lipomas

DEFINITION: Benign neoplasm of mature white adipose tissue, usually encapsulated by a thin fibrous capsule. Most common soft tissue tumor in adults (peak 40-60 years). The malignant counterpart is liposarcoma (rare).

PATHOPHYSIOLOGY/GENETICS: - Often sporadic with clonal chromosomal rearrangements, most commonly involving 12q13-15 (HMGA2 gene, a chromatin architecture protein). Other changes include 6p21-23 and deletions of 13q. - Contrast with liposarcoma: well-differentiated/dedifferentiated types show MDM2 and CDK4 amplification (12q13-15 amplification); myxoid liposarcoma has t(12;16) FUS-DDIT3 (CHOP).

CLINICAL FEATURES: - Soft, rubbery, mobile, non-tender, slow-growing subcutaneous mass; "slip sign" (edges slip under finger pressure). - Sites: trunk, back, neck, shoulders, proximal extremities. - Usually less than 5 cm; typically solitary but may be multiple. - Rarely cause symptoms unless compressing nerves or located in a confined space.

KEY ASSOCIATIONS/VARIANTS: - Familial multiple lipomatosis (autosomal dominant, multiple lipomas). - Dercum disease (adiposis dolorosa): painful lipomas, usually in obese postmenopausal women. - Madelung disease (multiple symmetric lipomatosis): symmetric neck/upper trunk fat deposition, associated with alcohol use disorder. - Angiolipoma: painful, with vascular components, in young adults. - Hibernoma: brown fat tumor. - Spindle cell/pleomorphic lipoma: posterior neck/back of older men. - Lipomas in PTEN hamartoma tumor syndromes (Cowden, Bannayan-Riley-Ruvalcaba) and Proteus syndrome. - Intradural/spinal lipomas in spinal dysraphism (tethered cord). - Lipomatosis of the colon: submucosal lipoma may serve as a lead point for intussusception (typically at the ileocecal valve). - Cardiac: lipomatous hypertrophy of the interatrial septum.

DIAGNOSIS: - Clinical in typical cases. - Ultrasound or MRI if large (>5 cm), deep (subfascial), rapidly growing, painful, firm, or fixed, to rule out liposarcoma. MRI shows fat signal that suppresses on fat-saturated sequences without thick septations or nodular enhancing components. - Biopsy/excision for atypical features; histology shows uniform mature adipocytes without atypia or lipoblasts.

RED FLAGS FOR LIPOSARCOMA: size >5 cm, deep location, rapid growth, firm or fixed mass, thick septa/nodularity on imaging, retroperitoneal or thigh location.

MANAGEMENT: - Asymptomatic: reassurance and observation. - Excision for symptoms, cosmetic concern, or diagnostic uncertainty. Liposuction is an option. - Recurrence is uncommon after complete excision. Malignant transformation is extremely rare.

BOARD PEARL: A soft, mobile, painless subcutaneous mass with a slip sign is a lipoma. Large, deep, or rapidly growing fatty masses, particularly in the retroperitoneum or thigh, should prompt concern for liposarcoma and MRI evaluation.