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Interstitial lung disease (ILD) is an umbrella term for a diverse group of pulmonary disorders that affect the interstitium, have similar clinical, radiologic, physiologic, or histologic features and may ultimately lead to fibrosis of the lungs.
The most common ILD is idiopathic pulmonary fibrosis (IPF), a fatal chronic progressive fibrotic lung disease. IPF, a rare disease with an incidence ranging from 3 to 9 per 100,000 persons per year, affects elder individuals with a mean age of 65 years and is more commonly seen in males. Historically the average survival time of IPF from the time of diagnosis was 2-3 years but is now reaching 3-5 years, likely due to earlier diagnosis and use of antifibrotic treatment that slows progression. Early and accurate diagnosis of IPF and differentiation from other types of ILD is critical not only for its dismal prognosis but also in providing the patients with the opportunity to receive antifibrotic treatment and be assessed for lung transplantation. Moreover, immunosuppressive medications used for other types of ILD have been reported to be harmful for IPF patients.
The Radiologists play a critical role in the diagnosis and management of IPF by determining the pattern on high resolution computed tomography (HRCT). A usual interstitial pneumonia (UIP) pattern on HRCT is essentially diagnostic in the appropriate clinical context after excluding other known causes of ILD. Recently updated guidelines for the diagnosis of IPF defined four diagnostic categories based on CT (9, 10): typical UIP pattern, probable UIP pattern, indeterminate for UIP and CT features most consistent with non-IPF diagnosis. A typical or probable UIP pattern in the appropriate clinical context are sufficient for diagnosis of IPF. Both CT patterns are characterized by subpleural, basal predominant reticular pattern with traction bronchiectasis and bronchiolectasis and absence of features to suggest an alternative diagnosis. The only difference is the presence of subpleural honeycombing in the typical UIP CT pattern. The indeterminate for UIP pattern is also subpleural and basal predominant, however characterized by subtle reticulation, mild ground glass opacity (GGO) or distortion and may have features or distribution that do not suggest any specific etiology. Although surgical lung biopsy may be needed to establish the diagnosis if the patient can tolerate it, a confident diagnosis of IPF can still be made in a multidisciplinary discussion (MDD) meeting in the appropriate clinical context. Finally, the “inconsistent with IPF” pattern is characterized by CT features that are suggestive of an alternative diagnosis including, extensive mosaic attenuation pattern, cysts, predominant GGO, profuse microndules, peribronchovascular, perilymphatic distribution or upper/mid lung predominance, subpleural sparing, pleural plaques, extensive lymphadenopathy or dilated esophagus.
Diagnosis of IPF cannot be based solely on imaging. Integration of detailed history, clinical findings and serological testing is essential. Incorporation of clinical probability of IPF which is increased in patients above 60 years, current or former smokers and absence of history of known causes of ILD is also critical. Known causes of ILD that need to be excluded are grouped into systemic and exposure related disorders including connective tissue diseases, sarcoidosis, immune system abnormalities, hypersensitivity pneumonitis, occupational diseases, drug-induced lung diseases and familial fibrosis.
A surgical lung biopsy can be considered in the last 2 categories (indeterminate for UIP and inconsistent with IPF) and any CT pattern with indeterminate clinical context for IPF should be decided in an MDD. MDD including a radiologist, pulmonologist and when necessary, a pathologist is the gold standard and has shown to increase the diagnostic confidence and change the histological diagnosis. An MDD is necessary in the above scenarios and post biopsy to integrate the clinical, imaging, and histologic findings, to revisit a former MDD diagnosis in patients with discordant longitudinal course, and to establish a working diagnosis in case of lack of biopsy.
In conclusion, the Radiologist plays a crucial role in the ILD team by determining a prompt diagnosis of UIP and suggesting IPF, recommending an alternative diagnosis of ILD, monitoring the progression of ILD especially in a patient with a working diagnosis of IPF, assessing response to treatment and diagnosing an acute process or complication, such as acute exacerbation, infection, drug toxicity or lung cancer.
If you want to learn more, consider joining us at the CAR ILD Masterclass on Tuesday, September 20.