In radiotherapy, the 'linear-quadratic (LQ) model' is used to describe cell killing. The alpha/beta ratio for late-responding normal tissues (like spinal cord, rectum) compared to early-responding tissues (like mucosa, tumor) is characteristically:
- A Lower for late tissues (alpha/beta ~3 Gy) than early (alpha/beta ~10 Gy) ✓
- B Higher for late tissues (alpha/beta ~10 Gy) than early (alpha/beta ~3 Gy)
- C Equal for all tissues (alpha/beta ~7 Gy)
- D Higher for tumor tissue (~15-20 Gy) than all normal tissues
Explanation
In the linear-quadratic model (S = e^-(αD + βD²)), the alpha/beta ratio characterizes tissue radiation response. Late-responding tissues (spinal cord, kidney, rectum, bladder) have low alpha/beta ratios (~3 Gy), meaning they are more sensitive to fraction size — large fractions cause disproportionately more late damage. Early-responding tissues (mucosa, skin, most tumors) have high alpha/beta ratios (~10 Gy), responding more to total dose than fraction size. This is the radiobiological basis for conventional fractionation (2 Gy/fraction) — it protects late-responding normal tissues while effectively treating tumors. Prostate cancer is an exception with low alpha/beta (~1.5-3 Gy), justifying hypofractionation (stereotactic body radiotherapy).
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.