Safety · Radiobiology2 min read5-question check
Cell Radiosensitivity, LET and RBE
Young, dividing cells are most radiosensitive. High-LET radiation does more damage per gray than x-rays.
The law
The law of Bergonié and Tribondeau (1906) states that cells are more radiosensitive when they divide often and are less specialized. Immature stem cells and cells with high metabolic rates are the most sensitive. Lymphocytes, spermatogonia, red cell precursors and intestinal crypt cells rank high. Mature nerve and muscle cells rank low. The fetus, with rapidly dividing cells, is more sensitive than an adult.
LET, RBE and OER
Linear energy transfer (LET) is the energy deposited per unit length of a radiation's path, in keV per micrometer. Diagnostic x-rays are low LET, about 3 keV/µm. Alpha particles are high LET, about 100 keV/µm. Relative biologic effectiveness (RBE) compares how much dose of 250 kVp x-rays and of a test radiation produce the same effect. Diagnostic x-rays have an RBE of 1. As LET rises, RBE rises. The oxygen enhancement ratio (OER) shows that oxygenated tissue is more sensitive. It is about 3 for x-rays and near 1 for high-LET radiation.
The numbers
RBE equals the 250 kVp x-ray dose divided by the test radiation dose for the same effect. Worked example: if 10 Gy of x-rays and 2 Gy of neutrons kill the same share of cells, the RBE is 10 divided by 2, or 5. OER equals the dose without oxygen divided by the dose with oxygen. If hypoxic cells need 6 Gy and oxygenated cells need 2 Gy for the same effect, the OER is 3.
How damage happens
Direct action means radiation ionizes DNA itself. Indirect action means it ionizes water, and the products then damage DNA. The body is about 80 percent water, so most x-ray damage is indirect. In radiolysis of water, a photon splits water into an ion pair. These form free radicals such as OH•, and two OH• can join to make hydrogen peroxide. Target theory says a cell dies only when a key molecule, its DNA, is inactivated. Hits are random and can be direct or indirect. High-LET radiation makes direct hits more likely.
Clinical Example
A patient receiving pelvic radiation therapy develops diarrhea within days, while nearby muscle shows no early change. The intestinal crypt cells divide constantly and are highly radiosensitive. The mature muscle cells divide rarely and are resistant.
Common Mistakes
- Reversing the law: thinking mature, specialized cells are most sensitive.
- Assuming direct action is the main mechanism for x-rays.
- Stating that OER rises with LET; it falls toward 1.
- Calculating RBE upside down, test dose over x-ray dose.
Why It Matters
Radiation biology makes up part of the Safety category. The registry asks about cell sensitivity order, the LET and RBE relationship, and free radical damage.
Key Relationships
- More division, less differentiation → more radiosensitive
- Higher LET → higher RBE
- Higher LET → lower OER
- Low-LET x-rays → mostly indirect action through free radicals