Radiotherapy

Definitions:

• Gray (Gy): radiation dose that produces absorption of 1 J/ kg of tissue
  • 1 Gy = 100rad
• rad: dose of radiation that will produce absorption of 100ergs of energy per gram of tissue
• Roentgen: unit of charge produced by x-rays or gamma rays that ionize a specific volume of air
  • 1gm of tissue exposed to 1 roentgen of gamma rays = 93 ergs
• rem: the dose of radiation that causes a biologic effect equivalent to 1 rad of x-rays or gamma rays
• Sievert (Sv): dose of radiation that causes a biologic effect equivalent to 1 Gy of x-rays or gamma rays
• 1 Sv corresponds to 100rem

Types

• External beam
• Brachytherapy – sealed radioactive sources placed precisely in the area under treatment
• Systemic radio-isotope therapy – infusion or oral
  • Yttrium - neuroendocrine
  • Radioiodine
  • Monoclonal antibodies tagged to Yttrium
• Ionizing radiation – liberates an electron from an atom which
  • Either directly damages the DNA or
  • Produces free radicals which damages DNA
    • Oxygen needed as most common free radical = OH
    • So tumour hypoxia, say in a tumour that is big dense & perhaps outgrown its blood supply, can confer 2-3x resistance to radiotherapy– a lot of research going into making them more radiosensitive
    • Cells in the G2 & mitotic phases of cell cycle are most sensitive to ionizing radiation
  • Ionizing radiation can take an electromagnetic form (eg high-energy photons) or particulate forms (eg electrons, protons, neutrons, alpha particles)
• Principle: small frequent doses allow normal cells to repair (ie sub-lethal damage) between fractions, while cancer cells die
• Fractionation of radiation spares normal tissue (because of their greater ability to repair sublethal damage)
• Typical daily fraction: 1.8-2.5 Gy
• Chemotherapy can be used to make cancer cells more responsive to radiotherapy (ie to “radiosensitise”)

Effects of radiation

• Skin changes = most common (BJS 08; 95:699)
  • usually appear after 2-3/52 of Rx & continues beyond end of Rx
  • Seen when a critical number of cells within a tissue are killed, above a certain threshold dose (usually ≈ 2Gy)
  • 10-12Gy → dermal trophy and necrosis
  • 14-28 Gy → desquamation

  • 24Gy → ulceration

  • Telangiectasia – uncommon
  • Fibrosis or fat necrosis – rare
• ↑ed lymphedema (especially it pt has had axillary dissection)
• Tiredness
• Hair loss
  • Transient hair loss may manifest at 3 Gy
• (Acutely) oedema & (Delayed) fibrosis effects on irradiated organs (lungs, GI tract)
• ±↑ed CA risk in long-term
• Bowel effects: bleeding, strictures, fistulas