 1.5mm lead-cover are 89.8% from measurement and 89.2%from Monte Carlo. For dose calculation using patient CT, Monte Carlo results show differences in the absolute dose althoughthe isodose lines remain almost the same with and without inhomogeneity corrections. The average dose with inhomogeneitycorrections is (3.86�0.90) % lower than the dose without inhomogeneity corrections. The maximum effect observed is 5.9%.The average skull density based on patient CT is between 1.42 and 1.81 g/cc. However, the inhomogeneity effects at differentlocations for the same patient geometry are within 20%.

Conclusions: The Monte Carlo method has been applied to dose comparison based on CT data with and without inhomogeneitycorrections. Monte Carlo simulations matched measurements very well for a spherical QA phantom, demonstrating that ourimplementation of Monte Carlo simulations for Gamma Knife dose calculation was accurate. The dose difference inheterogeneous patient geometry mainly comes from the photon attenuation by the skull bone. The inhomogeneity effectincreases with skull density in general. However other factors such as thickness and shape of skull also have a significant effecton the dose difference. The results show that the inhomogeneity effect should be corrected in gamma knife treatment planning.

Author Disclosure: W. Xiong, None; D. Huang, None; L. Lee, None; J. Feng, None; K. Morris, None; E. Calugaru, None; C.Burman, None; J. Li, None; C. Ma, None.

2804 Variability of Gross Tumor Volume Delineation in Head-and-Neck Cancer Using PET/CT Fusion, PartII: The Impact of a Contouring Protocol

A. C. Riegel1, A. M. Berson1,2, S. Destian3,2, T. Ng1,2, L. B. Tena1,2, R. J. Mitnick3,2, N. Stein1, S. Heiba4

1St. Vincent’s Comprehensive Cancer Center, New York, NY, 2New York Medical College, Valhalla, NY, 3St. Vincent’sHospital, New York, NY, 4Mount Sinai Medical Center, New York, NY

Purpose/Objective(s): To assess the efficacy of a gross tumor volume (GTV) contouring protocol on interobserver variabilitybetween 4 physicians in PET/CT treatment planning.

Materials/Methods: A GTV contouring protocol for PET/CT treatment planning was developed collectively by the radiationoncology, radiology, and nuclear medicine departments. The protocol consisted of 4 stages: Preliminary contouring on CTalone, determination of appropriate PET activity windowing (lower window of 0, upper window at level where tumor activityheterogeneities can be observed), accurate image registration (emphasizing monochromatic PET display and proper imageblending), and modification of CT contouring with correctly formatted PET/CT display and rules for modality disagreement.Two neuroradiologists and two radiation oncologists (labeled A,B,C,D respectively, identical to the previous work1) were givena tutorial of PET/CT coregistered imaging by a nuclear medicine physician instrumental in contouring protocol development.The tutorial was individualized to their skill level and included a step-by-step explanation of the contouring protocol withclinical examples. Opportunities for questions and hands-on practice with the protocol were given. The physicians were askedto re-contour 16 head-and-neck patients from the first part of the study on PET/CT fusion imaging. Fusion GTVs produced bythe physicians were compared with each other and with fusion GTVs from Part I. Differences in volume magnitude wereanalyzed for statistical significance by analysis of variance (ANOVA) and paired t-tests (��0.05). Volume overlap, defined bythe ratio of common volume to encompassing volume for all 4 physicians, was analyzed for statistical significance using aWilcoxon signed rank test (��0.05).

Results: Volume overlap increased significantly from Part I to Part II (P�0.05). Although no significant differences inmagnitude were seen from Part I to Part II for any physician’s volumes, the mean fusion volume of Physician A did not showany significant difference to that of Physician D, which is an improvement from Part I. The mean fusion volume of PhysicianC, however, was significantly larger than that of Physician D (P�0.01). This result is unchanged from Part I.

Conclusions: The multidisciplinary contouring protocol significantly improved the coincidence of GTVs contoured by multiplephysicians. The magnitudes of the volumes showed marginal improvement in conformity. Developing an institutionalcontouring protocol for PET/CT treatment planning is highly recommended to reduce interobserver variability.

1Riegel AC, et al. Variability of gross tumor volume delineation in head-and-neck cancer using CT and PET/CT fusion. IntJ Radiat Biol Oncol Phys, [in press].

Author Disclosure: A.C. Riegel, None; A.M. Berson, None; S. Destian, None; T. Ng, None; L.B. Tena, None; R.J. Mitnick,None; N. Stein, None; S. Heiba, None.

S660 I. J. Radiation Oncology ● Biology ● Physics Volume 66, Number 3, Supplement, 2006