radioresistance

Radioresistance is the relative capacity of biological systems to withstand ionizing radiation. It describes how effectively cells, tissues, or organisms repair radiation-induced damage and survive. Radiosensitivity is the converse. The concept is central to radiobiology and radiotherapy, where the goal is to maximize tumor control while minimizing normal-tissue injury.

Variation by cell type and physiological state; the cell cycle influences sensitivity: cells are most sensitive in the G2/M phases and most resistant in late S phase. Oxygen presence increases radiation effectiveness (oxygen enhancement ratio). Hypoxic cells exhibit reduced radiosensitivity, contributing to radioresistance in many tumors.

Key cellular mechanisms include efficient DNA double-strand break repair by non-homologous end joining and homologous recombination, robust DNA damage checkpoints, and the ability to undergo senescence, apoptosis avoidance, or autophagy. Microenvironmental factors such as hypoxia, nutrient deprivation, and pH modulate sensitivity.

Contexts: intrinsic resistance refers to innate properties; acquired resistance develops after exposure. Tumor cell populations often show heterogeneity in radiosensitivity; cancer stem cells tend to be more resistant; some normal tissues also display relatively high intrinsic radioresistance.

Clinical implications and approaches: radioresistance can limit tumor control and promote normal-tissue injury. Assessment uses clonogenic survival and dose–response modeling. In treatment, fractionation exploits differential repair and reoxygenation; strategies to overcome resistance include hypoxia modification, radiosensitizers, and combination therapies.