Radiotherapy: skin sparing with megavoltage photons: practice question

A Radiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Radiology · Radiotherapy: skin sparing with megavoltage photons

Sample item

Why do patients treated with cobalt-60 or linear accelerator photons show less severe skin reactions than those treated with orthovoltage X-rays?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Maximum dose is deposited at a depth below the skin surface, the build-up effect

With megavoltage photons, secondary electrons build up over a short distance, so the dose maximum lies beneath the skin. Orthovoltage beams give maximum dose at the surface. Bolus is used when the skin must be treated deliberately.

Option by option

  • A. The beam is fully absorbed by the surface layer

    The surface is not the site of full absorption.

  • B. Maximum dose is deposited at a depth below the skin surface, the build-up effect (correct)

    Correct. The dose peak lies below the skin because of electron build-up.

  • C. The dose rate is much lower

    Dose rate is unrelated to the skin sparing effect.

  • D. Megavoltage beams do not interact with skin

    Megavoltage photons do interact with skin but deposit less dose there.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Radiology question

What is the answer to this Radiology question?

The best answer is B, Maximum dose is deposited at a depth below the skin surface, the build-up effect. With megavoltage photons, secondary electrons build up over a short distance, so the dose maximum lies beneath the skin.

Why is "The beam is fully absorbed by the surface layer" not the answer?

The surface is not the site of full absorption.

Why is "The dose rate is much lower" not the answer?

Dose rate is unrelated to the skin sparing effect.

Why is "Megavoltage beams do not interact with skin" not the answer?

Megavoltage photons do interact with skin but deposit less dose there.

What concept does this question test?

It tests Radiotherapy: skin sparing with megavoltage photons in Radiology. Orthovoltage beams give maximum dose at the surface. Bolus is used when the skin must be treated deliberately.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Radiotherapy: skin sparing with megavoltage photons?

Learn signs with the image, and the disease they point to. Then use the flashcards and the other Radiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.