1. Objective To study the role of bystander effect in the cytocidal activity of TK/GCV against tumor cells.

2. Conclusion Bystander effect plays an important role in the cytocidal activity of TK/GCV against tumor cells, and is quite relative to intercellular contact.

3. The study was conducted to clarify the cytocidal effect of combination therapy consisting of administration of Acridine orange (AO), which is a photosensitizer, and radiation therapy using in vitro and in vivo mouse osteosarcoma models