Radiation Exposure from CT Scans
GIST Support International asked questions about radiation exposure from CT exams to Donald P. Frush, MD of Duke University Medical Center. At Duke Dr. Frush is a Professor of Radiology, a Pediatrics Faculty member, as well as Chief of the Medical Physics Graduate Program, Division of Pediatric Radiology. Dr. Frush’s research interests are predominantly involved with pediatric body multidetector CT, including techniques, assessment of image quality, and radiation dosimetry. Dr. Frush has published very widely and has served as a guest editor and invited reviewer for numerous medical journals. He is currently the associate editor (North American) of the journal Pediatric Radiology.
Here are Dr. Frush's responses to our questions.
1. What is the radiation dose from CT scans of the chest/abdomen/pelvis? How does this compare to chest X-rays and to normal daily-life annual background radiation?
Radiation dose from CT scans of the chest, abdomen, or pelvis varies depending on the individual patient, and the technique used. In general, in adults, most abdomen CTs are performed at approximately 10 mSv. The dose is less for a chest CT. The doses should be the same-to-less, if size adjusted, for children. Head CT doses are generally less than about 2-4 mSv. As a rough approximation, one abdomen pelvis CT in an adult is equal to 100-250 chest x-rays. The average background radiation (just from living…) that individuals get is about 3-3.5 mSv per year.
2.What is the risk to health from quarterly CT scans of the chest/abdomen/pelvis over many years?
The risk of low-level radiation, such as that used in CT, is unknown. There are established scientific data that show that doses > 100-200 mSv have a significant association with the risk of developing cancer but we do not know for sure about doses, such as from infrequent CT, which are under that amount. The risk is either zero or very small. In general, radiologists and healthcare providers should assume that any unnecessary amount of radiation should be avoided and that the benefit of the CT examination (for example the probability of detection of recurrent tumor, or the satisfaction of knowing that there is no recurrence) outweigh the risks (small, at most with CT examinations).
In general, the dose of CT (or any medical radiation, such as an x-ray) is cumulative. That is, 4 examinations at 5.0 mSv each done over three years is the equivalent of 20 mSv of dose (4x5=20). This is accumulated over the lifetime. Similarly, if quarterly abdominal CT scans (at 10 mSv each) were given for 5 years, the cumulative dose would be 4x10x5 mSv = 200 mSv.
3. What is the latency period for development of radiation-induced cancers?
The latency for development of solid tumors can be more than a decade.
4. Is sensitivity to radiation exposure greater in growing children and adolescents in whom tissues are still developing?
When discussing a potential risk of CT scan, the radiation dose risk is higher for children than young adults. While there is no agreed upon age cut-off where the potential risk is zero, generally the younger the patient (i.e. under 30 years of age) the risk is higher due to potential accumulation of multiple CT examinations over a longer lifetime and increase radiation sensitivity of developing tissues, particularly in children. Sensitivity for children is 2-10 times that for adults; most favor the lower part of the range.
5. What strategies exist for minimizing the radiation dose in CT scans by altering the technique or settings?
There are multiple strategies for minimizing radiation risks. First, when imaging is necessary, imaging evaluation considerations should be those that have the least amount of risk, including radiation. For example, if MR imaging (or sonography) will answer the question, then these should be performed. MR may not be the best evaluation, for example if lung parenchyma needs to be assessed. The decision about the type of frequency of examinations needs to come from a discussion between the individual patient and the healthcare team caring for the patient.
When a CT is indicated, only as much radiation as is necessary for evaluation should be used. For example, protocols should based on patient size in the pediatric population. Only the necessary region should be scanned, and repeat scans through the area during the CT examination (multiphase examinations) should be minimized in frequency. Additional technical considerations for certain regions need to be considered, such as slightly lower dose to the chest as opposed to the abdomen during CT examinations.
6. Can MRI yield equally useful images to monitor growth or shrinkage or density change of existing tumors, and to detect new metastases in the liver and peritoneum? Are there any disadvantages of MRI?
MR examination of the abdomen and pelvis is often an excellent modality for detecting solid organ tumors, as well as tumors elsewhere in the abdomen. The decision about whether this type of study needs to be performed versus a CT, again, needs to be arrived at through discussions between the healthcare team and the patient.