• The Review Course in Family Medicine

"Doctor, how much radiation is in this scan?" Here's how to answer.

Updated: Aug 13, 2020

by Dr. Chinmay Dalal MD CCFP

  • Everything is Risk vs Benefit - Will this imaging aid in the diagnosis AND change management! This is key, we don’t test just to find an answer, only if it will change the management

  • Absorption of radiation is affected by body habitus

  • Modern radiology equipment is VERY focused with very little bleeding of radiation outside the targeted area

Review of the terms:

  • Millisieverts (mSv) - derived unit of ionizing radiation dose in the International System of Units (SI) and is a measure of the health effect of low levels of ionizing radiation on the human body - 1 rad = approx 10 mSv

  • Background Radiation = including radon, cosmic rays, terrestrial, and internal sources - Annually, about 3 mSv

  • The International Commission on Radiological Protection (ICRP) estimates a 4 to 5% increased relative risk of fatal cancer after an average person receives a whole-body radiation dose of 1000 mSv

ESSENTIALLY (in very general terms):


1 Chest X-Ray (PA and Lat) = 0.1 mSv of radiation = 12 days of background radiation

  • This calculation can then be used to discuss with the patient the risk vs benefit of the test and seek informed consent ie. CT Abdo/Pelvis in a patient with abdominal pain NYD ≅ 10 mSv (or 3 years of background radiation or a 0.04% increased relative risk of fatal cancer)




Photo by: Mikael Häggström / CC0, https://commons.wikimedia.org/wiki/File:Normal_posteroanterior_(PA)_chest_radiograph_(X-ray).jpg



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