ADAPTATION AND CROSS-RESISTANCE - AN EXPLANATION FOR THE EXISTENCE OFDIFFERENT RADIOSENSITIVITIES AMONG MALIGNANT-MELANOMA CELLS

A hypothesis is presented to explain the apparent difference in the ra dioresponsiveness of melanoma lesions whether they are located on the skin or in other parts of the body The hypothesis states that the radi osensitivity of a cell may change when the cell adapts to live and gro w in a different environment. The most important environmental factor that affects the radiosensitivity of cutaneous melanoma cells appears to be the partial pressure of oxygen in their immediate environment. B y virtue of adapting to grow in an environment having a high partial p ressure of oxygen, the melanoma cells located on the skin may have dev eloped a better antioxidant defense mechanism than cells that metastas ize to, and grow in, other parts of the body having lower partial pres sures of oxygen such as lymph nodes, brain and viscera. Because some o f the cell-damaging effects of both oxygen and ionizing radiation are mediated through a similar mechanism, the melanoma cells on the skin b ecome cross-resistant to ionizing radiation because of their higher to lerance to oxygen toxicity.