The fake fat phenomenon in organizing pleuritis: a source of confusion with desmoplastic malignant mesotheliomas

Our plain-English summary of The fake fat phenomenon in organizing pleuritis: a source of confusion with desmoplastic malignant mesotheliomas, by A. Churg, P. Cagle, T.V. Colby, et al (The American Journal of Surgical Pathology, 2011).

The study

• Authors: A. Churg, P. Cagle, T.V. Colby, J.M. Corson, A.R. Gibbs, S. Hammar, N. Ordonez, V.L. Roggli, H.D. Tazelaar, W.D. Travis, M. Wick, US-Canadian Mesothelioma Reference Panel
• Journal: The American Journal of Surgical Pathology
• Published: 2011;35(12):1823-9
• Cited by: 21 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

We report 9 patients with pleural biopsies referred because of concern about infiltration of what appeared to be chest wall fat by pan-keratin-positive spindled cells, a finding that led to a consideration of desmoplastic mesothelioma. All patients showed pleural effusions/pleural thickening on computed tomographic scan. Pleural biopsy showed a greatly thickened and fibrotic paucicellular pleura with circular fat-like spaces and, sometimes, adjacent oblate spaces mostly deep in the fibrotic area. Indistinct, keratin-positive, spindle cells arranged parallel to the pleural surface coursed between these fat-like spaces. S-100 stains were negative around the fat-like spaces. Vimentin stains showed that the spaces did not have a cellular lining of any kind. Sometimes the spaces contained faintly hematoxyphilic material that was Alcian blue positive, and similar material was seen in the fibrotic stroma. Follow-up with periods ranging from 6 to 30 months revealed that 8 cases had stable disease on chest imaging or by clinical findings. One case had slowly progressive pleural thickening. These observations suggest that spaces resembling fat may be encountered in fibrotic pleurae and that horizontally oriented keratin-positive spindled cells between the fat-like spaces deep in the fibrotic portion of a thickened pleura represent a benign finding seen in some cases of organizing pleuritis/fibrothorax. The spaces themselves are probably artifacts derived from the biopsy procedure and/or cutting artifacts. In contrast, in true desmoplastic mesotheliomas there is downward, rather than horizontal, growth of keratin-positive spindled cells running between clearly definable fat cells.

In plain English

This report described 9 patients whose pleural biopsies were sent to the US-Canadian Mesothelioma Reference Panel between February 2008 and March 2011 to ask whether they showed desmoplastic malignant mesothelioma. All were male. All but one (aged 36) were in their 60s or 70s, and all had a pleural effusion and thickened pleura. The biopsies showed a thickened, scarred pleura containing round spaces that looked like fat cells. Keratin-positive spindle-shaped cells ran between these spaces parallel to the pleural surface, mimicking invasion of fat. Stains showed that most of the spaces had no cellular lining, and the authors suggested they may be traction artifacts from pulling the pleura during biopsy. Five patients had stable chest findings at 6 to 30 months, and one had slowly increasing pleural thickening at 6 months. One patient was lost to follow-up, and two were recent cases. The authors concluded that these were unusual variants of organizing pleuritis, and that attention to the direction of cell growth can help avoid overdiagnosing desmoplastic mesothelioma. The article did not report the patients' asbestos exposure.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Case series of consultation cases (pathology review with immunostaining and clinical follow-up)
• Who was studied: 9 patients whose pleural biopsies were referred to the US-Canadian Mesothelioma Reference Panel (based at the University of British Columbia, Vancouver) and accessioned between February 2008 and March 2011. All were male, and all but one (aged 36) were in their 60s or 70s.
• Exposure: Not reported. The article does not describe asbestos or other exposures for these patients.
• Main results:
  • In all cases, fat-like spaces lay well within the fibrotic pleura and were arranged parallel to the pleural surface.
  • In all cases, keratin-positive spindled cells ran between the fat-like spaces.
  • Vimentin stains showed that most of the spaces had no cellular lining.
  • The authors suggested the spaces may be traction artifacts produced by pulling the pleura during biopsy.
  • Per Table 1, five patients (cases 2, 3, 4, 5 and 7) had stable chest findings at follow-up of 6 to 30 months. Case 7 continued to be short of breath.
  • In case 6, a slow increase in pleural thickening was reported at 6 months. The authors stated this case was not overtly malignant.
  • Case 1 was lost to follow-up, and cases 8 and 9 were recent cases with no follow-up duration reported.
  • Over the same period, the Panel accessioned 18 definite or probable desmoplastic mesotheliomas, compared with these 9 cases of organizing pleuritis with fake fat.
• The authors' conclusion: The authors concluded that, when keratin-positive spindled cells are seen between fat-like spaces in a fibrotic pleura, careful attention to the direction of spindle cell growth and to the characteristics and location of the 'fat' can help avoid overdiagnosis of desmoplastic mesothelioma.
• Limitations the authors state:
  • The comparison of case numbers with desmoplastic mesotheliomas may be misleading because of unknown referral bias.
  • The cause of the artifact is not clear.
  • One patient was lost to follow-up.
  • Case 9 was recent, so it had little follow-up.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Mesothelioma.

Medical terms in this study, explained in our Lexicon: Desmoplastic Mesothelioma, Malignant Mesothelioma, Pleural Effusion, Pleural Thickening, Pleural Biopsy, Pleura, Spindle Cell, S-100.

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