In the study by Argani et al, (7) the TFE3-positive PEComas were found in younger patients without evidence of tuberous sclerosis, and the lesions were composed of predominantly epithelioid cells in an alveolar pattern with minimal immunoreactivity for muscle markers.
Microscopically, sections from the appendiceal nodule showed large, epithelioid cells arranged in nested and alveolar patterns (Figure, A through C).
The heart appeared normal in thoracic radiographs, but the pulmonary vasculature was mildly enlarged, and a marked diffuse alveolar pattern
occurred throughout the lungs.
These features included an alveolar pattern
of the tumor nests in a rich stroma background, with clear cell components and a palisading arrangement of the peripheral basal cells.
The "zellballen" pattern is a characteristic feature in many tumors, but in some cases it may be obscured by broad anastomosing bands, an alveolar pattern
, a predominance of round to oval cells, and the presence of giant multinucleated cells.
These tumours were formerly recognized as "juvenile carcinomas" in the paediatric literature, where they were considered as aggressive tumours displaying papillary and/or alveolar patterns
with "voluminous" eosinophilic and/or clear cells (6).