Although radiation can interfere with many cellular processes, many experts feel that a cell must undergo a double-strand DNA break from radiation in order to be killed. The factors that influence tumor cell killing include the D0 of the tumor (the dose required to deliver an average of one lethal hit to all the cells in a population), the Dq of the tumor (the threshold dose—a measure of the cell's ability to repair sublethal damage), hypoxia, tumor mass, growth fraction, and cell cycle time and phase (cells in late G1 and S are more resistant). Rate of clinical response. | Chapter 081. Principles of Cancer Treatment Part 5 Although radiation can interfere with many cellular processes many experts feel that a cell must undergo a double-strand DNA break from radiation in order to be killed. The factors that influence tumor cell killing include the D0 of the tumor the dose required to deliver an average of one lethal hit to all the cells in a population the Dq of the tumor the threshold dose a measure of the cell s ability to repair sublethal damage hypoxia tumor mass growth fraction and cell cycle time and phase cells in late Gi and S are more resistant . Rate of clinical response is not predictive some cells do not die after radiation exposure until they attempt to replicate. Therapeutic radiation is delivered in three ways 1 teletherapy with beams of radiation generated at a distance and aimed at the tumor within the patient 2 brachytherapy with encapsulated sources of radiation implanted directly into or adjacent to tumor tissues and 3 systemic therapy with radionuclides targeted in some fashion to a site of tumor. Teletherapy is the most commonly used form of radiation therapy. Radiation from any source decreases in intensity as a function of the square of the distance from the source inverse square law . Thus if the radiation source is 5 cm above the skin surface and the tumor is 5 cm below the skin surface the intensity of radiation in the tumor will be 52 102 or 25 of the intensity at the skin. By contrast if the radiation source is moved to 100 cm from the patient the intensity of radiation in the tumor will be 1002 1052 or 91 of the intensity at the skin. Teletherapy maintains intensity over a larger volume of target tissue by increasing the source-to-surface distance. In brachytherapy the source-to-surface distance is small thus the effective treatment volume is small. X-rays and gamma rays are the forms of radiation most commonly used to treat cancer. They are both electromagnetic nonparticulate waves that cause the ejection
