Background Mosaic attenuation on computed tomography (CT) has been identified in international guidelines as an important diagnostic feature of fibrotic hypersensitivity pneumonitis (FHP) as opposed to idiopathic pulmonary fibrosis (IPF). However, mosaic attenuation comprises several different radiological signs (low-density lobules, preserved lobules, air trapping and the so-called headcheese sign) which may have differing diagnostic utility. Furthermore, the extent of mosaic attenuation required to distinguish these two diagnoses is uncertain and thresholds of mosaic attenuation from international guidelines have not been validated.
Methods Inspiratory and expiratory CT scans were evaluated by two readers in 102 patients (IPF n=57; FHP n=45) using a semiquantitative scoring system for mosaic attenuation. Findings were validated in an external cohort from a secondary referral institution (IPF n=34; FHP n=28).
Results Low-density lobules and air trapping were a frequent finding in IPF, present in up to 51% of patients. A requirement for increasing extent of low-density lobules and air trapping based on guidelines (American Thoracic Society and Fleischner Society) was associated with increased specificity for the diagnosis of FHP (0.96 and 0.98, respectively) but reduced sensitivity (0.16 and 0.20, respectively). The headcheese sign was found to be highly specific (0.93) and moderately sensitive (0.49) for a high-confidence diagnosis of FHP. The high specificity of the headcheese sign was maintained in the validation cohort and when patients with other CT features of FHP were excluded.
Conclusion Mosaic attenuation is a frequent finding in IPF. However, the headcheese sign can be confidently considered as being inconsistent with a diagnosis of IPF and specific for FHP.
The headcheese sign is the form of mosaic attenuation most specific for distinguishing fibrotic hypersensitivity pneumonitis from idiopathic pulmonary fibrosis
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Conflict of interest: J. Barnett has nothing to disclose.
Conflict of interest: P.L. Molyneaux has, via his institution, received industry academic funding from Roche, Boehringer Ingelheim and Galapagos, and has received lecture fees from Roche.
Conflict of interest: B. Rawal has nothing to disclose.
Conflict of interest: R. Abdullah has nothing to disclose.
Conflict of interest: S.S. Hare has nothing to disclose.
Conflict of interest: R. Vancheeswaran has nothing to disclose.
Conflict of interest: S.R. Desai is co-organiser of an educational course convened by Boehringer Ingelheim. This is a “hands-on” workshop for pulmonologists and radiologists working in the field. The aim of the course is purely educational and does not focus on any particular disease (and specifically not IPF or hypersensitivity pneumonitis), nor does it promote any specific treatment or management.
Conflict of interest: T.M. Maher has, via his institution, received industry academic funding from GlaxoSmithKline R&D and UCB, and has received consultancy or lecture fees from Apellis, AstraZeneca, aTyr Pharma, Bayer, Biogen Idec, Boehringer Ingelheim, Galapagos, GlaxoSmithKline R&D, Indalo, Pliant, ProMetic, Roche, Samumed and UCB, and has stock options in Apellis.
Conflict of interest: A.U. Wells reports personal fees from Intermune, Boehringer Ingelheim, Gilead, MSD, Roche, Bayer and Chiesi, outside the submitted work.
Conflict of interest: A. Devaraj reports personal fees from GlaxoSmithKline, Roche and Boehringer Ingelheim, outside the submitted work.