The previous blog posting described the Second Key to the PA Castle: USE Evidence-Based Guidelines that can be found on the radiology benefits management (RBM) websites, copy to your computer desktop and refer to whenever you are uncertain which imaging study to request. The third Key to the Castle is an extension to the Guidelines but focuses on Patient Safety - Radiation and Contrast Risks.
It has long been recognized that pregnant women should avoid CT scans due to radiation effect on the fetus. Ultrasound or Non-contrast MRI are preferred substitutes for CT scan during pregnancy. Our pediatrician colleagues remind us of the long term effects of radiation in children and suggest substitute imaging studies in place of CT scans. Recently the medical literature has reported a causal relationship between radiation exposure and the incidence of some cancers.
So just how much Radiation do Patients receive from CT scans and how risky is that radiation?
The Radiation Reference Table shows how much radiation patients receive from various imaging studies. The imaging study is listed on the left column, while the two columns on the right show the amount of radiation measured in miligrays (mGy) and milisieverts (mSv). For physicians who are not physicists, an easier way of understanding radiation dose is seen in the center column, which reports radiation by relative comparison to a known index: a single chest X-ray as ‘one unit of radiation’. All imaging studies are compared to ‘one chest x-ray unit’ of radiation. For example, a Chest PA and Lateral would expose a patient to 2 “chest x ray units.” According to this table, a Chest CT with contrast (71260) would expose a patient to 400 ‘chest x-ray units.’ A dual study, (e.g. chest CT without AND with contrast (71270)) would subject a patient to 800 ‘chest x ray units,’ twice the radiation of the single CT scan.
Is a dual CT scan any more valuable than a single CT scan?
To answer this question we must turn to the ACR appropriateness criteria. ACR lists those conditions that exclusively support a Chest CT without contrast (71250), including screening for lung cancer, following a solitary pulmonary nodule, and diagnosing interstitial lung disease (with a high resolution non-contrast CT scan). ACR also lists many diseases supporting a CT chest with contrast. However, the appropriateness criteria lists only one condition for which a dual study (CT chest without AND with contrast) is recommended and that disease is rare. And yet many providers request dual study CT chest, doubling the radiation but with no additional diagnostic value. These requests are usually denied because they are not supported by any guideline.
What CT scan subjects patients to the most ionizing radiation?
Near the bottom of the Radiation Exposure table is a test called CT abdomen and pelvis without and with contrast (74178). This study is technically 4 different imaging studies, exposing patients to 1600 or more ‘chest x-ray units.’ According to the ACR appropriateness criteria, the only indication for this test is in the evaluation of hematuria in older patients when urological malignancy is suspected. But this CT scan is often requested by providers for reasons other than hematuria. These requests are usually denied because they are not supported by any guideline and subject patients to excessive radiation.
Some diseases or conditions are best diagnosed without contrast, while others are best performed with contrast. A few conditions in the abdomen may still require a dual (or even a triple) study to confirm a diagnosis. ACR appropriateness criteria or RBM Guidelines should be consulted to determine the best contrast decision.
The next blog posting will demonstrate how technological advances in CT scanning and resolution have made most dual studies nearly obsolete.