Lipomatous pseudohypertrophy of the pancreas

Lipomatous pseudohypertrophy of the pancreas is a rare, benign entity characterized by focal or diffuse enlargement of the pancreas due to the replacement of exocrine parenchyma with adipose tissue.

Terminology

Arguably lipomatous pseudohypertrophy may be considered a distinct clinicopathologic entity ¹. However, many authors refer to a spectrum of pancreatic lipomatosis with lipomatous pseudohypertrophy representing the extreme, mass-like variant ^2,11,14.

Epidemiology

The entity is very rare, with fewer than 50 cases reported in the literature, across all ages (neonate to 80 years) ¹. No sex predominance exists ¹.

Associations

Associations with rare childhood syndromes have been suggested ¹:

• cystic fibrosis ^5-11 (although some authors exclude this arbitrarily by definition ¹²)
• Shwachman-Bodian-Diamond syndrome
• Bannayan syndrome
• Johanson-Blizzard syndrome

Unlike typical pancreatic fatty infiltration, lipomatous pseudohypertrophy may be seen without association with obesity, diabetes mellitus, or chronic pancreatitis ^1,8.

Clinical presentation

It is usually asymptomatic and incidentally discovered during workup for other disorders. When symptomatic, the most common presenting symptom is abdominal pain ¹. Due to exocrine insufficiency, patients may present with steatorrhea ¹³. Occasionally, it causes jaundice due to mass effect on the bile duct ¹.

Pathology

Etiology

The exact etiology is unknown. Postulated etiologies include parenchymal atrophy due to ductal obstruction, viral infection, toxin exposure, and congenital anomalies ¹.

Location

The disorder usually affects the pancreas diffusely (70%), with a minority localized to the head (20%) or to the body and tail (10%) ¹.

Macroscopic appearance

The pancreas is replaced with a large volume of adipose tissue, which shows mass effect on adjacent structures ¹. The shape of the pancreas is maintained ¹.

Microscopic appearance

The pseudotumor is composed of mature adipose tissue, amongst scattered, well-preserved acinar structures ¹. The lesion is well demarcated but lacks a capsule ¹. The pancreatic ductal system and islets of Langerhans are preserved ^1,13. No atypia, fibrosis, or fat necrosis is seen on histology ¹.

Radiographic features

• focal or diffuse mass-like enlargement of pancreas (pseudotumor) with preservation of the shape of the pancreas
• could be as large as 20-25 cm
• typically has well-defined border without any well-formed capsule
• no dilatation of the pancreatic ductal system, which remains visible ²

CT

• fat attenuation

MRI

• high signal on T1 and T2 weighted images
• signal suppressed with fat suppression techniques

History and etymology

The entity was described in 1931 by German pathologist Walther Hantelmann, who coined the term "lipomatöse pseudohypertrophie" of the pancreas as an analog to the pseudohypertrophy of the muscles seen with childhood muscular dystrophies ⁴.

Differential diagnosis

The imaging and pathologic differential includes

• pancreatic lipomatosis
  • pancreas not enlarged
  • most common in older adults with metabolic syndrome
• pancreatic lipoma
  • multilobed, encapsulated fatty tumor that is demarcated from the surrounding pancreas, rather than replacing parenchyma in the shape of the pancreas ¹
• retroperitoneal liposarcoma
• pancreatic ductal adenocarcinoma​
  • infiltrative hypoattenuating mass but not fat attenuation