Fractionation

Fractionation in radiotherapy is the process of dividing the total radiation dose for a particular treatment into multiple smaller doses or fractions, which are typically administered over several days or weeks. The primary reason for fractionation is to minimize damage to healthy tissues while effectively targeting cancer cells. By dividing the total dose of radiation, healthy cells have time to recover between treatments, and a larger total radiation dose can be delivered over time, which can help to kill more cancer cells. Fractionation also allows radiation therapy to be delivered with greater precision, which can help to minimize the risk of side effects and improve outcomes for patients.
Fractionation in radiotherapy works based on several mechanisms, such as:

• Redistribution: The radiosensitivity of cells depends on their stage in the cell cycle. Cells are most sensitive to radiation in the M and late G2 phase of their cycle and most resistant in the late S phase. Since a group of malignant cells are at various points in their cell cycle, delivering the entire dose of radiation in a single fraction is ineffective against a proportion of the tumor cells. Dividing the total dose of radiation into multiple fractions maximizes the probability of irradiating cells when they are in the most radiosensitive period of their cell cycle.
• Re-oxygenation: When tumor cells are hypoxic they are less susceptible to the indirect effects of radiation. Fractionating radiotherapy allows cells which are closer to sources of oxygen to be killed first, and the intervening time between fractions allows the relatively hypoxic cells to improve their oxygen supply. These cells are then more sensitive to subsequent doses of radiation.
• Repair: Fractionation increases the destructive effect on tumor cells while minimizing damage to healthy cells due to the different ability of normal cells and malignant cells to repair DNA damage. Healthy cells have a greater ability to repair DNA damage than malignant cells. As such, splitting the total radiation dose allows healthy cells an opportunity to repair this sublethal damage between fractions. Meanwhile, malignant with impaired DNA repair pathways are less able to recover from radiation damage to their DNA
  

There are different methods or ways of dividing the total radiation dose into fractions, such as:

• Conventional fractionation: the most common type of fractionation and is used when treating most types of cancer. Typically, conventional fractionation uses doses range from 180cGy to 200cGy; is delivered once a day, 5 days a week over a period of 6-7 weeks.
• Hyperfractionation: divides the same total dose into more fractions by giving smaller doses per fraction, twice a day treatment (6 hours or more apart) and the same number of treatment days and weeks as conventional fractionation.
• Hypofractionation: the treatment course is shorter by delivering higher treatment doses per fraction, treatments once a day or even with gaps between days. Consists in fewer days and weeks of treatment than conventional radiation. Used for any type of cancer treated with stereotactic radiation therapy (SBRT)