Radiobiology Flashcards

What is the standard dose per fraction in conventional radical radiotherapy?

Answer: 1.8–2.0 Gy

Conventional fractionation uses 1.8–2.0 Gy per fraction, balancing tumor cell kill while allowing repair of sublethal damage in normal tissues.

Which of the following best describes hyperfractionation in radiotherapy?

Answer: Multiple smaller fractions per day to increase total dose while limiting late toxicity

Hyperfractionation uses multiple daily fractions with smaller doses per fraction, exploiting differential α/β ratios to increase total dose and improve tumor control while protecting late-responding tissues.

A treatment delivers 2 Gy × 30 fractions to a tumor with α/β = 10 Gy. What is the Biologically Effective Dose (BED)?

Answer: 72 Gy

BED = nd × (1 + d/[α/β]) = 30 × 2 × (1 + 2/10) = 60 × 1.2 = 72 Gy.

Accelerated repopulation of tumor cells during radiotherapy is generally thought to begin after approximately:

Answer: 3–4 weeks

Accelerated repopulation begins around 3–4 weeks into radiotherapy as surviving tumor cells increase their proliferative rate in response to radiation-induced cell loss.

Prostate cancer has an unusually low α/β ratio estimated at approximately:

Answer: 1.5 Gy

Prostate cancer has an α/β ratio of ~1.5 Gy, similar to late-responding normal tissues, making it particularly sensitive to high dose-per-fraction hypofractionated schedules.

Stereotactic body radiotherapy (SBRT) is characterized by which fractionation schedule?

Answer: 6–20 Gy × 3–5 fractions

SBRT uses ablative hypofractionated schedules of 3–5 large fractions (6–20 Gy each), delivering high doses with precision to minimize normal tissue exposure.

Which statement correctly compares late-responding and early-responding tissues in fractionated radiotherapy?

Answer: Late-responding tissues have lower α/β ratios and are more sensitive to high doses per fraction

Late-responding tissues have lower α/β ratios (1–3 Gy), making them more sensitive to increased dose per fraction; conventional fractionation protects them by using small daily doses.