Case 73. Actinomycosis of lung (50 y-o F)
    Surgical specimen of an irregular-shaped lung nodule in a previously healthy individual    
Key words : Actinomyces israelii, lung abscess
Formation of grains within the abscess cavity is pathognomonic of actinomycosis (HE, low power). If grains are not included in the specimen, the histopathological diagnosis may be inflammatory pseudotumor of the lung.   The grains are composed of clustered thin bacteria and homogeneous matrix called Splendore-Hoeppli material (HE, high power). Neutrophilic reaction is evident.
The grain, patrticularly its matrix, is strongly PAS-positive.   Grocott stain identifies numbers of thin filamentous bacteria, Actinomyces israelii, within the grain.