3D-Segmentation of the 18F-choline PET signal for target volume definition in radiation therapy of the prostate

Volumetric assessment of PET signals becomes increasingly relevant for radiotherapy (RT) planning. Here, we investigate the utility of F-18-choline PET signals to serve as a structure for semi-automatic segmentation for forward treatment planning of prostate cancer. F-18-choline PET and CT scans of ten patients with histologically proven prostate cancer without extracapsuiar growth were acquired using a combined PET/CT scanner. Target volumes were manually delineated on CT images using standard software. Volumes were also obtained from F-18-choline PET images using an asymmetrical segmentation algorithm. PTVs were derived from CT F-18-choline PET based clinical target volumes (CTVs) by automatic expansion and comparative planning was performed. As a read-out for dose given to non-target structures, dose to the rectal wall was assessed. Planning target volumes (PTVs) derived from CT and F-18-choline PET yielded comparable results. Optimal matching of CT and F-18-choline PET derived volumes in the lateral and cranial-caudal directions was obtained using a background-subtracted signal thresholds of 23.0+/-2.6%. In antero-posterior direction, where adaptation compensating for rectal signal overflow was required, optimal matching was achieved with a threshold of 49.5+/-4.6%. 3D-conformal planning with CT or F-18-choline PET resulted in comparable doses to the rectal wall. Choline PET signals of the prostate provide adequate spatial information amendable to standardized asymmetrical region growing algorithms for PET-based target volume definition for external beam RT.