Fleischner Guidelines for Pulmonary Nodules

by Onno Mets and Robin Smithuis

the Academical Medical Centre, Amsterdam and the Rijnland Hospital, Leiderdorp, the Netherlands


Pulmonary nodules are frequently encountered incidentally on chest CT.

This article summarizes the basics of indeterminate pulmonary nodules with current management recommendations.

Fleischner guidelines

Solid Nodules

In 2005 the Fleischner Society published their literature-based guidelines for the follow-up of solid pulmonary nodules [1].
The aim was to provide practical guidelines for the management of small nodules detected on CT examinations performed for purposes other than lung cancer screening.

The guidelines separate low- and high-risk patients and provide management strategies based on lesion size (see Table).

Sub-Solid Nodules

In 2013 the Fleischner Society released their management recommendations for SSNs [2], which aims to provide practical guidelines for the management of SSNs in clinical practice.

The guideline separates solitary and multiple SSNs, both pure non-solid or ground-glass, part-solid with a solid component <5 mm or part-solid with a solid component >5 mm (see Table).

For part-solid nodules > 10 mm FDG-PET should be considered.

In all other SSPNs PET is of limited value and potentially misleading.
It is therefore not recommended in those cases.


A lesion which measures 8 x 5 mm has an average of (8 + 5) : 2 = 6.5 mm.

Lung nodule measurements should be obtained as the average of the diameters in the transverse plane [2].
In a part-solid SSN both the sub-solid component as well as the solid component should be measured this way.
These measurements will downsize oblong lesions.

In SSNs this is probably of minor importance, since one should differentiate lesions smaller or larger than 5 mm.
In solid nodules where follow-up is based on several size categories, this can make a difference.
A solid nodule of 7mm has a different follow-up to a 5.5mm nodule (figure).

Nodule characterization is obtained on thin-slice unenhanced scans only, since a small solid nodule may appear to have groundglass density on a thick slice.

Solid Nodules

Most data on small indeterminate pulmonary nodules originate from lung cancer screening in a high-risk population.

The likelihood of malignancy is different for an incidentally found small solid nodule in a relatively young patient than for a high-risk heavy smoker screened for lung cancer, or a patient with a known malignancy.

For this reason the Fleischner guidelines for the management of solid pulmonary nodules separates high- and low--risk patients, excludes those with malignancies and only apply to patients older than 35 years [1].

Malignant characteristics

There are some features that can help to differentiate between a benign and a malignant lesion.

Unfortunately there is a considerable overlap, and often no definitive answer based on morphology. Follow-up is therefore a commonly used strategy.

Click here to read more on benign versus malignant characteristics in solitary pulmonary nodules.

Recently it was shown that the risk of lung cancer in those with a solid nodule <5mm was no different from the risk in high-risk patients without pulmonary nodules in lung cancer screening participants [3].

This is an interesting area of research, since raising the threshold would dramatically reduce radiation exposure and costs of follow-up.

However, until future management adjustments for the clinical practice are issued, it is advisable to act uniformly according to the current available guidelines.

Perifissural nodules

Perifissural nodules are a separate and benign entity.

Although the current available Fleischner guidelines do not separately denominate the perifissural opacity or nodule (PFO or PFN), it is now well known that these are benign entities. A typical PFN is attached to a pulmonary fissure, is homogeneous, and solid with smooth margins. The shape is oval, lentiform or triangular (Figure ).

A nodule with these specific characteristics needs no follow-up and is probably an intrapulmonary lymph node.

In a study by de Hoop none of the 919 typical and atypical PFNs were found to be malignant in 5.5 year follow-up [4].

Thus, follow-up of a perifissural nodule is indicated only when a non-PFN lesion is found.
Typical or atypical PFNs should be left alone.

Typical PFNs can show significant growth rates on serial imaging comparable to malignant nodules.
This is not a sign of malignancy, but merely a result of their lymphatic origin.

Typical PFNs. Images from Ref [4].

Here are two examples of typical PFNs.

No follow-up is needed.

Non-PFN lesions. Courtesy of M. Prokop

The images show lesions that do not fulfill the criteria for PFNs.

These non-PFN lesions proved to be an HCC metastasis (left) and an adenocarcinoma (right).

Sub-Solid Nodules

Some points to emphasize:

  • Contrary to the solid nodule guidelines, there is no separation in low- or high-risk subjects and there is no age limit.
    There are concerns that adenocarcinoma tends to occur more often in younger and non-smoking individuals.
  • These rules may be applied in subjects with a known or suspected underlying malignancy.
    SSNs are rarely metastatic.
    Multiple lesions are typically synchronous primary cancers.
  • Lung nodule measurements should be obtained as the average of the diameters in the transverse plane, as in solid nodules. Other measurement techniques may lead to overestimation and associated overmanagement.
  • Optimal nodule characterization is obtained on thin-slice unenhanced scans only, since small solid nodules may appear to be of groundglass density on thicker slice scans.
  • Follow-up is preferably performed according to an unenhanced low-dose protocol.
Transient subsolid nodules

Most SSNs are transient and of no relevance.

This is why there is a short initial follow-up of 3 months to see if they disappear.

These nodules are probably of infectious origin.

The images show a 7 mm pure groundglass SSN in the right upper a lobe.
On 3-months follow-up this proved to be a transient SSN.

Persistent malignant subsolid nodule.

Persistent SSNs have a much slower growth rate than solid nodules, but they carry a much higher risk of malignancy.

This is why there is a long follow-up of 3 years.

The images are of a patient with a pure groundglass SSN in the right lower lobe.
This lesion demonstrated growth in a two year interval and proved to be malignant after resection.

In a study by Henschke et al., part-solid nodules were malignant in 63%, pure non-solid nodules in 18% and solid nodules only in 7% [4].

Persistent SSNs often represent an adenocarcinomatous malignancy.

This entity was formerly known as bronchoalveolar carcinoma or BAC - a term which should no longer be used in radiology reports.

A new pathology-based classification was introduced in 2011 and this current classification makes distinction between:

  1. Adenocacinoma in situ.
  2. Minimally invasive adenocarcinoma.
  3. Invasive adenocarcinoma.

No reliable distinction can be made radiologically, although studies suggest that larger size and a solid component are associated with more invasive stages (figure).