ADAPTATION - A NEW CONCEPT IN RADIATION BIOLOGY AND ONCOLOGY AND ITS CLINICAL RELEVANCE

A new concept of adaptation and cross-resistance in radiation biology and oncology is introduced. It explains the apparent difference in the radiosensitivity of cells of tile same type (both normal and tumor) t hat are located in different parts of the body. Oxygen tension, which varies considerably from one part of the human body to another, ranges from 20 mmHg to 159 mmHg (2.7-21%). When cells are adapted to live in areas with high physiologic oxygen tension (high normoxic) such as th e epidermis and upper and lower airways, they develop better antioxida nt mechanisms, which include free radical scavengers and DNA repair, t han cells that live in lower physiologic oxygen tension (low normoxic) . Because the mechanisms I of cell-damaging effects of ionizing radiat ion are similar to those of oxygen, the cell that is resistant to oxyg en toxicity also becomes resistant to ionizing radiation (and certain cancer chemotherapeutic agents). Based on this concept, it could be ex pected that some human tumors such as primary cutaneous malignant mela noma and lung cancers contain cells that are relatively resistant to i onizing radiation as a consequence of living in a high normoxic condit ion and the treatment of these tumors may require different strategies .