A fissural nodule, more precisely called a perifissural nodule (PFN), is a small solid spot that shows up on a chest CT scan right along or near one of the thin membranes (fissures) that separate the lobes of the lung. In the vast majority of cases, these nodules are benign intrapulmonary lymph nodes, not cancer. Their distinctive appearance and location have made them one of the most reliably harmless findings in lung imaging, and current screening guidelines treat them accordingly.
What a Perifissural Nodule Looks Like on a CT Scan
Radiologists identify PFNs by a specific combination of features. The classic PFN sits within about 15 millimeters of a fissure or the pleural surface, has an oval, lentiform, or triangular shape, appears solidly homogeneous, and has smooth, well-defined borders.1PubMed Central. Incidental perifissural nodules on routine chest computed tomography: lung cancer or not? That lentiform shape, somewhat like a lens or a flattened disk, is a hallmark. It reflects how the nodule sits sandwiched against the fissure rather than growing outward in all directions the way a tumor tends to.2PubMed. Pulmonary perifissural nodules on CT scans: rapid growth is not a predictor of malignancy Think of dough pressed against a flat surface: it flattens on one side and rounds on the other. A cancerous nodule, by contrast, usually looks more spherical and often has irregular or spiculated edges.
Most PFNs are quite small. In one large lung cancer screening study, the average PFN measured about 4.4 millimeters across, with a range from roughly 3 to about 11 millimeters.3Radiology. Pulmonary perifissural nodules on CT scans: rapid growth is not a predictor of malignancy At that size, many people would never know they have one unless they happened to get a CT scan for some other reason.
Why They Show Up So Often
PFNs are far from rare. In lung cancer screening programs, they account for a significant fraction of all the nodules detected. In the NELSON trial, one of the largest lung cancer screening studies ever conducted, about one in five baseline nodules were classified as perifissural.4European Respiratory Journal. Avoiding overdiagnosis in lung cancer screening: the volume doubling time strategy A separate review of screening CT images found that roughly 28 percent of non-calcified nodules in a cohort of participants were adjacent to a fissure and classified as PFNs, with some individuals harboring more than a dozen of them.5PubMed. Perifissural nodules seen at CT screening for lung cancer
The reason they appear so frequently is anatomical. The lung’s lymphatic drainage system runs heavily through the interlobular septa and subpleural space, the same tissue that forms the fissures between lobes.6PubMed Central. Morphometric analysis of intralobular, interlobular and pleural lymphatics in normal human lung Lymph nodes naturally cluster there, and CT scanners have become sensitive enough to pick them up even when they are only a few millimeters across. Decades ago, before high-resolution CT was standard, these tiny lymph nodes went unnoticed. Now they are one of the most common incidental findings on chest imaging.
What They Actually Are Under a Microscope
When PFNs have been biopsied or removed surgically and examined by pathologists, they consistently turn out to be intrapulmonary lymph nodes. These are normal components of the lung’s immune and drainage system, positioned along the routes where lymph fluid flows from the lung tissue toward the larger lymph nodes in the chest. In a pathological study of resected intrapulmonary lymph nodes, all of them showed heavy anthracotic pigment, a dark carbon-based deposit from inhaled particles, and about 60 percent also had silicotic changes.7PubMed. Intrapulmonary lymph nodes. Clinical, radiologic, and pathologic features That anthracosis is not a disease in itself; it is just a sign that the lymph node has been doing its job filtering out particulates from the air over a lifetime. It does, however, explain why PFNs sometimes appear denser than surrounding tissue on a scan.
The Cancer Question
This is the question most people really want answered: can a fissural nodule be cancer? The accumulated evidence across multiple large studies is remarkably consistent. In the NELSON screening trial, none of the 794 nodules classified as PFNs at baseline turned out to be malignant during years of follow-up.8Radiology. Pulmonary perifissural nodules on CT scans: rapid growth is not a predictor of malignancy That zero-malignancy rate held true even though a meaningful fraction of PFNs grew over time. About 15 percent showed some increase in size between the baseline and the first follow-up scan, and roughly 8 percent had volume doubling times fast enough that, for a non-PFN nodule, they would have triggered concern for possible cancer. Yet none of them were malignant.
Long-term follow-up data from other screening cohorts have reinforced this. Researchers involved in the NELSON study have described PFNs as nodules that “can be considered benign, as proven by long-term follow-up, even in the case of rapid growth in high-risk patients.”9European Respiratory Journal. Avoiding overdiagnosis in lung cancer screening: the volume doubling time strategy This is an unusual level of confidence in radiology, where hedging is the norm. The shape and location of a PFN are reliable enough indicators of benignity that growth alone is not a reason to escalate to biopsy.
When a Nodule Near a Fissure Is Not a True PFN
The reassuring data above apply specifically to nodules that meet the full set of PFN criteria. Not every nodule found near a fissure qualifies. Researchers in the NELSON study re-examined 97 fissure-attached nodules and classified them into three groups: typical PFNs (43 percent), atypical PFNs (17 percent), and non-PFNs that simply happened to touch a fissure (40 percent). Among the non-PFN group, 10 percent were malignant. Among the typical and atypical PFNs, zero were malignant.10Journal of Thoracic Oncology. New Fissure-Attached Nodules in Lung Cancer Screening: A Brief Report From The NELSON Study
The distinction matters. An atypical PFN meets most of the criteria but might not be directly attached to a visible fissure, or it may be convex on one side and round on the other rather than cleanly lentiform. These still carried a zero malignancy rate in the NELSON data. A non-PFN that touches a fissure, on the other hand, is a round or irregularly shaped nodule that simply happens to be in a fissural location. Its shape has not been influenced by the fissure. That kind of nodule gets treated like any other lung nodule in terms of follow-up, because location alone does not make it safe.
The malignant non-PFNs in this study were also significantly larger than PFNs and benign non-PFNs.11Journal of Thoracic Oncology. New Fissure-Attached Nodules in Lung Cancer Screening: A Brief Report From The NELSON Study Size is always worth paying attention to with lung nodules, but for a true PFN with classic morphology, size has not been shown to elevate cancer risk.
PFNs in People Who Already Have Cancer
One scenario that makes both patients and doctors more cautious is when PFNs are found in someone who already has a known malignancy. Could a fissural nodule in an oncology patient represent a metastasis rather than an innocent lymph node? A study that followed 62 cancer patients with PFNs for a median of about five to six years found that roughly 95 percent of PFNs remained stable or decreased in size. Only about 5 percent enlarged, and none of the clinical features or CT characteristics of those nodules predicted which ones would grow.12PubMed Central. Clinical Significance of Perifissural Nodules in the Oncologic Population
That study’s sample was relatively small, and an enlarging PFN in an oncology patient probably still warrants close attention. But the evidence so far suggests that the morphological criteria that mark a nodule as a PFN hold up even in people with active cancer. The shape tells you more than the patient’s medical history does.
How Screening Guidelines Handle PFNs
The strong benignity data have pushed guidelines toward treating PFNs differently from other solid lung nodules. The Lung-RADS system, used by radiologists to categorize findings on low-dose CT lung cancer screening, now classifies typical PFNs as category 2, meaning benign. That classification means no short-term follow-up CT is recommended; instead, the patient simply returns for their routine annual screening scan.13Radiology. How I Do It: Management of Pleural-attached Pulmonary Nodules in Low-Dose CT Screening for Lung Cancer Earlier versions of the guideline were less explicit, and many PFNs ended up triggering unnecessary short-interval follow-up scans. The updated approach saves patients from repeated imaging visits, radiation exposure, and the anxiety that comes with each round of waiting for results.
For patients who leave a screening program, for example because they quit smoking long enough ago to no longer qualify, there are no consensus guidelines on what to do with known PFNs. Many practices fall back on the Fleischner Society’s general nodule management recommendations for high-risk patients in those situations.14Journal of the American College of Radiology. ACR Lung-RADS v2022: Assessment Categories and Management Recommendations In practice, though, a nodule that has already been identified as a classic PFN and remained stable over at least one screening interval is very unlikely to need further attention.
Artificial Intelligence and Automated PFN Detection
Recognizing PFNs requires a trained eye, and radiologists reading hundreds of screening CTs inevitably face judgment calls on borderline cases. Researchers have developed deep-learning algorithms to automate PFN classification, and the results so far are promising. One deep-learning classifier tested on screening CT data achieved an area under the curve of about 96 percent, with sensitivity near 96 percent and specificity around 88 percent on its test set. The agreement between the algorithm and human expert readers was comparable to the agreement between pairs of expert readers themselves.15PubMed Central. Evaluation of a novel deep learning-based classifier for perifissural nodules
Earlier machine-learning approaches using convolutional neural networks applied to 2D nodule views also showed performance approaching that of human experts, though with somewhat lower accuracy than the more recent deep-learning models.16PubMed. Automatic classification of pulmonary peri-fissural nodules in computed tomography using an ensemble of 2D views and a convolutional neural network out-of-the-box The practical appeal is clear: if software can reliably flag a nodule as a PFN, it could be routed straight to the “benign, no follow-up needed” category without consuming radiologist time for a detailed shape assessment. In large-scale screening programs where thousands of nodules are detected per year, that efficiency gain adds up quickly.
AI classification is not yet standard in routine practice, but integration into radiology workflows is an active area of development. The biggest challenge is handling the borderline cases, the atypical PFNs and the round nodules that happen to sit near a fissure. Getting those right is where human judgment still matters most.
Why Growth Does Not Mean What You Think It Means
If you have been told a lung nodule has grown, the natural reaction is alarm. Growth is one of the hallmarks of malignancy in most clinical contexts, and for non-PFN lung nodules, a short volume doubling time is indeed a red flag. But PFNs break this rule. As noted in the NELSON screening data, roughly one in six PFNs grew between scans, and a subset doubled in volume fast enough to mimic cancer growth rates, yet none of them turned out to be malignant.17Radiology. Pulmonary perifissural nodules on CT scans: rapid growth is not a predictor of malignancy
Why would a benign lymph node grow? Lymph nodes are reactive tissue. They swell in response to infection, inflammation, or increased particulate burden. A respiratory infection, a bout of bronchitis, or even seasonal allergen exposure could plausibly cause a small intrapulmonary lymph node to increase in volume transiently. The key insight from the screening studies is that rapid growth in a nodule that otherwise meets all the PFN morphological criteria should not, by itself, trigger a biopsy or a PET scan. That represents a meaningful departure from how lung nodules are managed in general, and it is one of the reasons the PFN classification exists in the first place: to prevent unnecessary invasive procedures on nodes that are doing normal immunological work.
The Anxiety Factor
Finding any lung nodule on a CT scan tends to generate worry, and for good reason: the word “nodule” sounds ominous, and lung cancer is a leading cause of cancer death. Research on patient responses to nodule detection has shown that being told you have a lung nodule can measurably increase anxiety and reduce quality of life, even when the nodule is almost certainly benign. Physicians have been advised to inform patients before a CT scan that pulmonary nodules are found frequently and that most turn out to be benign, because setting that expectation in advance reduces distress.18Journal of Pulmonary Medicine & Respiratory Research. Effects of Pulmonary Nodules on Patient Anxiety and Health-Related Quality of Life
For PFNs specifically, the news is especially reassuring, and clinicians who can clearly explain what a perifissural nodule is and why it is almost certainly a lymph node do their patients a real service. The trouble is that radiology reports are written for other doctors, not for patients. A report that says “4 mm perifissural nodule, Lung-RADS 2, recommend continued annual screening” is perfectly clear to another physician. To a patient reading their results on a portal, it may look like yet another finding that needs worrying about. If your report mentions a perifissural nodule classified as benign, that classification carries a strong evidence base behind it.
Intrapulmonary Lymph Nodes and Inhaled Particles
The anthracotic pigment found in nearly all surgically removed intrapulmonary lymph nodes points to a straightforward explanation for why these structures exist where they do.19PubMed. Intrapulmonary lymph nodes. Clinical, radiologic, and pathologic features The lung is constantly filtering inhaled particulate matter: carbon from combustion, dust, pollen, fine industrial particles. Lymph fluid carries these particles from the alveoli toward the lymphatic channels that run along the interlobular septa and pleural surfaces. The small lymph nodes sitting along those channels act as collection points. Over decades of breathing, the nodes accumulate pigment and sometimes develop silicotic nodules from silica exposure.
This process is not a disease. It happens in virtually everyone who breathes air in an industrialized environment, and it happens more visibly in people with occupational dust exposure such as miners, construction workers, and foundry workers. The degree of anthracosis may vary, but the nodes themselves are normal anatomy. Their visibility on modern CT scanners is a function of imaging technology catching up to structures that have always been there, not a sign that something has gone wrong.