Perguntas frequentes (FAQ)

There is currently no conclusive scientific evidence that there is a relationship between the increased likelihood of developing cancer and the application of ionizing radiation in low-dose diagnostic procedures. However, some studies, based on limited statistics, suggest that radiation levels applied in computed tomography procedures may slightly increase the risk of developing radioinduced cancer in the future, particularly in pediatric patients.

There is a consensus that the benefit of accurately diagnosing the most appropriate clinical approach for treating the patient far outweighs the potential risks of this radiological technique.

To maximize the benefits of diagnostic radiation applications, procedures should be well justified and optimized. In a radiological examination, the patient should be given the lowest possible dose, maintaining the image quality required for diagnosis.

Risks from CT scans - what do recent studies tell us? J. Radiol. Prot. 34 (2014) E1 – E5
UNSCEAR 2013: Effects of Radiation Exposure of Children
NCRP Report 171 (2012): Uncertainties in the Estimation of Radiation Risks
Radiol Clin North Am. 2009 January; 47 (1): 27–40. doi: 10.1016 / j.rcl.2008.10.006.
Mayo Clin Proc. • December 2010; 85 (12): 1142-1146 www.mayoclinicproceedings.com
Cynthia H. McCollough1. The Role of the Medical Physicist in Managing Radiation Dose and
Communicating Risk in CT. AJR 2016; 206: 1241-1244.

There is no established upper limit to the number of CT scans that an individual can undergo, provided there is a well-founded clinical indication with properly optimized protocols. Prior examinations and alternative imaging techniques should be considered before performing another CT scan. Special attention should be paid to pediatric patients who are more radiosensitive than adults.

CT radiation doses to the patient are higher than other radiological procedures. Currently, the radiation dose associated with a routine CT procedure varies from 1 to 14 mSv depending on the scan and is comparable to the annual dose received from natural radiation sources such as radon and cosmic radiation (1-10 mSv), depending on where the person lives. Thus, the risk for an individual exposed to radiation on a CT scan would be comparable to environmental radiation levels.

The effective dose of a CT scan can range from 50 to 400 times the dose of a chest x-ray, depending on the technical parameters used.

However, it is also necessary to consider the doses absorbed in organs, especially the most radiosensitive ones, such as the lens, the thyroid and the gonads. It is therefore recommended to shield these organs with shielding when possible or to reduce exposure factors.

In the case of nuclear medicine, in scintigraphy procedures, the values range from 1.2 mSv to 23 mSv, depending on the study. These effective dose values are comparable to those observed in tomography procedures. Tables 1 and 2 show the effective dose values for different procedures in nuclear medicine and computed tomography respectively.

Table 1: Average effective doses in nuclear medicine examinations

ProcedureEffective dose (mSv)
MDP-Technetium-99m Bone Scintigraphy3,6
Myocardial scintigraphy MIBI-Technetium-99m4,2
Renal scintigraphy DMSA-Technetium-99m2,5
Pulmonary perfusion MAA-Technetium-99m1,2
Thallium chloride myocardial scintigraphy 20123
Thallium Chloride Parathyroid18
Sodium iodide thyroid scan (I-123)3,4
International Atomic Energy Agency - Basic Safety Standards - BSS115
International Commission on Radiological Protection - Publications ICRP 53, ICRP80 and ICRP106

Table 2: Average effective doses on CT scans

CT examsMean effective dose (mSv)Equivalent number of PA chest radiographs (0.02 mSv each)
Calcium Count3150
Pulmonary angiography5,2260
Coronary angiography8.7435
Virtual colonoscopy10500
Thorax (pulmonary embolism)15750

Source: (https://rpop.iaea.org/RPOP/RPoP/Content/InformationFor/Patientspatient-information-computed-tomography/index.htm) radiol Clin North Am, 2009; 47 (1): 27-40.
Effective doses in Radiology and diagnostic nuclear medicine: A catalog, Radiology 248 1 (2008) 254-263.
Radiation exposure in multi-slice versus single-slice spiral CT: Results of a nationwide survey, Eur. Radiol. 13 (2003) 1979-1991.
Revised radiation doses for typical x-ray examinations, Br. J Radiol. 70 833 (1997) 437-439.
Radiation dose and cancer risk estimates in 16-slice computed tomography coronary angiography. J. Nucl. Cardiol. 15 2 (2008) 232-240.

Both techniques do not use x-rays for imaging. There is no scientific data to indicate that MRI and ultrasound are associated with any cancer risk.

Yes, all CT scanners provide two dose descriptors: Volume Computed Tomography Dose Index (CTDIvol) or Volumetric CTDI and Dose Length Product (DLP) or Dose Length. However, these quantities should be used only as a dose estimate in the procedure. As they are obtained from phantoms (16 and 32 cm), these quantities do not represent the characteristics of each patient and, therefore, cannot be considered as the doses received by the patient, especially in pediatrics.

To obtain a more accurate estimate according to the specific anatomical region of a patient, a methodology was developed that includes correction factors, considering the patient's dimensions. The method for this analysis can be found in the following publication:

Size Specific Dose Estimation (SSDE) in Pediatric and Adult body CT Examination - American Association of Physicists in Medicine report of task group 204 (2011)

Yes, it is possible to reduce radiation doses without compromising image quality for diagnosis. Initially, it should be ensured that the examination was properly justified and that alternative techniques such as ultrasound and magnetic resonance imaging were considered. The best balance between image quality and radiation dose should be ensured by adopting the following optimization strategies:

- Adjust protocols for patient groups considering age, biotype, gender and clinical indication;
- Reduce the number of scanning phases by using only the necessary ones (intravenous contrast exams);
- Avoid repetition of unnecessary exams;
- Use automatic dose control whenever possible;
- Reduce mAs as much as possible by considering the acceptable noise level for diagnosis;
- Limit the scan length by restricting it to the region of interest;
- Use immobilization and shielding devices for sensitive organs when possible;
- Avoid using low pitches;
- Use interactive reconstruction methods;
- It is recommended to compare the institution's CTDIvol and DLP values for a defined sample of patients with established international reference levels.

Diagnostic Reference Levels (NRD) and Achievable Doses (AD) for Adult and Pediatric CT at CTDIvol

ProcedureLateral dimension of the patientPhantom Diameter (cm)CTDIvolNRD (mGy)CTDIvolAD (mGy)
Head - (adult)16167557
Pelvis Abdomen (adult)38322517
Chest (adult)35322114
Head (pediatric - 5 years)15164031
Abdomen (pediatric - 5 years)20162014

Source: ACR – AAPM Practice Parameter for Diagnostic Reference Levels and Achievable Doses In Medical X-Ray Imaging (August 21, 2015)

CT examinations in pregnant patients are not prohibited, but there must be an accurate clinical indication for their performance. Alternative diagnostic techniques that do not use ionizing radiation should be considered. However, if the CT scan is properly justified, every effort must be made to optimize the procedure while minimizing fetal exposure.

There is a particular concern in examining a pregnant woman because of the risk of exposure of the fetus to ionizing radiation, particularly in the first trimester of pregnancy. Potential effects of radiation to the fetus include embryonic, neonatal or fetal death, congenital malformation, and functional changes such as mental retardation, reduced intelligence quotient, and childhood cancer. The risk is related to the dose rate and total radiation dose received by the fetus and the developmental stage at the time of exposure.
Tests that require direct exposure of the fetus to the primary bundle, such as abdominal exams, deserve the most attention and care. For examinations in regions away from the fetal area, the scattered radiation received by the fetus will be very small as long as the procedure is properly conducted.

Prior to the examination, the radiologist should discuss the referral with the requesting physician assessing the risks and benefits of the procedure. A qualified physicist or professional needs to estimate the dose absorbed by the fetus. Technique parameters need to be optimized and technique factors recorded. Unnecessary exposure of the abdomen and pelvis should be avoided by limiting the area to be exposed as much as possible by using precise, single phase collimation and, if possible, pelvic protectors. The fetal dose needs to be reduced to what is strictly necessary to obtain the diagnosis. All contrast media need to be used with caution. Repeated examination should be avoided.

Importantly, in properly optimized procedures the doses received by the fetus are much lower than 100 mGy, which corresponds to the threshold established in international recommendations. Therefore, the procedures should not be associated with an increase in abnormalities or fetal death.

Radiation Exposure and Pregnancy: When Should We Be Concerned? RadioGraphics 2007; 27: 909–918
Imaging in Pregnant Patients: Examination Appropriateness. RadioGraphics 2010; 30: 1215–1233 •
Report No. 174 - Preconception and Prenatal Radiation Exposure: Health Effects and Protective Guidance (2013)

A New Pregnancy Policy for a New Era. Pregnancy and Medical Radiation. ICRP Publication 84. Ann. ICRP 30 (1), 2000

When performed properly, the benefits of a CT scan far outweigh the risks. CT can provide detailed information to diagnose, plan treatment and assess the patient's clinical condition. In addition, it can eliminate the need for exploratory surgery.

The risk of cancer in children due to radiation exposure is approximately two to three times higher than in adults because pediatric patients have a longer life expectancy and their organs are more sensitive to radiation. For newborns, the risk of cancer induction is essentially the same as in the second and third trimester of pregnancy. Therefore, it is essential that the exam be optimized to obtain the diagnosis.

Pediatric protocols with specifically established exposure parameters should be used, as well as current modulation systems and low-kV techniques. As an international consensus, the examination needs to be performed in only one phase, avoiding, where possible, the pre-contrast phase.

UNSCEAR 2013: Effects of Radiation Exposure of Children

The risk of accompanying a patient on a CT scan is very low. However, any radiation exposure should be avoided. The stay of a companion in the room during the procedure should only be allowed when strictly necessary for the exam. When the presence of the companion is indispensable, all radiological protection measures should be adopted in order to minimize their radiation exposure. It is recommended that the companion, parent or family member, use the lead apron and the thyroid protector during the procedure. The technician should be advised if the companion is likely to be pregnant to avoid exposure.

Radiotherapy uses x-rays, gamma rays and other types of radiation to treat cancer and other diseases. The radiation energy deposited in the tissue is used to destroy tumor cells. Radiation doses applied to the tumor are thousands of times higher than those received by patients undergoing diagnostic X-ray examinations. While doses in radiotherapy are in the order of Gy (absorbed dose unit), in tomography the absorbed doses in organ are in the order of mGy, that is, a thousand times smaller.

When radiotherapy and tomography are performed, the effective doses received may be added by applying the appropriate calculation methodology. The quantities and units involved in each procedure must be carefully checked to properly calculate the dose received by the patient.

To make sure that the radiology service meets the radiation protection requirements, it is recommended to check whether:

a) Does the facility have a health permit
To obtain the license, the service must meet radiological protection requirements established in the current legislation of the Ministry of Health.

b) The facility has a Computerized Tomography Quality Seal of the Brazilian College of Radiology
To obtain the seal of quality, the facility undergoes an evaluation by the tomography committee, composed of a group of radiologists and a physicist. In this evaluation process the following requirements are observed: a) qualification of service professionals; b) image quality assessment: the committee's radiologists group analyzes tomography images of different procedures provided by the service and the respective reports; c) radiological protection criteria, including radiation doses, are observed by the committee physicist.
In addition, in 2015 CBR began the implementation of an accreditation program (PADI) for diagnostic imaging clinics, which includes auditing of tomography services.

c) Does the service have a radiation protection program
It is recommended that the patient or guardian check with the service which radiation protection measures are adopted for the patient, such as: if there are specific protocols for pediatrics and which measures are adopted by the service to optimize the exams.

There are many differences between an individual's radiation exposure in a CT procedure and radiation exposure from an atomic bomb. Exposure on tomography only involves a specific region of the body, whereas with the bomb explosion, in addition to the exposure being full body, there was also internal contamination (radioactive particles were inhaled, ingested and deposited in the body). In tomography, technical factors are duly selected to provide the lowest possible radiation dose to the patient without impairing the radiological image. In the case of the atomic bomb, the radiation level received by each individual varied according to their position relative to the epicenter without any control.

It should be noted that the atomic bomb explosion resulted in initial radiation (gamma and neutron emission) and residual radiation (gamma and beta emission). Between 150,000 and 200,000 people died during the explosions and in the months that followed. Most individuals within a kilometer of the bombing died of acute radiation poisoning, debris fall, or fires that erupted in the aftermath of the attack.

However, about 25,000 atomic bomb survivors were exposed to relatively low doses of radiation, comparable between one and three CT scans, and showed no significant increase in cancer risk. The number of cancer cases that have developed over the rest of their lives is not, however, large enough to provide the statistics needed to reliably predict the risk of CT-associated cancer in the general population today.

In the light of radiation data in the literature, there is still no consensus on the risk of developing cancer due to the low radiation doses of tomography.

Even groups that believe there is an increased risk would be very small compared to the possibility of a person developing cancer from natural causes. The Food and Drug Administration (FDA) concluded that 10 mSv (approximate dose of an abdominal scan) would increase the risk of cancer death by 0.05%. Given that the natural incidence of cancer death in anyone in the US is 20% (about 400 times higher), a single CT scan would increase the risk of developing a fatal patient's average tumor to 20.05%. Importantly, it should be noted that there is no consensus on these risk values.

Beyond the Bombs: Cancer Risks from Low-Dose Medical Radiation. Lancet 2012 August 4; 380 (9840): 455-457.

At low radiation doses, as with radiological procedures, the exact magnitude of the risk is a controversial issue. This is because for doses below 100 mSv the risks are too low to be measured directly.

Assuming that there is a slight increase in the risk of cancer with low radiation doses, it is recommended to keep the dose levels as low as possible while maintaining the appropriate image quality for diagnosis.

The risk associated with single or multiple CT scans is minimal. Every individual is daily exposed to background radiation levels, which can range from 3 to 10 mSv / year, depending on the region. No increase in cancer cases has been observed in regions where background radiation is highest. In tomography, depending on the type of procedure, radiation doses received by the patient may range from 2 to 10 mSv. In special procedures, these values may increase to 20 to 30 mSv, but radiation levels are still considered low. Thus, the risk for an individual exposed to radiation on a CT scan may be comparable to background radiation levels. A head scan and a chest scan would correspond, on average, to 8 and 36 months of background radiation exposure, respectively. On a transatlantic flight, for example, radiation exposure would correspond to 11 days of background radiation exposure.

The risk of mortality from a CT scan is significantly lower than the risk associated with various daily activities. For example, it has been estimated that in the US, the risk of dying walking on the street is 32 times higher than the risk of a CT scan and the risk of death while driving a car is 240 times higher than a CT. The following table compares other types of day-to-day radiation risks due to CT procedures.

Derived from: Fletcher JG, Kofler JM, Coburn JA, Bruining DH, McCollough CH.
Perspective on radiation risk in CT imaging. 2012 Jul 27; 38 (1): 22–31.