What Causes Apical Pleural Parenchymal Scarring?

Apical pleural parenchymal scarring, the fibrous thickening that develops at the very top of the lungs and the tissue lining them, most often results from ordinary aging. In the majority of people, these small scars have no connection to serious disease and are discovered incidentally on a chest X-ray or CT scan. But the same appearance on imaging can also be left behind by past infections like tuberculosis, by occupational dust exposure, by autoimmune diseases, and in rare cases by a progressive form of lung fibrosis that does warrant close follow-up. Understanding which cause is at work matters because the range stretches from completely benign to genuinely concerning.

Why the Lung Apex Is Prone to Scarring

The top of the lung sits in a uniquely stressful mechanical environment. Because the lung hangs inside the chest cavity, gravity pulls it downward. That creates higher negative pressure at the apex compared with the base, meaning the tissue at the top is stretched more tightly against the chest wall at all times. A biomechanical analysis published in Medical Hypotheses found that these pleural stress levels depend on chest wall shape but are highest at the apex in young males with low body mass index, a group whose tall, narrow rib cages amplify the mechanical load on that region.¹ That chronic stretch can promote small bullae (air-filled blisters) and, over decades, encourage scar tissue to form even without infection or inflammation.

The pleural space itself is remarkably thin. Experimental models have estimated the costal pleural space to be roughly 20 microns thick, with a vertical pressure gradient that is less than hydrostatic.² Liquid drainage through this space is slow, which may contribute to why debris, inflammatory byproducts, and small amounts of blood tend to linger at the apex rather than being cleared efficiently. All of this makes the lung tip a natural site for minor scarring to accumulate over a lifetime.

Aging and the “Apical Cap”

The single most common cause of apical pleural parenchymal scarring is simply getting older. Radiologists have a dedicated term for it: the apical cap. This is a crescent-shaped density visible at the top of the lung on a standard chest X-ray or CT scan, typically less than 5 mm thick. A landmark radiology review noted that apical caps, whether on one side or both, are a common feature of advancing age and usually represent subpleural scarring unassociated with other diseases.³ In routine clinical practice, a thin, smooth apical cap in a middle-aged or older adult is overwhelmingly benign.

An educational radiology exhibit from the European Society of Radiology reinforces this, stating that an apical pleural cap under 5 mm is frequently present on normal chest radiographs and most commonly represents nonspecific apical scarring or prominent extra-pleural fat.⁴ The key word there is “nonspecific”: the scar is real, but it does not point to any particular disease. It reflects decades of minor mechanical wear on tissue that sits in a high-stress zone.

Past Infections, Especially Tuberculosis

After aging, the next most recognized cause of apical scarring is a prior lung infection. Tuberculosis is the classic culprit. TB has a strong preference for the upper lobes of the lung, partly because the higher oxygen tension at the apex favors the growth of Mycobacterium tuberculosis. Even after the infection has been treated or contained by the immune system, the healing process leaves behind dense scar tissue and often pleural thickening that can persist for life.

Asymmetric pleural thickening and calcification, combined with apical scarring and cavities or cystic bronchiectasis, should strongly suggest tuberculosis as the underlying cause.⁵ Histoplasmosis, another granulomatous infection caused by a soil-dwelling fungus, can leave an almost identical footprint. Apical pulmonary cavities with associated pleural thickening are characteristic of prior granulomatous infection from either organism.⁶

Apical thickening can also result from other past lung infections or from chronic lung inflammation that did not necessarily involve a named pathogen.⁷ In many parts of the world, TB is the first diagnosis that clinicians consider when they see significant apical scarring, especially if the scarring is thick, unilateral, or accompanied by cavitation. In regions where TB rates are low, prior fungal infections or even severe bacterial pneumonias may be more likely culprits, though the radiographic result looks similar.

Ankylosing Spondylitis and Autoimmune Disease

One of the more surprising causes of apical scarring is ankylosing spondylitis (AS), an inflammatory condition primarily known for affecting the spine and sacroiliac joints. AS is a common cause of pulmonary apical fibrocystic disease: early involvement may be one-sided or uneven, but most cases eventually produce bilateral apical fibrobullous lesions that can be progressive, with coalescence of nodules, formation of cysts and cavities, fibrosis, and bronchiectasis.⁸

The mechanism is twofold. The disease restricts chest wall motion because of spinal fusion, which reduces ventilation especially at the lung apices. At the same time, a direct inflammatory process attacks the lung parenchyma. A case report and literature review described this as apical fibrobullous disease, with or without cavitation, representing one of the severe and unique lung manifestations of AS.⁹ Because AS-related apical scarring can look nearly identical to tuberculosis on imaging, misdiagnosis is a real problem, particularly in areas where TB is endemic.

Other connective tissue diseases can contribute as well. A 2025 study found that connective tissue disease-associated pleuroparenchymal fibroelastosis (a pattern of scarring concentrated in the upper lobes) is linked to faster lung function decline, increased pneumothorax risk, and poorer outcomes, particularly in systemic sclerosis and rheumatoid arthritis.¹⁰ Distinguishing this pattern from other types of apical scarring remains difficult because the diagnosis relies heavily on imaging without strong pathological confirmation in most clinical settings.

Occupational Exposures and Asbestos

Asbestos exposure is a well-documented cause of apical pleural and parenchymal scarring, though it tends to develop slowly. In a series of 40 patients who developed upper-lobe-predominant lesions after occupational asbestos exposure, the average time from first exposure to the appearance of lesions was 34 years.¹¹ All were men, aged 41 to 78, and the average apical pleural thickening measured about 21 mm. Biopsies showed varying degrees of asbestosis combined with nonspecific pleuritis, and the lesions tended to progress over time, eventually becoming part of a more diffuse pleural and parenchymal fibrosis affecting the rest of the lung.

The long latency period is what makes asbestos-related apical scarring tricky. A person may have worked in construction, shipbuilding, or insulation installation decades ago and have no current occupational exposure. Without a detailed work history, the scarring can be mistakenly attributed to aging alone. For this reason, clinicians evaluating apical scarring routinely ask about past occupational and environmental exposures even when the imaging findings look benign.

Radiation and Chemotherapy

Medical treatments themselves can leave scars at the lung apex. Radiation therapy directed at the chest, particularly the supraclavicular and mediastinal fields used for lymphoma or breast cancer, can cause fibrosis in the nearby lung tissue. The apical cap review from the American Journal of Roentgenology listed post-radiation fibrosis after mantle therapy for Hodgkin disease or supraclavicular radiation in the treatment of breast cancer as a recognized cause of apical density.¹² Radiation-induced apical scarring and radiation pneumonitis sequelae are well documented, although they are rarely reported as late-onset complications appearing years after treatment.¹³

Certain chemotherapy drugs add another layer of risk. A review in the European Respiratory Journal reported that roughly 10% of cases of pleuroparenchymal fibroelastosis, an upper-lobe-predominant scarring pattern, developed in patients who had previously been exposed to chemotherapy agents, sometimes many years after treatment. Alkylating drugs, particularly cyclophosphamide and BCNU, were the common denominators across these cases, raising the possibility that these agents may trigger or directly cause the scarring process.¹⁴ The gap between treatment and the appearance of fibrosis ranged from six months to 16 years, which means the connection is easy to miss if nobody thinks to look back at a patient’s cancer treatment history.

Pleuroparenchymal Fibroelastosis

Most causes of apical scarring are either benign or the healed aftermath of a past insult. Pleuroparenchymal fibroelastosis (PPFE) is the exception. It is a rare, progressive form of lung fibrosis that specifically targets the upper lobes and can gradually worsen over time. Patients tend to be slender, often with a flat rib cage or an abnormally narrow chest dimension from front to back. The first symptoms are usually shortness of breath or a dry cough, though in some patients the first sign is chest pain from a spontaneous pneumothorax.¹⁵

What sets PPFE apart from more common forms of lung fibrosis like idiopathic pulmonary fibrosis (IPF) is where the damage concentrates. IPF predominantly affects the lower lobes, whereas PPFE homes in on the upper lobes. On lung function tests, both diseases produce similar declines in capacity and gas exchange, but imaging tells them apart clearly.¹⁵

Under the microscope, PPFE shows a distinctive pattern: intense elastic fibrosis of the visceral pleura, with short, curled, randomly oriented elastic fibers creating a thick band-like thickening. This elastic scarring extends into the alveolar walls that anchor into the pleura, with a sharp transition to normal lung tissue beyond. Unlike IPF, PPFE does not produce the honeycombing pattern characteristic of that disease, and the histological picture is more uniform in time rather than showing the mixture of old and new scarring typical of IPF.¹⁶

PPFE can occur without any identifiable trigger, in which case it is called idiopathic PPFE. But as noted earlier, it can also develop after chemotherapy, bone marrow transplantation, or in the setting of connective tissue diseases. Recent genetic research has uncovered a potential biological link: in a small study of six patients diagnosed with PPFE, five had lymphocyte and granulocyte telomere lengths at or below the 10th percentile for their age, and four of those five carried genetic variants in telomere-related genes.¹⁷ Shortened telomeres are associated with premature cellular aging and impaired tissue repair, which fits with the idea that PPFE may represent an accelerated scarring response in genetically susceptible people.

Less Common Causes

Beyond the major categories above, a surprisingly varied list of conditions can produce apical density on imaging. The classic AJR review catalogued several of these:

Trauma: A ruptured aorta can dissect blood into the extrapleural space at the apex. Rib or spinal fractures can do the same, as can hemorrhage from subclavian line placement.
Neoplasm: A Pancoast tumor (superior sulcus lung cancer) is a well-recognized cause of unilateral, asymmetric apical thickening. Lymphoma extending from the neck or mediastinum and metastatic disease can also mimic benign apical scarring.
Vascular abnormalities: Coarctation of the aorta with dilated collateral vessels over the apex, or a fistula between the subclavian artery and vein, can create apical density that mimics scarring.
Fat deposition: Mediastinal lipomatosis, where excess fat extends over the lung apices, can look like an apical cap on a standard chest X-ray.

These causes are individually uncommon, but they illustrate why radiologists do not automatically dismiss every apical opacity as harmless. The shape, thickness, symmetry, and clinical context all matter.¹²

When Apical Scarring Deserves a Closer Look

For most people who see “apical scarring” or “apical pleural thickening” on a radiology report, the finding is incidental and harmless. But certain features should prompt further investigation. An apical cap that is nodular, irregular, thick, markedly asymmetric, or accompanied by other abnormalities warrants consideration of more serious causes like a tumor or hemorrhage.⁴

Change over time is another red flag. A stable apical cap that has looked the same on imaging for years is reassuring. One that is new, enlarging, or developing cavitation justifies additional workup, which may include CT imaging, sputum testing for tuberculosis, or in some cases biopsy. Context matters too: apical scarring in a 70-year-old with no relevant exposure history means something very different from the same finding in a 35-year-old who recently immigrated from a country with high TB prevalence, or in someone with a known diagnosis of ankylosing spondylitis.

What the Microscope Shows

When apical scars are biopsied or examined in surgical specimens, they have a recognizable architecture regardless of their original cause. These subpleural scars are typically pyramid-shaped, with overlying pleural adhesions and hyaline pleural plaques. The lung tissue within the scar is replaced by dense, mature collagen, and the elastic skeleton of the alveoli is contracted in an accordion-like fashion with reduplicated curls of elastic fibers.¹⁸ This fibroelastic pattern is common across benign apical caps and more pathological forms of scarring, which is part of what makes the pathologist’s job difficult: the microscopic appearance does not always reveal the original insult. Clinical history and imaging context often provide more diagnostic clues than the biopsy alone.

In progressive conditions like PPFE, the elastic fibrosis is more extensive and intense, spreading deeper into the parenchyma, but the basic character of the scar is similar. An elastin stain can help pathologists confirm the diagnosis when the microscopic picture is ambiguous.¹⁶ The challenge is that apical caps are frequently encountered incidentally during surgery for other conditions, and pathologists who are not familiar with the entity may not recognize or report it, leading to underdiagnosis of conditions like PPFE that could benefit from monitoring.

The Telomere Connection and Emerging Genetics

Research into why some people develop progressive apical fibrosis while others accumulate only benign scars is still in its early stages, but the telomere findings are intriguing. The small study described earlier found that the vast majority of PPFE patients tested had abnormally short telomeres and carried variants in genes responsible for telomere maintenance.¹⁷ Telomere shortening has also been implicated in IPF and other fibrotic lung diseases, suggesting a shared vulnerability in how the lung repairs itself after injury.

If these findings hold up in larger studies, genetic testing could eventually help clinicians distinguish patients whose apical scarring is likely to remain stable from those at risk of progression. For now, PPFE remains a clinical and radiologic diagnosis, but the genetic work points toward a future where a blood test might flag who needs closer surveillance. It also raises the possibility that some people who develop upper-lobe fibrosis after chemotherapy or bone marrow transplant were genetically predisposed all along, with the medical treatment serving as the trigger rather than the sole cause.

1
PubMed. A biomechanical hypothesis for the pathophysiology of apical lung disease
2
American Physiological Society (J Appl Physiol). Liquid thickness vs. vertical pressure gradient in a model of the pleural space
3
PubMed. The apical cap
4
European Society of Radiology. Roadmap in normal looking abnormal chest radiograph: How to approach in exam and Review areas in routine practice
5
ScienceDirect. Pleural Thickening
6
ScienceDirect. Pleural Thickening and Pleural Calcification
7
Exon Publications. Pleural Thickening: Education for Patients and the Public
8
PubMed. Pulmonary manifestations of ankylosing spondylitis
9
PubMed Central. Apical fibrobullous lung disease in ankylosing spondylitis: case report and literature review
10
PubMed Central. Pleuroparenchymal Fibroelastosis in Connective Tissue Disease-Related Interstitial Lung Disease
11
PubMed. Pleural and parenchymal fibrosis mainly affecting the upper lung lobes in persons exposed to asbestos
12
American Journal of Roentgenology (AJR). The apical cap
13
BMJ Case Reports. Pulmonary apical fibrosis in a patient treated earlier for breast cancer
14
European Respiratory Journal. Pleuropulmonary fibroelastosis: one more walk on the wild side of drugs?
15
PubMed Central. Pleuroparenchymal Fibroelastosis: Its Clinical Characteristics
16
Modern Pathology. Idiopathic pleuroparenchymal fibroelastosis: an unrecognized or misdiagnosed entity?
17
PubMed Central. Genetic Testing Goes Beyond Imaging and Histological Evaluation in Pleuroparenchymal Fibroelastosis
18
PubMed. Pulmonary apical cap: a distinctive but poorly recognized lesion in pulmonary surgical pathology