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Biological Effects of Radiation

Course Number: 572

Effects of High-dose Ionizing Radiation

High-dose ionizing radiation is an effective alternative to surgery or a valuable adjunct to surgery and/or chemotherapy in the locoregional treatment of head and neck malignancies. Oral healthcare providers can expect to be called on to care for head and neck cancer patients undergoing radiotherapy or who may have previously completed radiotherapy. Consequently, it is essential to understand the deterministic biological changes due to high-dose ionizing radiation.5,6,11

The therapeutic benefit of ionizing radiation is related to its effects on tumor cell DNA. Radiation can interact and damage DNA directly by causing DNA breaks or indirectly by interacting with water or oxygen molecules, resulting in ion pairs and reactive oxygen metabolites such as H2O2 and hydroxyl radicals. (Figure 5).13 DNA damage, if not repaired, will likely cause cell death and eliminate malignant activity.13

hydroxyl radicals

Figure 5.

It is estimated that two-thirds of radiotherapy-induced damage to DNA is caused by hydroxyl radicals.

Malignant cells have a decreased capacity to repair radiation damage. They can fall into one of three categories: (1) lethal damage, which occurs when no DNA repair is possible and leads to cell death, (2) sublethal damage, which is repairable as a function of time provided no further radiation damage is incurred prior to the repair, and (3) potentially lethal damage, which is a condition in which cells may survive without repair, depending on post-radiation conditions.13

Radiation dose is expressed as the absorbed energy by the irradiated tissue. The unit of the absorbed dose is the Gray (Gy=1 J/Kg). A conventional radiotherapy regimen for head and neck neoplasms consists of delivering 60–70 Gy, fractionated at a rate of 2.0 Gy/day, 5 days/week, for 6 to 7 weeks. However, newer and often more aggressive protocols and techniques may be employed to improve tumor control and/or reduce side effects.13

Theoretically, any malignant tumor can be destroyed by ionizing radiation if the dose delivered is sufficient. The limiting factor is the amount of radiation the adjacent normal tissues will tolerate.13 These adverse effects can be either direct, i.e., radiation-induced destruction or damage to susceptible cells causing a loss or disruption of tissue function, or indirect, i.e., radiation-induced decrease in vascularity and associated tissue changes.5,6,8

Short-term effects - Undifferentiated cells of irradiated tissues or organs will be severely damaged. It will lead to acute but transient disruption of the integrity and function of affected tissues or organs. If the doses are relatively low, after some time, the stems cells will be able to differentiate, and healing will take place, followed by at least partial re-establishment of tissue or organ function.5,6,8

Long-term effects - In radiation therapy, sometimes the radiation doses are high enough to cause damage to the microvasculature of tissues and organs without damaging the differentiated cells. The outcome of this damage will be noticed 6-8 weeks post irradiation. The damage to the microvasculature will lead to a loss of nutrient flow into the organs leading to their degeneration and necrosis. These effects progress for the remainder of the irradiated patient’s life.5,6,8