Answer to Question #13535 Submitted to "Ask the Experts"

Category: Medical and Dental Patient Issues

The following question was answered by an expert in the appropriate field:


I had two lower back (lumbar) x rays 7.5 months apart. Each time, two views were taken. The procedures were taken by different providers and I forgot to mention to the second provider that I already had the first x ray (at a different clinic) so maybe the second set of x rays were not needed. In addition to the lumbar x rays, I have dental x rays every year and have had the following x rays in my lifetime: sinus, chest, tympanic membrane, foot (three views), shoulder (two views) and knee (three views). In regard to these x rays, I have the following questions and concerns.

  1. Is it safe to have two lumbar x rays 7.5 months apart, considering my understanding they have the highest radiation dose among all x rays?
  2. I don't remember the number of views for sinus, chest, tympanic membrane x rays as they were done many years back; any suggestions regarding the normal number of views and the dose?
  3. What is my likely total radiation dose from medical procedures and what is the likely effect?

Yes, is it safe to have two lumbar x rays 7.5 months apart? While lumbar studies use more radiation than a foot x ray, it is by no means the highest diagnostic imaging exposure. To address your second and third question, we refer to sources such as for estimated exposures. All together you may have received approximately 6 milliSievert (mSv) over a period of years. 

You will find that many references cite 100 mSv as the important number to stay below. A 100 mSv dose is not intended to indicate that doses below that are "safe" and doses above it are "not safe." As stated in the Health Physics Society's position paper on "Radiation Risk in Perspective:" "The references to 100 mSv should not be read as implying that health effects are well established for doses exceeding 100 mSv." The position paper notes that there is considerable uncertainty for stochastic effects (cancer induction) of radiation exposure between 100 mSv and 1,000 mSv, depending on multiple variables, such as the population exposed, the exposure rate, and the organs and tissues affected. The risks associated with protracted exposures such as your 6 mSv exposure have very high uncertainties and may not definitively impose a risk. It is important to understand that a theoretical small increase in population-based cancer risk above 100 mSv is very small in comparison to the natural incidence of cancer development in the United States which is 439.2 per 100,000 men and women each year based on 2011–2015 cases as stated on the National Cancer Institute website.

Additionally, the exposures you received are not all additive, in part because the exams expose different areas of the body. The only exams that might be additive are your lumbar studies; however, these would still be significantly below 100 mSv (approximately 3 mSv total). At low doses, such as those received from your x rays, the dose rate or time between exams has an insignificant effect. So, spreading doses over five years vs. every year or two is irrelevant, low dose is low dose.

Kendall Berry, MSPH, CMLSO

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 7 July 2020. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.