Radiation Therapy (RT) Basics




Radiobiology is the study of how radiation therapy interacts with cancer cells and normal cells.

Radiotherapy damages the DNA causing double strand breaks which are un-repaired or mis-repaired. This leads to cell death.

  • Some tumors are very sensitive to radiation treatment, or radiosensitive - for example lymphomas and seminomas.
  • Some tumors are quite insensitive, or radioresistant - for example melanoma.
  • Lack of Oxygen (hypoxia) in the center of tumors leads to radioresistance.
  • Chemotherapy acts as a radiation sensitizer (makes the RT more effective) and can also make side effects from radiation more severe.
  • All cancer cells vary in sensitivity depending on their position in the cell cycle.


Gap 0 (G0) = resting phase. Cells not dividing

Gap 1 (G1) = post mitotic phase. Enzymes necessary for DNA synthesis produced. Protein and RNA synthesis

Synthesis (S) = Cellular DNA duplicated

Gap 2 (G2) = premitotic phase. Precursors of mitotic spindle produced.

Mitosis (M) = Cell division in 4 step process:

  • prophase
  • metaphase
  • anaphase
  • telophase

Cells are most sensitive to radiotherapy in mitosis (M) and early in the DNA synthesis phase (S).

Cells in G2 and late S phase have the least sensitivity to radiation.

Normal tissues can repair damage from RT better than cancer cells.

Waiting at least 6 hours between radiotherapy treatments gives normal tissue the chance to repair and this is why radiation therapy is split up into small amounts or "fractions."




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