The alpha/beta ratio used in BED calculations reflects tissue sensitivity to fractionation. Which statement is true?

• A. Late-responding tissues have a high alpha/beta ratio
• B. Early-responding tissues have a high alpha/beta ratio
• C. Alpha/beta ratio is irrelevant to fractionation
• D. It states that late-responding tissues typically have a low alpha/beta ratio

Answer: (D)

The key idea is how fraction size affects tissue damage through the alpha/beta ratio in the linear-quadratic model. A low alpha/beta means a tissue’s response grows more with larger doses per fraction, making it more sensitive to how you split the total dose. Late-responding tissues (like spinal cord and other normal tissues) typically have a low alpha/beta ratio, around 2–3 Gy, so they show greater sparing with smaller fraction sizes and more damage with larger fractions. This is why the statement that late-responding tissues typically have a low alpha/beta ratio is correct. In contrast, early-responding tissues tend to have higher alpha/beta values, and saying they have a high alpha/beta would’t apply to late-responding tissues, while claiming alpha/beta is irrelevant contradicts the whole fractional-dose framework.