Histiocytosis With Raisinoid Nuclei: A Unifying Concept for Lesions Reported Under Different Names as Nodular Mesothelial/Histiocytic Hyperplasia, Mesothelial/Monocytic Incidental Cardiac Excrescences, Intralymphatic Histiocytosis, and Others

We report 50 cases of peculiar histiocytic proliferations occurring in diverse body sites that currently bear various names, including nodular mesothelial/histiocytic hyperplasia, nodular histiocytic aggregates, mesothelial/monocytic incidental cardiac excrescences, reactive eosinophilic pleuritis, histioeosinophilic granuloma of the thymus, and intralymphatic histiocytosis. They can sometimes cause considerable differential diagnostic difficulties by resembling a metastatic carcinoma or Langerhans cell histiocytosis. Several previous publications have established a link between some of these conditions, suggesting that these are merely variations within a histopathologic spectrum, affecting different organs and bearing different names based on a particular location. However, no publication has ever comprehensively addressed all of these lesions together in one study in an attempt to explain and discuss their striking analogy. Having studied a large series of cases we provide evidence that all these lesions share the same morphologic, immunohistochemical, and pathogenetic properties, thus they all represent the same pathologic process and should be referred to as such. Taking into account their typical nuclear features we propose a collective term "histiocytosis with raisinoid nuclei" for this spectrum of conditions.