Radiation Exposure from CT, PET/CT Will Be Tracked at the NIH

Current Issues & Debates

• Controversies exist whether low-dose (less than 150 mSv) medical radiation tests are related to development of cancer.
• Model used to extrapolate the cancer risk from low-dose medical radiation exposure is a "linear-no-threshold hypothesis", which implies that any amount of ionizing radiation (even small) has a finite probability of inducing cancer.
• This approach is widely accepted and used for radiation protection regulations and guidelines by the International Commission on Radiological Protection.
• Several recent studies and news reports have raised concerns regarding radiation exposure from medical devices (particularly CT and nuclear cardiology)
• A recent study published in the Archives of Internal Medicine estimated that radiation from CT might cause 29,000 new cancers and 14,500 deaths a year. Another study in the same journal pointed out that patients may have received much higher radiation from imaging tests than previous believed. Read synopsis in the older RiT post.

Addressing These Issues by NIH

• Radiology and Imaging Sciences at the National Institutes of Health (NIH) Clinical Center will incorporate radiation dose exposure reports into the electronic medical record (EMR)
• The process will be developed in corporation with major equipment vendors beginning with exposures from CT and PET/CT
• Radiation dose will be recorded, entered into DICOM header for CT and PET/CT and stored either in radiology information system or preferably hospital-based EMR. It should be trackable by patients in their own personal health records

What Will It Do?

• It is the first step toward monitoring patient dose
• It is the basis for future research on this subject
• Who knows, in the future this may be required in all institutions...

Occupational Radiation Dose Limits

Facts:

• International Commission on Radiological Protection (ICRP) issues periodic recommendations on radiation protection. The Commission was founded in 1928.
• ICRP's latest publication was Publication 103 (2007)
• ICRP effective dose limit for radiation worker (occupational dose limit) = 20 mSv per year when averaged over 5 years; any year limit to 50 mSv
• The most highly exposed workers are unlikely to receive regular annual effective doses more than 5 mSv
• Radiation workers should be monitored using personal dosimeter (film badges, TLDs)
• In emergency situations, occupational exposure can exceed these dose limits if lifesaving actions are involved. Older workers with low lifetime accumulated effective doses should volunteer for emergencies.

Nephrogenic Systemic Fibrosis Disappeared After Restrictive Use of Gadolinium?

According to a large (50 000+) retrospective cohort of patients who underwent contrast-enhanced MR examinations at a single academic institution pre- and post-adoption of strict gadolinium guidelines:-

• No new cases of nephrogenic systemic fibrosis (NSF) were diagnosed
• During the pre-guidelines adoption and transitional period, the incidence of NSF was 3 cases per 10,000 contrast-enhanced MRI
• After the adoption of guidelines, the incidence was 0 per all examinations

The Guidelines for Imaging Adult Patients

• Based on renal disease severity
• eGFR 60 or greater - GBCA can be administered as indicated
• eGFR 30-59 - weight-based dose of GBCA (0.2 mL/kg) can be administered with maximal dose of 20 mL allowed within 24 hours
• eGFR less than 30 - GBCA cannot be administered except in cases of medical necessity; informed consent required; nephrology consultation required; hemodialysis should be considered
• Very rarely that any patients with eGFR less than 30 would get contrast-enhanced MR exams (36 in 52 954 exams; 0.07%)

eGFR = estimated glomerular filtration rate; GBCA = gadolinium-based contrast agent

CT Radiation Exposure in Real Clinical Practice and Cancer Risk Estimation

Two recent studies published in the Archives of Internal Medicine in December 2009 could serve as an eye opener for every physicians ordering CT scans as well as radiologists.

Smith-Bindman et al collected CT dose data of 11 most common CT study types performed in 4 hospitals in San Francisco Bay Area. They found that there were wide variations in dose within each study type and between different types. For example, routine chest CT dose ranged from 2 to 24 mSv, routine abdomen-pelvis CT with IV contrast dose ranged from 4-45 mSv. Median effective doses for each exam were higher than they were commonly quoted in the literature, for example, 8-10 mSv is common quote for a chest CT examination. They also estimated the risk of developing cancer related to CT in several patient groups according to patient's age at the time of CT. Based on their calculation, 1 in every 80 women who undergo a chest CT for suspected pulmonary embolism at age 20 will develop cancer. Similarly, 1 in every 270 women who undergo coronary CT angiography at age 40 will develop cancer.

Berrington de Gonzalez et al utilized Medicare claim data and IMV Medical Information Division survey to project estimated age-specific cancer risk from CT studies performed in the U.S. in 2007. Excluding CT studies done for cancer diagnosis and within the last 5 years of life, 2% (29,000) excess cancers caused by CT scans in 2007 were predicted.

What is needed?
Optimization and standardization of CT protocols and techniques to limit radiation
Reduction of number of CT scans
Collection of dose information at patient level to educate patients and health care providers about radiation exposure

Effects of radiation in our body

• 0 - 5 rem received in a short period or over a long period is safe—we don’t expect observable health effects.
• 5 - 10 rem received in a short period or over a long period is safe—we don’t expect observable health effects. At this level, an effect is either nonexistent or too small to observe.
• 10 - 50 rem received in a short period or over a long period—we don’t expect observable health effects although above 10 rem your chances of getting cancer are slightly increased. We may also see short-term blood cell decreases for doses of about 50 rem received in a matter of minutes.
• 50 - 100 rem received in a short period will likely cause some observable health effects and received over a long period will increase your chances of getting cancer. Above 50 rem we may see some changes in blood cells, but the blood system quickly recovers.
• 100 - 200 rem received in a short period will cause nausea and fatigue. 100 - 200 rem received over a long period will increase your chances of getting cancer.
• 200 - 300 rem received in a short period will cause nausea and vomiting within 24-48 hours. Medical attention should be sought.
• 300 - 500 rem received in a short period will cause nausea, vomiting, and diarrhea within hours. Loss of hair and appetite occurs within a week. Medical attention must be sought for survival; half of the people exposed to radiation at this level will die if they receive no medical attention.
• 500 - 1,200 rem in a short period will likely lead to death within a few days.
• >10,000 rem in a short period will lead to death within a few hours.

The health effects listed above are for a radiation dose to the entire body. If the radiation is given to a smaller area of the body, there are other effects that may occur, but illness or death is not expected unless noted:

• 40 rem or more locally to the eyes can cause cataracts.
• 100 rem - 500 rem or more can cause hair loss for a section of the body that has hair.
• 200 rem or more locally to the skin can cause skin reddening (similar to a sunburn).
• 1,000 rem or more can cause a breakdown of the intestinal lining, leading to internal bleeding, which can lead to illness and death when the dose is to the abdomen.
• >1,500 rem or more locally to the skin can cause skin reddening and blistering.

Fundamental dose reduction concepts in CT

• optimize scan parameters by reducing tube current, either as a user-selectable option in the scan prescription or by selecting scanners that feature angular and z-axis modulation of mA according to tissue thickness along a given projection;
• develop tabulated guidelines for height- and weight-adjusted current settings;
• increase table increment or pitch (axial and helical scans, respectively) whenever possible without reducing image quality;
• reduce the number of multiphase scans; and
• reduce inappropriate referrals and recommend less dose-intensive modalities—such as MRI, ultrasound, and radiography—whenever possible.

Factors contribute to the dose intensity of CT

1) Mode of irradiation. In contrast to planar radiography, which is planar by nature, CT involves multi-angular irradiation of the patient. As a result, dose is distributed with essentially uniform intensity throughout the scan plane rather than with the decreasing intensity with depth characteristic of radiography. In general, a CT examination of a given section of anatomy delivers a dose that is substantially higher than its radiographic equivalent. For example, the effective doses delivered by radiography and CT of the (PA) chest are approximately 2 and 800 mrem, respectively. Order-of-magnitude dose disparities between CT and radiography exist for other examination types.

2) Dose to extraneous tissues. Relatively high doses are delivered to tissues included in the scan plane but not of clinical interest, such as the breast in thoracic CT. Breast dose during such procedures lies in the range of 2 to 10 rad—in comparison with an average mean glandular dose of approximately 200 millirad per view in mammography. Dose to all tissues in the field of view at CT as well as those irradiated by secondary radiation (internal and external scatter and tube leakage) contribute to the patient's effective dose.

3) Irradiated tissue volume. With the advent of helical MDCT and subsecond gantry rotation times, and the option of contiguous or overlapping scans, greater scan lengths are achievable in increasingly less time—resulting in a concomitant increase in the average total volume of irradiated tissue. Further, the requirement for interpolation of transmission profiles from neighboring scans in helical scanning in turn necessitates additional rotations of the gantry at the extremes of the scan range, such that the exposed tissue volume is greater than the reconstructed volume. This type of dose augmentation is exacerbated as the aperture width increases.

4) Nature of CT image formation. Modalities that use image-recording media with limited dynamic range—such as screen-film radiography—have associated with them limits to the dose that can be recorded without loss of information. CT, however, is an inherently digital-imaging modality for which there is no such dose penalty. Image quality in CT will increase with increasing dose as the level of Poisson-distributed noise decreases.

5) Nonoptimized scanning protocols. The NRBP UK CT dose survey1 demonstrated that patient-efficient doses for the same examination could vary by up to a factor of 10 among institutions. However, this magnitude of variability represents a significant improvement in the findings of the 1991 survey by the same group in which dosewise variation on the order of a factor of 40 was found. This change is, in part, the result of emergent awareness of the radiation burden imposed by CT and the application of dose-mitigating strategies based on patient age, body habitus, and the tissue type to be imaged.