6.1 Cell & Tissue Radiosensitivity

The Law of Bergonie and Tribondeau

Radiosensitivity is the relative susceptibility of a cell, tissue, or organ to injury from ionizing radiation. In 1906 two French scientists, Jean Bergonie and Louis Tribondeau, irradiated rodent testes and formulated the classic law of radiosensitivity that still anchors ARRT radiobiology items. Their law states that cells are most radiosensitive when they are: (1) highly mitotic (rapidly dividing), (2) undifferentiated or immature (stem cells), (3) have a long mitotic future (many future divisions), and (4) possess a high metabolic rate. Cells that are mature, specialized, and non-dividing are comparatively radioresistant. Ancel and Vitemberger later refined this by noting that expressed damage also depends on the biologic stress placed on the cell and the conditions after exposure, but the original four criteria are what the exam tests.

Most and Least Radiosensitive Cells

Apply the law by ranking cells. Lymphocytes are the classic exception and a favorite exam trap: although they do not divide frequently once mature, they are extremely radiosensitive and are the first blood cells to fall after exposure, making the lymphocyte count an early biologic dosimeter. Spermatogonia (immature sperm-forming cells) and erythroblasts (immature red cells) are likewise highly sensitive. At the opposite end, mature nerve and muscle cells are the least radiosensitive because they are highly differentiated and no longer divide.

This ranking explains why the bone marrow, gonads, and gastrointestinal epithelium are the tissues of greatest concern in radiation protection, and why a developing embryo or fetus, a sea of dividing undifferentiated cells, is dramatically more sensitive than an adult.

Target Theory and DNA as the Critical Target

Target theory proposes that each cell contains a critical target molecule whose inactivation causes cell death or malfunction. That target is DNA. A radiation "hit" is an ionizing event deposited within the target. If the hit inactivates DNA, the outcome may be cell death, a repairable break, or a viable mutation that is passed to daughter cells. Because there is only one master copy of the genome per cell, DNA damage is far more consequential than damage to abundant cytoplasmic molecules.

Direct Versus Indirect Action

Radiation damages DNA by two routes. In direct action the photon or particle deposits energy straight into the DNA macromolecule, ionizing it. Direct action dominates with high-LET radiation. In indirect action the radiation first ionizes a water molecule (the body is roughly 60-80% water), a process called radiolysis of water. Radiolysis yields highly reactive free radicals, chiefly the hydroxyl radical (HO-) and the hydrogen radical (H-), which carry an unpaired electron. These migrate and chemically attack DNA; two hydroxyl radicals can also combine to form toxic hydrogen peroxide (H2O2). Because water is so abundant, indirect action accounts for roughly two-thirds of the biologic damage from diagnostic x-rays, which are low-LET. Recognizing that free radicals, not the photon itself, cause most low-LET injury is a commonly tested concept.

LET, RBE, and the Oxygen Effect

Three physical factors modify how much biologic harm a given absorbed dose produces.

• LET (Linear Energy Transfer) is the amount of energy a radiation deposits per unit length of its track, expressed in kiloelectron volts per micrometer (keV/um). Diagnostic x-rays and gamma rays are low-LET (sparsely ionizing, about 3 keV/um), whereas alpha particles and neutrons are high-LET (densely ionizing). Higher LET packs ionizations close together, making DNA double-strand breaks and cell killing more likely.
• RBE (Relative Biological Effectiveness) compares radiations. It is the dose of a reference radiation (250 kVp x-rays) needed to produce a given effect divided by the dose of the test radiation needed for the same effect. RBE rises as LET rises, peaking near 100 keV/um and then declining (the "overkill" effect). High-LET alpha radiation has an RBE around 20.
• The oxygen effect describes how oxygenated tissue is more radiosensitive than hypoxic tissue. It is quantified by the oxygen enhancement ratio (OER), the dose under hypoxic conditions divided by the dose under oxygenated conditions for the same effect. For low-LET radiation OER is about 2.5-3; for high-LET radiation it approaches 1. Oxygen "fixes" free-radical damage, making it permanent, which is central to radiotherapy but also explains why well-perfused tissues respond more strongly.

Cell-Cycle Sensitivity and Tissue-Level Effects

Radiosensitivity also shifts within a single cell across the cell cycle. Cells are most sensitive during mitosis (M phase) and the G2 phase, when the chromosomes are condensed and least able to repair breaks, and most resistant during the late S phase, when DNA repair enzymes are most active. This is why rapidly cycling tissues, which have many cells passing through mitosis at any moment, show the greatest injury, tying the cell-cycle concept back to Bergonie and Tribondeau.

At the tissue and organ level, the same principles predict which structures are protected in imaging. Continually renewing tissues, the hematopoietic bone marrow, the germinal epithelium of the gonads, and the crypt cells of the intestine, are the most radioresponsive because their stem-cell compartments divide constantly. The embryo and fetus represent the extreme case: a mass of undifferentiated, rapidly dividing cells that is far more radiosensitive than any adult tissue, with the greatest vulnerability during organogenesis (roughly weeks 2-8 of gestation). A frequent exam trap reverses the ranking: remember that lymphocytes are radiosensitive despite not dividing, while nerve and muscle are radioresistant despite being metabolically active.