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Radiology of the Chest--Chapter Nine

Introduction and Interesting Links
Chapter One - Methods of Examination

Chapter Two - Normal Radiographic Anatomy
Chapter Three - Normal Variants and Congenital Anomalies
Chapter Four - Signs of Disease
Chapter Five - Acute Pulmonary Infections
Chapter Six - Pulmonary Tuberculosis
Chapter Seven - Pulmonary Sarcoidosis
Chapter Eight - Metastatic Disease and Related Conditions
Chapter Nine - The Solitary Pulmonary Nodule
Chapter Ten - Lung Carcinoma and Related Conditions

Chapter Eleven - Pneumothorax
Chapter Twelve - COPD
Chapter Thirteen - Heart and Great Vessels


Chapter Nine: The Solitary Pulmonary Nodule

The solitary pulmonary nodule, or SPN, can pose a formidable diagnostic challenge. Also known as "coin lesions", solitary pulmonary nodules are defined as single nodules under 4 centimeters in diameter. (When the lesion is over 4 cm it is referred to as a "mass".) SPNs typically do not contain a large amount of calcium, though CT scans often depict density levels (Hounsfield units) which are consistent with calcification. This feature will be discussed in more detail below.

The term "SPN" implies a lesion that has indeterminate characteristics. In other words its origins need to be investigated to exclude the possibility of a malignancy. They are usually found by accident since their size causes few, if any, symptoms.

Over half of all SPNs turn out to be granulomas which are, of course, benign leftovers of an old histoplasmosis, coccidioidomycosis or tuberculosis infection. Though that statistic is comforting, keep in mind that 30% are primary malignancies such as bronchial carcinoma or carcinoid tumors. Metastatic lung disease, though extremely common, does not usually manifest as an SPN. Patients usually present with multiple pulmonary nodules instead.

There are important considerations after an SPN is found.

The two most important prognostic indicators are:

  • growth
  • calcification

Less predictive prognostic indicators are:

  • edge
  • size of lesion
  • age of patient
  • smoking history
  • location within the chest

Both malignant and benign nodules can grow. It is the rate of growth which is most important. Malignancies are characterized by uninhibited, exponential growth, and benign nodules should grow more slowly. By comparing serial films, it is possible to calculate the rate of growth. A lesion which takes 18 months to double in volume, or one which takes less than 1 month to double, is generally benign. (You might be wondering which benign process could demonstrate such fast growth. The answer is that cysts and infections can enlarge very quickly.)

The average time it takes for a malignant nodule to double in volume is about 110 days. The "malignant window" of the time in which malignancies typically double is between 1 and 18 months (or 30 to 500 days).

How this works: take a measurement of the diameter of the lesion on comparable chest x-rays from different dates. A doubling time calculator is provided in a link below. It is very important to remember that because of the spherical nature of a nodule, a 9mm lesion that increases to 11mm in diameter has doubled in volume. By the time a lesion of 5mm has increased to 10mm it has more than quadrupled in size.
Doubling time calculator

48 year-old woman with 30 pack-year history of smoking. Note dense nodule in right lung base.

Same patient with an x-ray from 9 years previous. Note there has been no change in size.

Calcification is a good thing. Less than 10% of all malignant SPNs contain appreciable calcium. Most lesions which are calcified are benign and they include granulomas, hamartomas and some rarer entities.

Calcification, however, is sometimes hard to determine on a chest x-ray. A nodule may contain a good amount of calcium and still not be discernible on the x-ray. When in doubt a low kVp x-ray can help, however it is a CT scan which is most important in making a determination of calcium content.

The unit of measurement for density used in CT scanning is the Hounsfield Unit (HU), after Sir Godfrey Hounsfield, the British Nobel laureate who developed the CT scanner. The scanner is calibrated to reflect water as having 0 HU. Air measures at -1000 HU and bone at +1000 HU. Plain films are able to resolve only large differences in tissue attenuation, e.g. air is more lucent than fat which is more lucent than soft tissue, etc. CT computers can measure subtle differences within soft tissue as the chart to the right depicts. Also, the operator can manipulate an image to highlight certain tissues. These variables are called "windows". A single study of the chest may include "soft tissue windows" and "lung windows".

Lung window

Soft tissue window

A spiculated edge is ominous. It implies encroachment into previously normal tissue. A well-defined border is a good thing usually, but not always. Remember that cannon ball-type metastatic nodules tend to have fairly well-defined edges.

Unknown Case 1

Unknown Case 2

Above are two cases of SPNs in different patients, both adult women. (Apical lordotic projections are shown.) Try to discern which has benign characteristics and which has malignant characteristics.

Size of Lesion
The larger the nodule, the more likely it is to be malignant. Lesions under 1cm are most likely to be benign and those over 3cm are likely to be malignant. Those between 1cm and 3cm are a toss-up and, of course, there can be very small malignancies and very large benign lesions.

Age of Patient
A person under the age of 30 is very unlikely to have cancer. Of course there are exceptions. The incidence of cancer increases with age so that someone over 70 with an SPN has a great probability of cancer.

Click image to enlarge

Smoking History
Pack-year calculation: Average number of cigarette packs smoked per day, multiplied by years of smoking. The length of time spent smoking is important, too. The longer a person smokes, the greater the risk. Example:

Person "A" smoked 1 pack of cigarettes a day for 40 years. Person "B" smoked 2 packs of cigarettes daily for 20 years. Both have smoked for 40 pack-years. However, person "A" is 8 times more likely to develop lung cancer.

CT scan showing SPN and a pack of cigarettes in the upper right portion of the image.

Location within the Chest
A nodule within the upper lobe has a greater probability of malignancy compared with nodules discovered elsewhere.

Summary of Features

Favors Malignant
Favors Benign
Most valuable indicators
Doubles in size between 1 and 18 months No growth; slow growth (over 18 months to double); or very fast growth (under 1 month to double)
No -- 90% have no calcification Yes
Less valuable indicators
Spiculated edge, or lobulated Well-defined, smooth edge
Over 3cm Under 1cm
Older Younger
Yes No
Upper lobes Middle lobe and lower lobes

Differential Diagnosis of SPNs

Management of SPNs

As nearly everyone knows, an early diagnosis is associated with better survival odds for most cancers. Prompt determination of the cause of an SPN is essential and treatment options develop from a knowledge of the origin of the lesion. A small well-defined, partially calcified nodule in an otherwise healthy 24 year-old who has never smoked can usually be safely watched at intervals of 3-6 months. Conversely, a large, poorly defined, uncalcified nodule in a 40 pack-year smoker requires decisive action. Usually a work-up will include noninvasive procedures like CT initially. If the index of suspicion goes up with those results, a biopsy (often an excisional biopsy) is undertaken. Cancer treatments depend of the stage at which it is found, i.e. a cure is not realistic when cancer has spread to distant parts of the body, so life extension with care to consider quality of life issues is often the path taken.

If you find a solitary pulmonary nodule, the first thing to do is to determine whether or not films of the same region are available which will help you determine the rate of growth.

If, like the case above, serial films show no change over time, it is safe to declare the nodule benign. There may be other compelling clinical reasons to go forward with further tests, however. When plain films leave important questions unanswered, a CT scan can characterize the lesion in terms of Hounsfield Units.

Note small nodule seen with tomography

Same case with CT scan

Note cursors placed for accurate measurement


At this point it is a good idea to reflect on whether the chiropractor can efficiently manage an SPN with the current state of affairs in health care. As chiropractors, we are not always well connected to the larger health care community, and taking responsibility for collecting all the data and assembling it to make critical treatment decisions is probably not realistic for most of us.

Also, SPNs are very common litigation cases in medicine for lesions missed, misinterpreted or mismanaged. Nearly half of the cases are settled in favor of the litigant.


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