• Tumor cells packed with mitochondria with tubulovesicular cristae typical of steroid-producing cells
  
• Rare; usually non-functioning, usually benign
  
• Mean age 45 years, range 27-74 years
  
• Confirm adrenocortical origin by immunoreactivity to 3-beta-hydroxysteroid dehydrogenase, the enzyme that converts pregnenolone to progesterone and dehydroepiandrosterone to androstenedione
  
• Clinical behavior not entirely predictable
  
• Weiss system for malignancy not applicable because all tumors have eosinophilic tumor cytoplasm, diffuse architecture and nuclear atypia; better to use criteria of mitotic activity, necrosis, invasion and MIB1 activity
  
• 7 cases (Am J Surg Pathol 1998;22:603), 3 cases (Am J Surg Pathol 1991;15:949)
  

• 40 year old woman with retroperitoneal oncocytoma of apparent adrenocortical origin (Arch Pathol Lab Med 2002;126:1118)
  
• 55 year old woman with 13 cm tumor (Arch Pathol Lab Med 2003;127:e167)
  

• Light to dark brown, solid, well-circumscribed, up to 20 cm
  

Ectopic tumor: solid with smooth external surface

• Alveolar, tubular or solid patterns composed exclusively of oncocytes (epithelial cells with abundant eosinophilic, granular cytoplasm and sparse lipid), nuclear atypia common
  
• No / rare mitotic activity, no necrosis
  
• Tumors of uncertain malignant potential: increased mitotic activity and necrosis, but no invasion
  

Abundant eosinophilic granular cytoplasm; PTAH+

3 beta-hydroxysteroid dehydrogenase immunohistology

Figure 1: solid tan-brown mass with variegated cut surface; 2: sheets of large polygonal cells with abundant eosinophilic granular cytoplasm and small central nucleus; 3: EM shows dense mitochondria with platelike cristae, scattered non-membrane bound electron-dense granules

• Keratin (CAM 5.2, AE3), anti-mitochondrial antibody, PTAH, NSE, synaptophysin, low MIB1 (except in tumors of uncertain malignant potential) and variable vimentin
  

• Chromogranin A, EMA, S100 and p53
  

• Numerous mitochondria with stacked tubulo-vesicular and lamellar cristae, smooth endoplasmic reticulum, lysosomes and small electron dense inclusions
  

• Adrenocortical tumors with focal oncocytic change
  
• Eosinophilic variants of renal cell carcinoma: EMA+, NSE- and synaptophysin-
  
• Oncocytic carcinoma
  
• Oncocytic pheochromocytoma: chromogranin A+
  

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