Answer to Question #12761 Submitted to "Ask the Experts"

Category: Medical and Dental Patient Issues — Therapy - Radiation Oncology

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


I have searched and researched everywhere to try to find information I can understand. What I want is a more layman explanation of the following information provided regarding a radiation therapy treatment: 66 Gy/60 Gy54Gy in 30 Fractions. 


The 66 Gy, 60 Gy, and 54 Gy are radiation doses to areas being treated. A Gray (Gy) is a unit of radiation dose, called the "absorbed dose." Absorbed dose is the amount of energy from the radiation that is absorbed by whatever matter the radiation is going through. One Gy is equal to 1 Joule (J) of energy absorbed in 1 kg of matter; in this case, the matter is the tumor being treated. (For reference, 1 J is equal to the amount of energy used by a 40-watt lightbulb in 0.025 seconds.)

What you provided is the prescription for the radiation therapy. This prescription calls for the major tumor area to receive 66 Gy, while areas with microscopic disease or areas near critical structures, such as the spinal cord, will receive the lower doses of 60 Gy and 54 Gy. This technique is sometimes called "dose painting." 

These doses will not be delivered all at once, but will be spread over a period of six weeks/30 fractions. This is a common radiation therapy technique called fractionating the dose. On each day of treatment only a fraction of the total dose (in fact, 1/30 of the total dose) to each area will be delivered. Each day of treatment is called a fraction. So, for example, the major tumor area will receive a radiation dose of 2.2 Gy per fraction or day.     

If you have specific questions, the best people to consult would be your radiation oncologist, medical physicist or therapist providing treatment. 

Jacqueline Emrich, PhD, DABR
Kent Lambert, CHP, FHPS

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 5 February 2019. 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.