KLK3 and bone metastasis: In our series, we clarified that (1) 11C-choline PET/CT could directly detect viable bone metastases with abnormal 11C-choline uptake, regardless of various morphological types (osteoblastic, osteolytic, mixed, and invisible types on CT), (2) 11C-choline PET/CT could discriminate viable progressive osteoblastic bone metastasis (abnormal 11C-choline uptake) from benign osteoblastic change induced by the treatment effect (no 11C-choline uptake) on 11C-choline PET/CT, (3) treatment response (EORTC criteria) corresponded well with changes in the serum PSA level.