How Radiotherapy Works!

How Radiotherapy Works

Radiotherapy uses high-energy, ionizing radiation to damage cancer cells. Treatment is usually delivered in multiple smaller doses rather than one large dose.

How Radiation Damages Cells

• Visible light is radiation, but it does not have enough energy to ionize atoms. X-rays and gamma rays have much higher energy.
• Ionizing radiation can remove electrons from atoms, disrupting biological molecules and damaging cells.
• Treatment X-rays are commonly produced by a linear accelerator. High-speed particles, often electrons, strike a metal target and generate photons. The resulting beam is aimed at the tumor.
• Electrons and protons can also be used to treat cancer, though the video describes them as less commonly used.

Radiation damages DNA in two main ways:

• Indirect damage: Radiation interacts with water in or around cells, creating free radicals. Reactions involving oxygen can produce especially damaging superoxide radicals. The video says most current radiation techniques work this way.
• Direct damage: Radiation damages DNA directly. The video notes that heavier particles, such as protons, rely more on this mechanism.

A key treatment goal is to cause double-strand breaks—breaks in both strands of DNA at the same location. Accumulated damage can overwhelm a cell’s ability to repair itself and lead to cell death.

Why Treatment Is Fractionated

Cells are more vulnerable to certain types of radiation damage during the M phase of the cell cycle, when chromosomes are dividing. Tumor cells divide more frequently than normal cells, so they are more likely to be vulnerable to radiation. However, tumor cells are in different stages of the cell cycle at any given time.

Radiotherapy therefore commonly divides the total dose into smaller treatments delivered over multiple days. This is called fractionation. It gives more tumor cells a chance to be treated while vulnerable and gives normal cells time to recover between treatments.

The Four Rs of Radiotherapy

1. Repair

   • Normal cells generally repair radiation-induced DNA damage better than cancer cells.
   • The time between treatment fractions allows normal cells to repair damage.

2. Repopulation

   • Surviving tumor cells can multiply if they are not sufficiently damaged.
   • Each fraction needs to cause enough damage to limit tumor repopulation.
   • Normal cells can also repopulate between fractions.

3. Reassortment

   • Tumor cells occupy different stages of the cell cycle at any one time.
   • Between fractions, cells move through the cycle. Later treatments may catch more cells in the vulnerable M phase.

4. Reoxygenation

   • Cells farther from a tumor’s blood supply, often near its center, may be oxygen-poor.
   • Oxygen helps radiation-generated radicals cause more damaging effects.
   • As oxygen-rich tumor cells are killed, previously oxygen-poor areas may become better oxygenated, making those cells more susceptible to later fractions.

Speakers and Sources Featured

• Speaker: An unnamed narrator presents the whiteboard explanation.
• Source named in the video: Learn Oncology, including its website, learnoncology.ca.