*2.3.1 What is radiation?*

One needs to remember that discovery of many great things happened by accident. Same thing holds good for practice of radiotherapy, as its use was established when Henry Becquerel left 200 mg of Radium in his vest pocket for 6 hours and later noted ulceration along the adjacent skin. This event opened the eyes of the

scientific community towards the clinical effects of radiation & radioactive sources. From then on, there were many experiments done to establish the role of radiation in treatment of malignancy.

Radiation is nothing but energy in the form of waves or stream of particles which can be explained by emission, propagation and absorption of energy. In the present day, most common form of radiation in use is X-rays though there are utilisation of gamma in brachytherapy and proton in particle therapy but in order to restrict ourselves to common terminologies we are not going in detail about them. This form of radiation is considered to have the ability to produce ions in the cells of tissues it passes through, by dislodging the electrons from atoms, and hence it is commonly called as ionising radiation.

If one needed to know the most basic principle about radiation, it would be that - "radiation is effective only on dividing cells". There are 5 phases that each cell goes through from G0 ➔ G1 ➔ S ➔ G2 ➔ M. Among these G2 & M phases are the most radiosensitive and S phase is the most radioresistant phase. The knowledge of these help in deciding on combination therapy and use of radiosensitisers. Radiation is usually delivered in a fractionated manner and it would be paramount to have the knowledge of the 4R's of radiation therapy to understand the basis for this fractionation:


### *2.3.2 How is it delivered?*

One can divide this based on technology, based on setting and based on type. The following flowchart will briefly describe about it (**Figure 4**).

Description of these commonly employed terminology are as follows

**Figure 4.** *Aspects of radiotherapy in Oral cancers.*

