In conventional radiation therapy, photon beam is typically used for treatment. Photons are beams of electromagnetic radiation (just like visible light or radio waves but with much more energy).

As photon penetrates the patient’s body, the emitted dose by the photon beam gradually decreases.  As a result, a limited amount of the dose is delivered to healthy tissue and some to the tumor / cancerous tissue. Depending on the sensitivity of healthy tissues and the amount of irradiation, sides effects may occur. In recent years, with significant advances in conventional radiotherapy, it has possible to reduce the dose to healthy tissue. However, minimize the dose to healthy tissue especially organ at risk has always been the goal of researchers in this field.