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Thymolipoma

Thymolipoma is a rare, benign anterior mediastinal mass of thymic origin, containing both thymic and mature adipose tissue. 

Epidemiology

Thymolipomas comprises 2-9% of all thymic neoplasms, but are less common than a mediastinal lipoma of non-thymic origin. There is no recognised sex predilection and reported age range between 3-56 years, with a mean age of 22 years.

Clinical presentation

Most thymolipomas are asymptomatic and found incidentally, often due to imaging of respiratory tract infection 1. Symptoms, when present are attributable to displacement of mediastinal structures. Approximately 25% complain of non-specific symptoms such as cough, dyspnoea and chest pain, which may or may not be the result of the mass. 

Pathology

Thymolipomas are composed of a mixture mature adipose tissue with islands of thiymic tissue. The aetiology remains uncertain, with hypotheses proposed including 1:

  • true neoplasm of the thymus
  • variant of a thymoma
  • hyperplasia of mediastinal fat
  • neoplasm of mediastinal fat which engulfs thymic tissue

Irrespective of cause, these tumours are slow-growing, only gradually increasing in size and are usually large at the time of diagnosis.

Associations

Unlike thymomas, thymolipomas are only occasionally associated with myasthenia gravis 1. In addition, they are also described in association with:

Location

They occur mostly in the cardiophrenic angles.

Radiographic features

A thymolipoma can grow to a very large size before discovery.

Plain film

Typically these tumours appear as large anterior mediastinal masses. The larger tumours tend to hang down one or either side of the pericardium, and being soft, they mold themselves to the adjacent mediastinum and diaphragm and often mimic cardiomegaly 1. The predominantly fat density can be difficult to identify on plain radiography, however in larger masses that abut the diaphragm, the diaphragm can still be seen.

CT

On CT thymolipomas typically appears almost entirely fatty with some areas of inhomogeneous soft tissue density that represent thymic tissue.

MRI

Thymolipomas have fat and soft tissue signal characteristics:

  • T1: typically the adipose tissue within the tumour returns high T1 signal and thymic component a more intermediate T1 signal
  • STIR FS: shows complete suppression; consistent with subcutaneous fat

Treatment and prognosis

Surgically local resection curative, no reports of recurrence, metasasis or mortality.

History and etymology

First reported by Lange in 1916. The term was coined by Hall in 1948.

Differential diagnosis

The differential on plain radiography is that of an anterior mediastinal mass. In larger tumours which 'hang' down, causes of cardiomegaly should also be considered.

On CT the differential is much narrower and includes:

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