Radiation risks

Absorbed dose is defined as the amount of energy absorbed per unit mass and is measured in Gray (Gy), i.e., J/Kg. The energy deposition of 1 J/kg tissue is the equivalent of 1 Gy. There are different types of radiation and all types do not produce the same biological effect; therefore, the term equivalent dose is often used instead of the absorbed dose. The equivalent dose is the product of the absorbed dose and a radiation weighting factor (earlier termed as quality factor) and is expressed in Sieverts (Sv). Because the radiation weighting factor for X-rays and gamma rays is same, i.e., 1.0; thus, 1 Gy is equivalent to 1 Sv in medical imaging. Radiation doses in medical imaging are typically expressed in millisieverts (mSv).

In our routine life, we are getting exposed due to small doses of radiation from the natural sources all the time, i.e., from cosmic radiation, rock, building material and by food like banana we eat. Usually, the average estimated dose received per year due to natural background is of the order of 3 millisieverts (mSv) per year. Unless you are exposed to high doses of radiation during cancer treatment at young age, any increase in risk for you developing cancer due to medical radiation appears to be quite low as the effects of radiation damage typically take many years to appear.

There are many ways to diagnose ailments in the patient which may involve ionizing radiation used in diagnostic radiology by conventional X-ray, CT scan or by administering radiopharmaceuticals to patients in nuclear medicine. It is always preferable to use non-ionizing radiation like MRI and USG for the desired information required by the referring physician. There is always a risk of developing cancer, if the person is exposed to radiation more than the background level. Moreover, the risk to an individual exposed to ionizing radiation is not the same as it depends upon age, sex and many other parameters. As it is a well-known fact that in children the cells are dividing rapidly, they are more sensitive and also have longer life expectancy; thus, it becomes visible. It is seen that the risk is small in diagnostic imaging rather not treating the diseases. It is always better that there should be awareness amongst public that as and when they are referred for CT scan or any other examination in nuclear medicine, they need to discuss the benefit and risk with the referring physician.

The very purpose of using any diagnostic modality is to provide the relevant information to the referring physician so that future mode of treatment can be decided based on the information obtained after the study. It is a proven fact that the there are advantages of using X-ray for getting information as it is a noninvasive technique and has low risk and at the same time provides accurate information instantaneously. Since all the imaging modalities more or less provide the relevant information about the specific diseases, however, it is always preferred to use nonionizing modalities like MRI and USG, w.r.t the other modalities using ionizing radiation, if it can be.

During radiation exposure, due to the damage of the cell structure, either stochastic effect takes place or deterministic effect takes place. But the likelihood of stochastic effect increases with the increase of dose received. The evidence available on radiation-induced cancer risk is available from the data available from the atomic bomb survivors. It has been noted that the cancer risk is more prevalent for doses above 100 mSv, which is very unlikely in diagnostic radiology until multiple CT examinations take place as a single CT examination of abdomen is likely to deliver a dose of around 10 mSv. It is also observed that no epidemiological effect is noted at the dose levels below 10 mSv which are usually the doses received in general radiography or nuclear medicine imaging.

International Commission on Radiological Protection (ICRP) uses the concept of effective doses to assess the health risk. In order to reduce the radiation dose to the patient, either the alternative modality is to be used or the examination is carried out by imparting less dose to get the desired clinical information. It is always better that clinician explains the radiation risk and benefit of the radiological procedures to the patients who express concern about this issue. It is the mere responsibility of the medical practitioner to perform the scan, if no alternative modality is available to get the desired information.

Legislation and principles of radiation protection

Patient doses in diagnostic imaging

When radiation passes through the body, majority of radiation get absorbed, some of it get scattered and some of it get transmitted. The X-rays that are transmitted from the patient are used to create the radiographic image. The amount that is absorbed contributes to the patient’s radiation dose.

Medical practitioners always use the term effective dose when estimating the risk to the patient whilst using ionizing radiation for diagnosis or treatment of diseases, as the risk takes into account the possible side effects during the diagnosis/treatment later in life. ICRP Report-133 also states that as and when the term effective dose is used; it is to be noted that whether the organ and tissue has received whole body exposure, partial body exposure or heterogeneous exposure which is usually the case in diagnostic radiology, where the beam is collimated only to the organ of choice.