Mediastinal lipoma is a rare benign tumor composed of mature adipose tissue, accounting for 2-4% of mediastinal masses. It can occur in all three mediastinal compartments (anterior, middle, posterior) but is most common in the anterior mediastinum. Typically diagnosed between ages 40-60, with no significant gender difference but mild male predominance. Usually asymptomatic and incidentally discovered on chest radiography or thoracic CT. The characteristic imaging finding is homogeneous fat density on CT (-50 to -120 HU) with NO enhancement. On MRI, it shows signal isointense to fat on T1 and T2 and homogeneously loses signal on fat suppression sequences. It is well-circumscribed and may contain a thin fibrous capsule. No solid enhancing components, calcification, or thick septa — their presence should raise concern for liposarcoma or thymolipoma. Treatment is usually unnecessary; surgical excision for large or symptomatic cases.
Age Range
40-70
Peak Age
55
Gender
Equal
Prevalence
Uncommon
Mediastinal lipoma arises from clonal proliferation of mature adipocytes in mediastinal fat tissue. Histologically, it is difficult to distinguish from normal fat tissue — composed of mature adipocytes surrounded by a thin fibrous capsule. Cytogenetic studies may detect HMGA2 gene rearrangements at the 12q13-15 region — confirming that lipoma is a true neoplasm (not reactive fat hypertrophy). The fat density of lipoma on CT (-50 to -120 HU) is explained by the low X-ray attenuation of triglyceride-containing adipocytes with low atomic number components (carbon Z=6, hydrogen Z=1, oxygen Z=8). T1 hyperintensity on MRI is explained by the short T1 relaxation of fat protons (the low motion frequency of C-H bonds is close to the Larmor frequency, enabling efficient dipole-dipole relaxation). Signal loss on fat suppression sequences is confirmed by selective suppression of fat protons — this feature is pathognomonic. Lipomas grow slowly, malignant transformation is extremely rare, and prognosis is excellent.
Homogeneous -50 to -120 HU fat density and absence of enhancement on CT is pathognomonic for lipoma. This combination is not seen in any other mediastinal pathology.
On non-contrast CT, well-circumscribed, homogeneous fat density (-50 to -120 HU) mass in the mediastinum. Density values are identical to subcutaneous fat tissue. A thin hyperdense capsule may help distinguish from surrounding tissues. No internal septa or solid components. The mass displaces but does not invade surrounding structures. Density measurement should be confirmed by placing multiple ROIs — if any area shows soft tissue density (above +20 HU), liposarcoma should be excluded.
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Well-circumscribed mass with homogeneous fat density (__ HU) in the mediastinum, consistent with lipoma; no solid component or enhancement is observed.
On contrast-enhanced CT, the lesion shows no enhancement. No significant difference between pre-contrast and post-contrast density values (<10 HU). Lipoma does not contain functional vascular structures. Absence of enhancement helps differentiate lipoma from other fat-containing lesions (well-differentiated liposarcoma, thymolipoma, extramedullary hematopoiesis) — non-adipose components may enhance in these.
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No enhancement is observed in the lesion on contrast-enhanced series; consistent with pure fat tissue composition.
On T1-weighted images, lipoma shows homogeneous hyperintense signal isointense to subcutaneous fat. This bright T1 signal reflects the characteristic short T1 relaxation time of fat protons. The capsule may be visible as a thin hypointense line. No heterogeneity or solid components in internal structure.
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The lesion demonstrates homogeneous hyperintense signal isointense to subcutaneous fat on T1-weighted sequences; consistent with lipoma.
On T2-weighted images, lipoma shows intermediate to high signal — isointense to subcutaneous fat. Homogeneous signal distribution confirms presence of mature fat tissue. Signal loss on fat-suppressed T2 — confirms intrallesional fat content.
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The lesion shows signal isointense to subcutaneous fat on T2-weighted sequences with signal loss on fat-suppressed T2.
On fat suppression sequences (STIR or frequency-selective fat saturation), lipoma homogeneously loses signal and becomes hypointense/signal void. This complete signal loss definitively proves the entire mass is composed of fat tissue. If areas retain signal after fat suppression, they represent non-adipose components and should raise concern for liposarcoma. India ink artifact may be visible at mass margins on chemical shift (opposed-phase) sequences.
Report Sentence
Homogeneous and complete signal loss on fat suppression sequences confirms pure fat tissue composition; consistent with lipoma.
Lipoma is well-circumscribed with a thin capsular line separating from surrounding structures. Capsule appears as thin (<2 mm) smooth line of soft tissue density. Mass displaces but does not invade surrounding structures. Capsular integrity is a reliable indicator of benign lesion. Irregular borders or invasion suggest liposarcoma. Absence of worrisome features (thick septa >2 mm, nodular components, solid enhancing areas, globular non-adipose areas) supports lipoma diagnosis.
Report Sentence
The mass is well-circumscribed with a thin capsule separating it from surrounding structures; no thick septation or solid component; consistent with benign lipoma.
Criteria
Pure mature fat tissue; most common form
Distinct Features
Completely homogeneous fat density on CT, homogeneous fat signal on MRI, complete signal loss on fat suppression. No enhancement, no septa, no solid components.
Criteria
Increased fibrous component within fat tissue; density values may be mildly elevated
Distinct Features
Fat density on CT with thin fibrous bands (linear hyperdense lines). On MRI, bands retain signal on fat suppression. Fibrous bands may show minimal enhancement.
Criteria
Mixture of fat + thymic tissue; large mass in anterior mediastinum, usually >10 cm
Distinct Features
CT shows fat density alongside soft tissue density bands/islands (thymic tissue). Usually very large, extends toward diaphragm (drooping appearance). Thymic tissue presence distinguishes from pure lipoma.
Distinguishing Feature
Teratoma is heterogeneous with fat, calcification, and soft tissue triad. Fat-fluid level on CT is pathognomonic. Lipoma has homogeneous fat density without other components.
Distinguishing Feature
Thymoma is a solid soft tissue density mass (30-50 HU) with enhancement. Lipoma has fat density (-50 to -120 HU) without enhancement. ROI measurement provides definitive differentiation.
Distinguishing Feature
Pericardial cyst water density (0-20 HU) at cardiophrenic angle. Lipoma fat density (-50 to -120 HU). ROI measurement differentiates — both non-enhancing but density values completely different.
Distinguishing Feature
Well-differentiated liposarcoma shows thick septa (>2 mm), nodular enhancing components, and heterogeneous density. Lipoma has homogeneous fat density without thick septa or enhancing solid components. Non-adipose signal-retaining areas on fat suppression in liposarcoma.
Urgency
surveillanceManagement
conservativeBiopsy
Not NeededFollow-up
no-follow-upMediastinal lipoma is benign and malignant transformation is extremely rare. When typical imaging findings are present, diagnosis is definitive and biopsy is not needed. No treatment or follow-up required for asymptomatic cases. Large lipomas (>10 cm) may cause compressive symptoms — surgical excision performed. If atypical findings present (soft tissue components, thick septa >2 mm, enhancement, rapid growth), liposarcoma should be excluded and biopsy considered.
Mediastinal lipoma is a benign lesion that generally requires no treatment. Large lipomas may compress mediastinal structures. Liposarcoma should be excluded in the presence of enhancing components or thick septa. No follow-up is required.