Use of Fleischner Society criteria for assessment of pulmonary nodules on computed tomography: practice guidelines

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INTRODUCTION

These methodological recommendations present the terminology of the Fleischner Society, used for the standardized interpretation of pulmonary nodules based on computed tomography (CT) data.

The latest version of the Fleischner Society glossary was presented in 2024 and is intended for use in clinical practice, educational activities, and scientific research by the following specialist categories:

• practicing radiologists;
• physicians working with chest diseases;
• resident physicians;
• researchers.

These recommendations are a guideline for the radiology departments of the Moscow City Health Department.

REFERENCES

When using these guidelines, please check the validity of the documents listed below. If a referenced document is replaced or amended, follow the new (amended) document. If a referenced document is canceled without replacement, the regulation containing the reference applies only to the unaffected parts.

This document uses references to the following regulatory documents (standards).

• Federal Law No. 323-FZ of November 21, 2011, On the Fundamentals of Health Protection of Citizens in the Russian Federation¹.
• Moscow City Health Department order dated January 14, 2022, No. 16 On the management of medical care provision in the field of oncology in medical organizations of the state healthcare system of the city of Moscow².
• Order of the Ministry of Health of the Russian Federation dated June 09, 2020, No. 560n On the Approval of the Rules for conducting radiological examination³.

NOTATIONS AND ABBREVIATIONS

The following notations and abbreviations are used in this document:

• SBHCI RPCC D&TT MHCD — Research and Practical Clinical Center for Diagnostics and Telemedicine Technologies of the Moscow Health Care Department;
• UMIAS — Unified Medical Information and Analytical System of the city of Moscow;
• CT — computed tomography;
• PET-CT — positron emission tomography combined with computed tomography;
• RSRR — Russian Society of Roentgenologists and Radiologists.

TERMS AND DEFINITIONS

The following terms with their corresponding definitions are used in this document.

• Nodule — a lung nodule is a local opacity of lung tissue up to 1 cm in size, surrounded on all sides by lung tissue and/or visceral pleura. The term nodule is applied to solitary or few (no more than 6 in each lung) opacities of lung tissue. Multiple nodules, same as solitary ones, are less than 10 mm in size and are usually defined as pulmonary (nodular) disseminations and represent a different radiographic syndrome.
• Ground Glass Nodule — a lung tissue opacity having ground-glass density up to 1 cm in size, solitary or few. A distinguishing feature is the visibility of bronchial walls and vessels within the opacity zone.
• Solid nodule — a lung tissue opacity having consolidation density up to 1 cm in size. A distinguishing feature is the lack of visibility of bronchial walls and vessels within the opacity zone. Ground glass nodule areas are absent.
• Subsolid nodule — a lung tissue opacity having consolidation density but surrounded by a ground glass nodule area, with a total size of up to 1 cm. Air lumens of the bronchi may be visible within the consolidation zone.

HISTORICAL BACKGROUND ON TERMS USED IN THORACIC RADIOLOGY

In 1984, the Fleischner Society first published a glossary of terms for describing chest pathology identified using visualization methods [1]. English-language versions of the glossary were revised and published in 1996, 2008, and 2024. In 2022, the glossary of terms was included in the methodological recommendations Terminology for describing chest organs — radiography and computed tomography published by the SBHCI RPCC D&TT MHCD and in 2023, the glossary was presented in official documents in the Russian Federation, including RSRR publications, and published in professional medical journals [2, 3].

The glossary of terms used in thoracic radiology was first published in 1984. Attempt was made by Fleischner Society, led with W. J. Tuddenham [1].

The glossary provided definitions for terms from both morphological and radiological perspectives [1].

The term nodule:

1. morphologically: a small, nearly rounded area of pathologically altered tissues;
2. radiologically: a lung or pleural lesion in the form of rounded areas of altered density, with clear margins, from 2 to 30 mm in diameter.

The radiological description should include the location, size, margins, density, and number of nodules [1].

The term mass (synonyms: neoplasm, tumor):

1. morphologically: a collection of any tissues that differ from the surrounding ones;
2. radiologically: a lesion of the lungs or pleura in the form of rounded areas more than 30 mm in diameter.

The radiological description should include the localization, size, margins, density, and number of nodules. No English synonyms are provided in the glossary [1].

In 2008, the Fleischner Society, led with D. M. Hansell, released an update to the glossary of terms, in which definitions are given taking into account both the radiological and CT findings [4].

The term nodule is defined as follows:

1. radiological findings: a rounded opacity up to 3 cm in diameter;
2. CT findings: a rounded or irregular area of opacification up to 3 cm in diameter [4].

According to CT data, the following are distinguished:

• micronodules — less than 3 mm in size;
• acinar nodule — round or oval in shape, up to 5–8 mm; they represent an acinus that has undergone consolidation;
• centrilobular nodules located several millimeters from the pleura and interlobular septa;
• ground glass nodules — visualized as areas of opacification within which the pulmonary pattern is discernible;
• solid nodules — have a homogeneous soft-tissue density;
• part-solid nodules (subsolid nodules) — consist of ground glass and soft-tissue density components [4].

The term mass (synonyms: neoplasm, tumor) both radiologically and according to CT data, is defined as a lesion of the lungs, pleura, or mediastinum more than 3 cm in diameter, of a solid or subsolid type [4].

It should be noted that the international definition of a nodule is different from the Russian understanding of pulmonary nodules [5].

Thus, in the works of L. D. Lindenbraten et al. [6], one can encounter the terms nodule and mass without a description of clear characteristics and sizes of these changes. For example, in the book by L. S. Rozenshtraukh et al. [7] X-ray Diagnosis of Respiratory Diseases. A Guide for Physicians (1987), benign nodules up to 2 cm are described as small benign masses; furthermore, it is stated that the sizes of benign tumors can vary from 1–2 cm to masses occupying large spaces.

However, in the Encyclopedic dictionary for diagnostic radiology (2016), edited by L. S. Kokov et al. [8], there are translations and definitions of these terms that correspond to the definitions of international colleagues:

• Nodule — focal thickening;
• Mass — focal shadow in the lung parenchyma more than 3 cm in diameter.

It is necessary to take into account the fact that currently the maximum size of a pulmonary nodule is 3 cm. This value corresponds to the boundary between stages T1 and T2 of adenocarcinoma of the lung (peripheral lung cancer) [9].

The analysis of the number of nodules should begin with such a concept as Solitary Pulmonary Nodule. This term appeared in publications in 1956 and has since gained wide application in both international and Russian works [10]. The word "Solitary" translated from English means "single", therefore, "Solitary Pulmonary Nodule" translated into Russian means "single nodule in the lung" [11].

In the textbook by L. D. Lindenbraten et al. [12] Medical Radiology (2000), the radiological characteristics of peripheral lung cancer, which is presented as a single nodule in the lung, are described as following:

1. small dimensions (exact dimensions not specified);
2. low shadow intensity;
3. rounded shape;
4. soft (fuzzy) outlines.

The Encyclopedic dictionary for diagnostic radiology provides the following definition of the term Solitary Pulmonary Nodule: a single focal shadow, a rounded solitary shadow with well-defined margins, less than or equal to 3 cm in diameter, surrounded by normal lung tissue, not associated with atelectasis, lymphadenopathy, or pneumonia [8].

According to H. MacMahon et al. [13] (2005), as well as according to the data of I. E. Tyurin [5] (2008), a Solitary Pulmonary Nodule represents a distinct radiological syndrome and is characterized by the presence of a localized area of lung tissue densification, round or nearly round in shape, up to 3 cm in diameter.

Many works are dedicated to the solitary pulmonary nodule, based on which the Fleischner Society issued Guidelines in 2005 dedicated to the dynamic observation and diagnosis of small solitary pulmonary nodules [13]. Thus, according to the guidelines (H. MacMahon et al. [13], 2005), pulmonary nodules are very frequently detected during chest CT, and the ability to detect very small nodules improves with each new generation of CT scanners.

However, these guidelines for the observation and management of non-calcified nodules detected during non-screening CT were developed before the widespread use of multi-layer spiral CT and still indicate that for every indeterminate nodule, serial CT should be performed for at least 2 years.

Thus, when describing lung CT scan data in protocols, it is advisable to use the term nodule to describe lung lesions smaller than 3 cm, and the term mass (neoplasm, tumor) for lesions larger than 3 cm. The term single or solitary is recommended for use to describe a mass or nodule present in the lung tissue in a quantity of no more than one. A solitary pulmonary nodule is a distinct radiological syndrome, and its analysis is still relevant.

The recommendations were subsequently supplemented multiple times, and in 2017, the Fleischner Society, led by H. MacMahon, published their final version titled Guidelines for Management of Incidental Pulmonary Nodules Detected on CT Images: From the Fleischner Society 2017; they replace the recommendations for solid (2005) and subsolid (2013) pulmonary nodules [14].

These recommendations were intended to reduce the number of unnecessary follow-up examinations and to provide clear management strategies for detected nodules.

In 2023, the RSRR expert group prepared a dictionary of terms (glossary) in the field of visualization of respiratory diseases, which is based on the Fleischner Society recommendations [2].

These recommendations represent an adapted version of the Fleischner Society consensus regarding pulmonary nodules on CT. They are intended to reduce the number of unnecessary follow-up examinations and to provide clear management strategies for incidentally detected pulmonary nodules in patients outside lung cancer screening programs (Supplement A; B).

FLEISCHNER SOCIETY RECOMMENDATIONS WITH 2017 AND 2024 UPDATES

The Fleischner Society recommendations for pulmonary nodules relate to the follow-up observations and management of indeterminate pulmonary nodules incidentally detected on CT.

The recommendations do not apply to lung cancer screening, patients under 35 years of age, or patients with a history of primary cancer or immunosuppression.

Types of Pulmonary Nodules

According to the secondary pulmonary lobule on chest CT, three types of pulmonary nodules are identified, divided into two groups — solid and subsolid (Fig. 1).

 

Fig. 1. Types of lung nodules.

 

A nodule is called solid when it is impossible to distinguish the elements of the secondary pulmonary lobule. Subsolid nodules include ground glass nodules and, subsolid nodules themselves. Within a ground glass nodule, the secondary pulmonary lobule can be traced. A subsolid nodule is characterized by a combination of a solid component and a ground glass nodule area.

Etiology of Pulmonary Nodules

An incidentally detected pulmonary nodule can represent a wide spectrum of diseases, which can mostly be divided by etiology into inflammatory and neoplastic processes (Table 1).

 

Table 1. Possible causes of lung nodules

Figure	Nodule type and etiology
	Solid nodules Infections, benign granulomas, neoplasms, developmental disorders, areas of fibrosis, focal scar, intrapulmonary lymph nodes, primary cancers, metastases
	Subsolid nodules* Cancer (usually adenocarcinoma), infections, areas of fibrosis (rarely). Most nodules are transient and result from infection or hemorrhage. However, persistent nodules often represent pathology of the adenocarcinomatous spectrum
	Subsolid nodules of the adenocarcinomatous spectrum were previously known as bronchioloalveolar carcinoma, or bronchioloalveolar cancer. This terminology is outdated. In 2011, a new classification of adenocarcinoma based on histopathology was introduced, and this current classification distinguishes the following categories: adenocarcinoma in situ; minimally invasive adenocarcinoma; invasive adenocarcinoma
	Ground glass nodules** Atypical adenomatous hyperplasia, adenocarcinoma without signs of invasive growth (in situ), pulmonary eosinophilias, acute infections (rarely), area of fibrosis

Note. * It is impossible to make a radiologically reliable distinction, although studies show that larger size and a solid component are associated with a higher risk of invasive growth. Compared to solid nodules, persistent subsolid nodules have a slower growth rate but carry a much higher risk of malignant development. In a study conducted by C. I. Henschke et al. [15], subsolid nodules were malignant in 63% of cases, ground glass nodules in 18%, and solid nodules in only 7%. ** Usually represent infection or alveolar hemorrhage. To distinguish between transient or persistent ground glass nodules, a follow-up CT must be performed. Previously, it was recommended to repeat the CT in 3 months; however, this interval has been increased to 12 months. Due to slower growth rates, the total follow-up period for persistent subsolid nodules has been increased to 5 years.

 

Classification of Pulmonary Nodules

Management of a pulmonary nodule, according to the Fleischner Society recommendations, directly depends on its size and type. Tables 2 and 3 present the management of solid and subsolid pulmonary nodules, respectively.

 

Table 2. Management of solid pulmonary nodules

Nodule type	Size	Management strategy
Solitary solid nodule	< 6 mm (< 100 мм3)	low-risk patients: routine follow-up is not required; high-risk patients: CT in 12 months may be scheduled at the physician's discretion
	6–8 mm (100–250 mm3)	low-risk patients: CT in 6–12 months, then CT in 18–24 months may be scheduled at the physician's discretion; high-risk patients: CT in 6–12 months, then CT in 18–24 months
	> 8 mm (>250 mm3)	low- and high-risk patients: CT in 3 months, PET-CT or biopsy
Multiple solid nodules In the presence of multiple nodules, the final assessment focuses on the most suspicious one	< 6 mm (<100 mm3)	low-risk patients: routine follow-up observation is not required; high-risk patients: CT in 12 months may be scheduled at the physician's discretion
	>6 mm (> 100 mm3)	low-risk patients: CT in 3–6 months, then CT in 18–24 months may be scheduled at the physician's discretion; high-risk patients: CT in 3–6 months, then CT in 18–24 months

 

Table 3. Management of subsolid pulmonary nodules

Nodule type	Size	Management strategy
Ground glass nodule	< 6 mm (<100 mm3)	routine follow-up observation is not required
	≥ 6 mm (> 100 mm3)	CT in 6–12 months, then, if the lesion persists, CT every 2 years for up to 5 years
Subsolid nodule	< 6 mm (< 100 mm3)	routine follow-up observation is not required
	≥ 6 mm (> 100 mm3)	CT in 3–6 months, then, if a solid component < 6 mm persists, annual CT once a year for up to 5 years
Multiple subsolid nodules	< 6 mm (<100 mm3)	high-risk patients: CT in 3–6 months, then, if no changes are observed, CT in 2 and 4 years
	≥ 6 mm (> 100 mm3)	CT in 3–6 months, followed by patient management depending on the most suspicious nodule

 

Technical Parameters for the Visualization of Pulmonary Nodules

Visualization of a pulmonary nodule, according to the Fleischner Society recommendations, directly depends on compliance with technical parameters during CT performance. Table 4 lists the technical visualization parameters necessary for the correct assessment of pulmonary nodules based on CT data.

 

Table 4. Technical parameters of pulmonary nodule visualization on CT

Stages	Technical parameters
Obtaining and viewing images	CT should be performed on full inspiration; it is recommended to measure nodules in the axial (transverse) plane, although the coronal or sagittal plane may be used if the largest dimensions lie in these planes; nodules should be measured in the lung window setting, although the use of a soft-tissue window may help assess changes in nodule density during follow-up observations; assessment of small nodules (< 10 mm) should be performed on tomograms with a slice thickness of ≤ 1.5 mm and a lung reconstruction filter (Sharp) to avoid the partial volume effect and to identify areas of fat or calcification; automatic or semi-automatic 3D volumetry should be performed using the same software for both initial and follow-up measurements. In manual 2D measurements with a caliper, the nodule size is assessed along the long and short axes obtained on the same image in any plane, with the calculation of the average diameter and rounding to the nearest whole number. Alternatively, a volumetric method (3D volumetry) is used, while the scanning protocol must remain unchanged throughout the entire patient observation period
Description and Assessment of Nodules	measurements should be rounded off with a precision to the nearest whole number; in the presence of multiple lung nodules, only the largest or morphologically most suspicious ones should be measured, and their exact location should be specified; "dominant" — the most suspicious nodule; refers morphologically to a nodule that is not necessarily the largest in size
Exclusions from Recommendations	patients aged 35 years and younger. In general, the risk of developing lung cancer in such patients is considered lower than in other age groups; patients with known cancer; an incidentally detected lung nodule in such patients is more likely to be associated with cancer; immunocompromised patients. Increased risk of opportunistic lung infections; population-based lung cancer screening. These patients undergo active screening due to a high risk of developing lung cancer, and CT scan results should be interpreted in accordance with Lung-RADS criteria

 

Risk Factors⁴

Determining a high or low risk group is currently more difficult than it was in previous recommendations [16, 17].

Previously, a high-risk subject was identified based on the following factors:

• a history of long-term continuous smoking;
• a history of lung cancer in a relative;
• exposure to asbestos, radon, or uranium.

Current recommendations aim to separate high-risk nodules from low-risk nodules, considering more factors than just patient data.

Since risk factors are numerous and influence the risk of developing cancer in different ways, it is proposed to distinguish the following categories (Fig. 2).

 

Fig. 2. High-risk factors for lung nodules.

 

Since some categories are subdivided into low- and high-risk groups (e.g., for solitary solid pulmonary nodules < 6 mm), clinicians are asked to assess the risk of malignancy using factors other than the nodule descriptions included in the recommendations (size, multiplicity, and localization).

Risk factors to be considered include older age, heavy smoking, irregular or spiculed margins, and the location of the nodule in the upper lobe (Fig. 3).

 

Fig. 3. General risk group assessment for lung nodules.

 

According to The American College of Chest Physicians, risk assessment can be performed qualitatively and/or quantitatively using a validated prediction model.

When multiple nodules are detected that persist on follow-up scans, further management strategy is determined by the most "suspicious" nodule.

Suspicious CT features to consider include:

• an increase in the solid component of the nodule ≥ 6 mm shown during a follow-up observation;
• radiated margins;
• an increase in the density of the lung nodule or a new microcystic component.

Persistent subsolid nodules with a solid component ≥ 6 mm should also be considered highly suspicious. The presence of these features may prompt further examination (PET-CT, biopsy) rather than continued observation in accordance with these recommendations.

CONCLUSION

Currently, in the territory of the Russian Federation, chest CT is a typical tomographic study, and one of the most frequent findings is a lung nodule or lung node. The approach to interpreting a lung nodule depending on the clinical situation can be classified as a labor-intensive task for a radiologist.

Excluding lung cancer screening, the unified Fleischner Society terminology and standardized interpretation of chest CT findings enhance communication and understanding among healthcare professionals in clinical practice, education, and research.

ADDITIONAL INFORMATION

Author contributions: Yu.A. Vasilev: conceptualization; N.V. Tarasova: writing — original draft; D.M. Anikina: writing — original draft, writing — review & editing; I.A. Blokhin: writing — review & editing. All the authors approved the version of the manuscript to be published and agreed to be accountable for all aspects of the work, ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Acknowledgments: The authors express their sincere gratitude to the reviewers of the practice guidelines: Professor N.V. Nudnov, Dr. Sci. (Medicine), Deputy Director for Research, Head of the Research Department of Comprehensive Disease Diagnostics and Radiotherapy at the Russian Scientific Center of Roentgenoradiology of the Ministry of Health of the Russian Federation, and Professor D.V. Burenchev, Dr. Sci. (Medicine), Head of the Department of Diagnostic Radiology at A.K. Eramishantsev City Clinical Hospital of the Moscow Healthcare Department.

Ethics approval: Not applicable.

Funding sources: These practice guidelines were developed as part of the research project "Scientific Substantiation of Imaging Methods for Oncological Diseases Using Radiomic Analysis" (ESISU No. 123031500005-2), in accordance with Order No. 1184 of the Moscow Department of Healthcare dated December 17, 2024, "On Approval of State Assignments Financed from the Moscow City Budget for State Budgetary (Autonomous) Institutions Subordinated to the Moscow Department of Healthcare for 2025 and the Planned Period of 2026–2027".

Disclosure of interests: The authors have no relationships, activities, or interests for the last three years related to for-profit or not-for-profit third parties whose interests may be affected by the content of the article.

Statement of originality: Ths work incorporates excerpts from the previously published material (doi: 10.1148/radiol.2017161659), which is distributed under a restricted license © RSNA, 2017, and is reproduced with permission of the copyright holder. This article is a revised version of the previously published practice guidelines (EDN: AGKGZM) and is published with the permission of the copyright holder.

Data availability statement: All data obtained in this study are available in the article and its supplementary material.

Generative AI: No generative artificial intelligence technologies were used to prepare this article.

Provenance and peer-review: This article was commissioned by the Editorial Board and underwent prioritized internal peer-review.

 

SUPPLEMENTS

Supplement A. Physician's work algorithm with Fleischner Society recommendations

 

Supplement B. Example of examination description according to these recommendations

Structure of the examination description protocol	Example
Type of radiological examination	CT of chest organs
Clinical information	Male, 65 years old. No history
Primary/secondary examination	Primary
Examination quality	The examination was performed without technical defects
Description	In S9 of the left lung, a single solid ovoid nodule 9 mm in diameter with spiculed margins is identified peripherally and peribronchovascularly. In both lungs, moderate subpleural thickening of the peripheral interstitium is noted; small linear areas of ground-glass opacification of the lung tissue are also present — they are likely post-inflammatory changes. The trachea is not displaced and is patent. The main, lobar, and segmental bronchi are visible throughout their length; their lumens are of normal width. The mediastinum is not displaced. The heart chambers are not dilated. The ascending aorta is up to 39 mm. The pulmonary trunk is dilated up to 33 mm. Fragmentary calcification of the walls of the aorta and coronary arteries is noted. Intrathoracic lymph nodes are not enlarged. Axillary lymph nodes are not enlarged. The serous membranes are thin and contain no effusion. Consolidated fractures of the posterior segments of the X, XI, and XII ribs on the right. In the ThX vertebral body, an area of bone structure rarefaction with trabecular striation measuring 23 × 27 × 18 mm is identified, likely a hemangioma. The endplates of the ThXI–XII vertebral bodies are sclerosed — degenerative-dystrophic changes of the spine. At the margin of the study: multiple concretions up to 8 mm with a density of up to 800 HU are identified in the gallbladder lumen
Conclusion	According to the Fleischner Society eligibility criteria, CT appearance of a solitary solid nodule in S9 of the left lung, high risk. Post-inflammatory changes in the lungs Signs of pulmonary hypertension. Hemangioma of the ThX vertebral body. Pronounced aortocoronary sclerosis. Concretions in the gallbladder lumen
Recommended	Follow-up observation, CT control in 3 months

 

¹ Federal Law of the Russian Federation No. 323-FZ dated November 21, 2011, "On the Basics of Health Protection of Citizens in the Russian Federation" (as amended and supplemented). Available at: https://base.garant.ru/12191967/. Accessed on November 24, 2025.

² Order of the Moscow City Health Department No. 16 dated January 14, 2022. "On the management of medical care provision in the field of oncology in medical organizations of the state healthcare system of the city of Moscow" (with amendments and additions). Available at: https://base.garant.ru/403620046/. Accessed on November 24, 2025.

³ Order of the Ministry of Health of the Russian Federation No. 560n dated June 09, 2020. "On the Approval of the Rules for conducting radiological examination" (with amendments and additions). Available at: https://base.garant.ru/74632238/. Accessed on November 24, 2025.

⁴ Risk factors are assessed in each case of chest CT examination when the patient's medical documentation is available to the radiologist through UMIAS. In the absence of sufficient medical documentation, it is recommended to assign the patient to a high-risk group.

About the authors

Yuriy A. Vasilev

Research and Practical Clinical Center for Diagnostics and Telemedicine Technologies

Email: VasilevYA1@zdrav.mos.ru
ORCID iD: 0000-0002-5283-5961
SPIN-code: 4458-5608

MD, Dr. Sci. (Medicine)

Russian Federation, Moscow

Natalia V. Tarasova

Research and Practical Clinical Center for Diagnostics and Telemedicine Technologies

Author for correspondence.
Email: TarasovaNV20@zdrav.mos.ru
ORCID iD: 0000-0003-2769-8675
SPIN-code: 4196-4059

MD, Cand. Sci. (Medicine)

Russian Federation, Moscow

Darya M. Anikina

Research and Practical Clinical Center for Diagnostics and Telemedicine Technologies

Email: AnikinaDM@zdrav.mos.ru
ORCID iD: 0000-0001-6554-4779
SPIN-code: 1005-7000
Russian Federation, Moscow

Ivan A. Blokhin

Research and Practical Clinical Center for Diagnostics and Telemedicine Technologies

Email: BlokhinIA@zdrav.mos.ru
ORCID iD: 0000-0002-2681-9378
SPIN-code: 3306-1387

MD, Cand. Sci. (Medicine)

Russian Federation, Moscow

References

Supplementary files