A chest CT scan reveals a 1.6 cm by 2.7 cm irregularly shaped soft tissue density nodule or focus of consolidation in the posterior basal segment of the left lower lobe. A fusion positron emission tomography (PET)/CT scan of the chest demonstrates a standardized uptake value (SUV) of 5.5 for the left lower lobe lesion and no associated lymphadenopathy or increased uptake elsewhere. A CT-guided biopsy was performed and demonstrates organizing pneumonia, alveolar septal thickening due to lymphoplasmacytic infiltrate, and poorly defined granulomas with no evidence of organisms on acid fast bacilli (AFB) and giesma (GMS) staining.

Because of the concern for malignancy, a video-assisted thoracoscopic (VATS) wedge resection was performed. The pathology is pictured below.

The biopsy contains a prominent fibrotic nodule containing numerous microabscesses with brown pigmented, moniliform fungal elements within giant cells and granules characterized by a prominent Splendore-Hoeppli phenomenon.  It also demonstrates yeast-like cells that produce single buds or chains and spherical cells with septations.  This is consistent with phaeohyphomycosis.  The Splendore-Hoeppli phenomenon represents eosinophilic, pseudomycotic structures composed of necrotic debris.  It is commonly seen in infections with S. aureus and P. aeruginosa, but can also be found surrounding schistosome eggs, microfilariae and fungi (1).  The distinguishing characteristic of the dematiaceous fungi, including phaeohyphomycosis, is the presence of melanin in their cell walls (2).  The term dematiaceous, when related to fungus, means dark in color, which may include black, dark brown or pale brown.  The Fontana-Masson stain may be used to help identify these fungi histologically, as it is specific for melanin (2).

Phaeohyphomycosis is a chronic, slowly progressive infection caused by a number of dematiaceous fungi commonly found in the soil worldwide.  They uncommonly cause disease in humans, but when seen, may cause life-threatening illness (2, 3).  Infections are more commonly seen in immunocompromised patients, but can appear in immunocompetent patients as well (2).  The routes of infection most commonly encountered are from inhalation or percutaneous innnoculation (3).  Limited skin and subcutaneous infection is generally encountered in immunocompetent patients, while immunocompromised patients experience disease outside of the skin (i.e., brain, respiratory tract or disseminated disease) (4).

Not much is known about the mechanism of virulence of phaeohyphomycosis. However, this group has a cytoplasmic melanin-like pigment which may be partially responsible (2).  The melanin may assist by scavenging free radicals and hypochlorite produced by phagocytic cells (2).  These are produced in the oxidative burst that would normally kill many organisms.  Alternatively, the melanin may bind to hydrolytic enzymes and prevent their action on the plasma membrane (2).  The melanin also helps distinguish this group of fungi from other species, like Aspergillus.  Melanin has also been shown to reduce the virulence of some organisms to drugs like amphotericin B and caspofungin. However, this reduced virulence has not been seen with the azole drugs (2).

The lack of cells consistent with adenocarcinoma makes this diagnosis incorrect.  The histologic diagnosis of adenocarcinoma of the lung requires the presence of neoplastic gland formation or intracytoplasmic mucin.  Mucormycosis and Aspergillus are both incorrect because they have different appearances on the GMS stained slides.  Phaeohypomycosis is more fragmented and have irregular septate hyphae and yeast-like forms when compared to Aspergillus.  Aspergillus grows as septate dichotomous branching hyphae at acute (45°) angles. Mucor histologically appears as nonseptate hyphae.  There are no findings consistent with aspiration pneumonia on the pathologic specimen.  Such findings might include giant cells with foreign materials such as vegetable matter.