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Radiation Therapy 101

What is Radiation Therapy?

Radiation Therapy is the medical use of ionizing radiation as part of cancer treatment to control cancer cells.

iX

Varian 23 iX with OBI imaging

Uses of Radiation Therapy:

Radiation Therapy can be used for the primary course of treatment or in conjunction with surgery and/or chemotherapy. Depending on diagnosis and staging, radiation therapy is used for curative or palliative purposes.

Other factors that determine if radiation is used include tumor type, location, and general health of the patient. Radiation therapy is applied to the tumor, which also includes a margin. This margin compensates for uncertainties such as internal motion (i.e. filled bladder or respiration), movement of external skin marks, and may include the draining lymph nodes.

How Radiation Therapy Works:

Radiation therapy works by damaging the DNA of cancerous cells. Photons and charged particles can both be used to damage DNA strands. The most effective DNA break is a double strand break which effectively prevents the cells from reproducing, ultimately killing the population of the cancer cells. In
addition to killing off the “bad” cells, the healthy cells surrounding the area are protected with a complex shielding mechanism known as multi-leaf collimators (MLC).

 MLC Blocking                           RT101 3
 Multi Leaf Collimators   Types of DNA breaks 

 

Purpose for Fractionation (treatments over time):

The total course of radiation therapy is delivered over a period of time (as oppose to all in one day) for several reasons:

  1. Gives the healthy cells surrounding the tumor a chance to recover and repair, while tumor cells repair less efficiently.
  2. Oxygen rich cells are more radiosensitive to the treatments, and since tumor cells are more hypoxic when compared to normal cells, spacing out the treatments allows reoxygenation of the tumor cells.
  3. Fractionation also allows tumor cells that were in a relatively radio-resistant phase of the cell cycle during one treatment to cycle into a more sensitive phase of the cycle before the next fraction is given.

Fractionation schedules vary depending on physician preference, but are generally 1.8 to 2.0 Gy/fx (Gray/fraction) for 5 days a week. Other fraction schedules can also be used depending on the diagnosis.

Types of Radiation Therapy:
External Beam Radiation Therapy

External beam radiation therapy (EBRT) is the most common form of radiotherapy. The Cancer Center of Irvine utilizes a Varian 23 iX (universally known as a linear accelerator) capable of delivering X-ray
(photon) and electron radiation therapy. Photons are used at the megavoltage energy range, which allows for deep-tissue therapeutic treatment (e.g. bladder, bowel, prostate, lung, or brain). Electrons are used in the kilo-electron voltage range, which effectively treats superficial lesions such as skin cancer or scar tissue. Linear accelerators are able to effectively deliver dose from multiple angles, permitting an equalized homogeneous dose to a given treatment volume.

 RT101 4                           RT101 5
 Multiple Angles   Treatment Volume
 Brachytherapy

Brachytherapy (aka “close” therapy) is an internal form of radiation therapy that is performed by temporarily placing a radioactive source inside or next to the area requiring treatment. This form of radiation therapy is an effective treatment for cervical, prostate, and breast cancers but can also be used for a variety of other cancers.

Useful Terms in Radiation Therapy:
  • Ionizing Radiation - Any radiation that ionizes (releases electrons) as it passes through a medium (such as human tissue).
  • Malignant cells - Cells of uncontrolled growth which have the potential to invade or spread (metastasize).
  • Benign cells - Encapsulated cell, non-invasive, favorable, and very slim chance of spread.
  • Chemotherapy - A medication that is given either orally or intravenously that kills cancer cells throughout the body.
  • Palliative - Alleviate or decrease pain without curative intent.
  • Photons - Small packet of energy. More specifically, a quantum of electromagnetic radiation. This is the type of radiation that is given off by a linear accelerator.
  • Charged particles - A particle with a charge such as an electrons or proton.
  • Double strand break - A break which occurs on both sides of the DNA strand, which prevent it from reproducing.
  • Multi-Leaf Collimators (MLC) - A 120 leaf system, composed of tungsten, that are individually controlled to block off healthy surrounding tissue.
  • Radiosensitive - Cells or tissues that are sensitive to radiation (easily killed by radiation).
  • Radio-resistant - Cells or tissues that are not efficiently killed by radiation.
  • Hypoxic - The state of being low in oxygen levels.
  • Gray - SI unit of absorbed radiation dose of ionizing radiation. Defined as the absorption of one joule of ionizing radiation by one kilogram of matter.
  • Fraction - One treatment.
  • Megavoltage x-rays - X-rays produced by linear accelerators (linacs) operating at voltages in excess of 1000 kV (1 MV) range.
  • Kilo-electron volts - Electrons produced by linear accelerators (linacs) operating at electron voltages below the 1 MV range.
  • Superficial - Anatomically closer to the skin surface.
  • Homogeneous - A uniform distribution of dose (balanced throughout the treatment volume).