Why is radiotherapy administered in small, divided daily doses rather than as a single large dose?

Appeared in: NORCET-11 Prelims

Explanation

Fractionation allows normal cells to repair sublethal damage between doses.
Cancer cells have less efficient repair mechanisms, so repeated small doses maximize tumor cell kill.
Fractionation increases the therapeutic ratio by protecting normal tissues while effectively treating tumors.

Option B: Secondary relapse is not a reason for fractionating radiotherapy. Fractionation is about balancing tumor control and normal tissue protection, not about relapse risk.
Option C: A single dose does have a biological effect on tumors, but it is not used due to high risk of normal tissue toxicity, not lack of effect.
Option D: Immediate systemic leukopenia is a possible side effect of radiotherapy, but fractionation is not primarily aimed at preventing this. The main goal is to protect normal tissues overall, not just blood cells.

$quadratic cell survival curve showing difference in survival between normal and tumor cells with fractionated vs single large dose radiation.$

Clinical Relevance

Nursing practice connection: Knowing Principle of radiotherapy fractionation and its biological rationale helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
Nurses must understand why radiotherapy is fractionated to educate patients about the treatment schedule and expected side effects.
Monitoring for acute and late radiation toxicity is a key nursing responsibility.
What if? If radiotherapy were given as a single large dose, normal tissue toxicity would increase dramatically, leading to severe complications and limiting the total dose deliverable to the tumor.

How to Approach the Question

Identify the core concept: radiotherapy dosing and its biological rationale.
Eliminate options that do not address the mechanism of action or clinical purpose of fractionation.
Recall that fractionation is designed to maximize tumor kill while minimizing normal tissue injury.
Look for keywords like 'repair', 'sublethal damage', and 'therapeutic ratio' in the options.
Use process of elimination for distractors that focus on relapse, lack of effect, or unrelated side effects.

Concept Tested & Keywords

Concept Tested: Principle of radiotherapy fractionation and its biological rationale
Stem keywords: radiotherapy, small divided daily doses, single large dose
Lead-in keywords: Why

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E6 Medicine Harrison 22e Part 1 p. 595-597

Practise the full NORCET-11 Prelims

Attempt every question from this paper in a timed mock, then review the full solution for each one.