What's the Difference Between Organizing Pneumonia and Fibroblast Foci?

A classic question!  Both lesions are composed of immature fibroblasts with a mucopolysaccrhide rich stroma giving them a grey or blue appearance.  Both signify active injury/repair.  Both have connections to the interstitium.  The reality is their distinction is based on the background findings.  OP mostly appear to be in free standing in the FF are seen in the 

Organizing Pneumonia

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Fibroblast Foci

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