Lung Cancer Screening: It’s Kind of a Big Deal

Editor’s note: Due to the subject matter of this post, the tone is a wee heavy. With that caveat, let’s get down to business.

Before I became a doctor, I remember wondering why there were not more screening tests to detect cancer. Why doesn’t everyone get annual head-to-toe CT scans and extensive blood tests to ensure a clean bill of health? Why don’t we all get a colonoscopy starting at 18? Happy graduation!

I eventually learned, as with most things in life, it just ain’t that simple. For any screening test to be truly effective, it must meet a few important criteria:

1. The disease for which you are testing must be common in the population you are screening. Like it or not, it is simply not economically feasible to test entire populations for rare diseases.
2. The disease must be serious. Otherwise, why even test in the first place?
3. The screening test must be safe, easy to administer, reliable, and inexpensive.
4. Treatment before symptoms occur (e.g. chest pain or coughing up blood in the case of lung cancer) is more effective than treatment after those symptoms occur.

Many proposed cancer screening tests have met the first 3 criteria, but criterion 4 is more complicated and often requires extensive investigation to prove; a test should reduce mortality in the screening population to meet this standard. Despite years of attempts with numerous types of cancer, only 3 cancer screening tests have been shown to meet all of the above criteria:

• Mammography for breast cancer
• Pap smear for cervical cancer
• Colonoscopy for colon cancer

But following a large, nearly decade-long clinical trial by the National Cancer Institute, low-dose CT screening was added to the list of recommended cancer screening tests. Annual chest CT screening was shown to reduce mortality from lung cancer. Not exactly champagne-popping time, but big news indeed.

Why this is important

Part of the reason for the excitement surrounding this newest cancer screening test is the fact that lung cancer is a particularly aggressive and deadly disease. To make matters worse, it is one of the most common cancers in the world, and the leading cause of cancer-related death in the United States. Almost 90% of individuals diagnosed with lung cancer eventually die from the disease.

Before I get into the details of CT screening, some background explanation about lung cancer is in order. Lung cancer is divided into two main types: small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC). These slightly ridiculous names refer to the microscopic appearance of SCLC, which is composed of—you guessed it—smaller cells. Critically, SCLC and NSCLC behave very differently. SCLC is generally more aggressive; a majority of patients have widespread, incurable disease when it is discovered. NSCLC is more common (80-85% of cases) and is further classified into subtypes, but the important thing to remember is that all NSCLCs have a similar treatment and prognosis. If you remember one thing about the types of lung cancer, make it this: NSCLC has a better prognosis, and can be removed with surgery if detected early enough. Lung cancer screening generally focuses on detection and treatment of these early NSCLC.

How it works

Screening CT scans look for lung nodules, some of which may be early lung cancers. Cancers start as microscopic collections of abnormal cells, which grow out of control and eventually become big enough to visualize as small “spots” or nodules on a CT scan. Individuals who meet the criteria for lung cancer screening (see below) receive annual CT scans to look for any new or enlarging nodules that might be suspicious for a growing lung cancer. The radiologist looks at the CT scan and generates a report with recommendations based on the size and appearance of any nodules present. Weekly conferences are held to go over these CT scans with the team of doctors and staff caring for these patients.

Nodules fall into 4 categories of increasing concern:

• Category 1: Negative
• Category 2: Benign appearance or behavior
• Category 3: Probably benign
• Category 4: Suspicious for cancer

Patients whose scans fall into category 1 or 2 continue with routine annual screening CT scan in another 12 months. Category 3 patients get a follow up CT in 6 months to ensure the nodules are truly benign and not growing. Category 4 patients are further evaluated with either another CT scan in 3 months, PET/CT (a special test to better characterize the nodules), or sometimes biopsy of the concerning nodules.

This is not for everyone

For a screening test to meet the first criterion above—the disease must be common—we need to screen the appropriate patient population. For example, it would not make sense to screen 18-year-olds for colon cancer. The number of colon cancers found would be so vanishingly small that the risks of the colonoscopy procedure and false-positive cases (more on this below) would outweigh any benefit of finding the very few cancers present in this age group. Thus, we initiate screening colonoscopies at 50, an age at which the incidence of colon cancer begins to increase more sharply.

Before we define who exactly gets screened for lung cancer, bear with me as I explain how doctors count cigarettes. As we all know, smoking is the most important risk factor in the development of lung cancer; the longer and more one smokes, the higher the risk. Doctors use a term called “pack-years” to describe the number of cigarettes smoked combined with the number of years a patient has smoked. One pack-year means someone smoked one pack of cigarettes per day for one year. A 30 pack-year smoking history could mean a patient smoked one pack per day for 30 years, or two packs per day for 15 years (2×15=30 for those without a calculator).

With our new cigarette counting skills, we are now ready to talk about who gets screened for lung cancer. Based on previous data and using predictive models, it was determined that the highest risk population who would benefit the most from screening CT fit 3 criteria:

1. Age 55-80
2. 30 pack-year smoking history
3. Current smoker OR quit smoking within the past 15 years

The reason for criterion 3 is that individuals who quit smoking actually decrease their risk for lung cancer over time, and after 15 years the risk if low enough to stop screening.

This is not a perfect test (but pretty darn good)

Some of you may note that this type of screening can result in several decades of repeated annual CT scans, perhaps performed more often if concerning nodules arise. Doesn’t that result in a lot of radiation exposure? Are we at risk for creating another Incredible Hulk or Godzilla with this madness? Well, no. You see, this is where the “low-dose” part of the CT scan comes to the rescue.

Modern CT scanners have come a long way since they were invented in the 1970s. They are faster, produce better images, and use lower radiation doses. A normal chest CT on a modern scanner uses technology to reduce radiation, but special low-dose protocols have been developed to screen the lungs for nodules at very low radiation doses. The tradeoff for that very lose dose is more graininess in the CT images outside the lungs, but this does not compromise the ability to detect lung nodules.

Finally, as with any screening test, there is the problem of false-positives. With lung cancer screening, a false-positive result means that a lung nodule was flagged as a possible cancer, but ended up being something benign or otherwise insignificant (e.g. someone inhaled a Junior Mint). Most of these false-positive nodules can be followed on CT scans to ensure they don’t continue to grow. However, some may end up getting biopsied or even surgically removed before it is discovered they are benign, a situation that would ideally be rare in a high-quality lung cancer screening program.

So there you are: low-dose CT lung cancer screening in a nutshell. Well, in a CT scanner. I can honestly say I never hope to see your screening CT scan, because it means you have been smoking a lot. Nevertheless, it is exciting that we finally have an effective screening test for this aggressive and devastating disease.

References

• Aberle DR, Adams AM, Berg CD, Black WC, Clapp JD, Fagerstrom RM, et al; National Lung Cancer Screening Trial Research Team. Reduced lung-cancer mortality with low-dose computed tomographic screening. N Engl J Med. 2011; 365:395-409. [PMID: 21714641]
• Mover VA; U.S. Preventive Services Task Force. Screening for Lung Cancer: U.S. Preventive Services Task Force Recommendation Statement. Ann Intern Med. 2014 Mar 4;160(5):330-8. doi: 10.7326/M13-2771.
• Kazerzooni, EA, et al; American College of Radiology and Society for Thoracic Radiology. ACR-STR practice parameter for the performance and reporting of lung cancer screening thoracic computed tomography (CT). J Thorac Imaging. 2014 Sep;29(5):310-6. doi: 10.1097/RTI.0000000000000097.