Radiation therapy planning uses the concept of 'dose-volume histograms' (DVH). The linear quadratic (LQ) model describes cell survival after radiation. In fractionated radiotherapy, the alpha/beta (α/β) ratio is used to characterise tissue radiosensitivity. For late-responding normal tissues (e.g., spinal cord, kidney), the α/β ratio is:
- A Low (1–3 Gy), meaning these tissues are more sensitive to fraction size than tumour ✓
- B High (8–12 Gy), meaning they respond similarly to acute tissues
- C Zero, meaning they do not respond to radiation
- D Equal to that of most tumours (10 Gy)
Correct answer: A. Low (1–3 Gy), meaning these tissues are more sensitive to fraction size than tumour
Explanation
The α/β ratio in the linear quadratic model indicates sensitivity to fraction size. Late-responding normal tissues (spinal cord, kidney, bladder) have LOW α/β ratios (1–3 Gy), meaning they are more sensitive to large dose fractions — using smaller fractions (conventional fractionation) relatively spares these tissues. Acute-responding tissues and most tumours have HIGH α/β ratios (8–12 Gy). Stereotactic body radiotherapy (SBRT) exploits low α/β tumours (e.g., prostate, ~1.5 Gy) by using hypofractionation.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.