Validation of Automatic Target Volume Definition as Demonstrated for ¹¹C-Choline PET/CT of Human Prostate Cancer Using Multi-modality Fusion Techniques

Rationale and Objectives: Positron emission tomography (PET) is actively investigated to aid in target volume definition for radiation therapy. The objectives of this study were to apply an automatic computer algorithm to compute target volumes and to validate the algorithm using histologic data from real human prostate cancer. Materials and Methods: Various modalities for prostate imaging were performed. In vivo imaging included T2 3-T magnetic resonance imaging and ¹¹C-choline PET. Ex vivo imaging included 3-T magnetic resonance imaging, histology, and block face photos of the prostate specimen. A novel registration method based on mutual information and thin-plate splines was applied to all modalities. Once PET is registered with histology, a voxel-by-voxel comparison between PET and histology is possible. A thresholding technique based on various fractions of the maximum standardized uptake value in the tumor was applied, and the respective computed threshold volume on PET was compared with histologic truth. Results: Sixteen patients whose primary tumor volumes ranged from 1.2 to 12.6 cm³ were tested. PET has low spatial resolution, so only tumors > 4 cm³ were considered. Four cases met this criterion. A threshold value of 60% of the ¹¹C-choline maximum standardized uptake value resulted in the highest volume overlap between threshold volume on PET and histology. Medial axis distances between threshold volume on PET and histology showed a mean error of 7.7 ± 5.2 mm. Conclusions: This is a proof-of-concept study demonstrating for the first time that histology-guided thresholding on PET can delineate tumor volumes in real human prostate cancer.