Spindle cell lipoma: clinicopathologic characterization of 40 cases

Abstract

Objective: In view of the existence of multifarious pathologic subtypes of spindle cell lipoma (SCL), which is easily misdiagnosed as other benign and malignant soft tissue tumors, we performed this study and aimed to better define the category of SCL.

Methods: We collected and analyzed 40 cases of SCL with complete clinical and pathologic information from January 2010 to December 2018. Clinical and histopathologic analyses of SCL were performed, as well as immunohistochemical staining and fluorescence in situ hybridization (FISH) using probes for RB1 and MDM2, and the related literature was reviewed.

Results: In 40 cases, the male to female ratio was 3.4:1, and the mean age was 54 years old. SCL of our study included six pathologic subtypes: classic (25/40), fibrous (4/40), myxoid (4/40), low-fat (3/40), pseudoangiomatous (2/40), and fat-rich (2/40) changes. Microscopically, SCL showed distinctive morphology, with uniform spindle cells and a variably adipocytic component. The spindle cells were bland in morphology, without prominent atypia or pleomorphism, set in a myxoid or fibrous matrix. Immunohistochemically, CD34 and vimentin were positive in spindle cells, and spindle cells of 6 cases also expressed S-100 protein. FISH analysis of 10 cases revealed that heterozygous deletion of RB1 was in six samples with chromosome 13 aberrations and MDM2 gene amplification was not detected in any cases. Surgical resection is considered as the primary treatment for SCL, as there was no any recurrence or metastasis in our cases after 2-105 months of follow-up.

Conclusions: SCL is a rare benign lipoma, and the proportion of spindle cells and adipocytic component varies, which may form various pathologic changes. The diagnosis needs to be combined with clinicopathologic features, immunophenotypes, and genetics. It has to be differentiated from mammary-type myofibroblastoma, cellular angiolipoma, solitary fibrous tumor, and myxoid liposarcoma.

Keywords: MDM2; RB1; S-100; Spindle cell lipoma (SCL); differential diagnosis.

Figure 1

Gross findings of SCL. The texture can appear from soft to tough, because of the different ratio of collagen fibers and fat cells (A, B).

Figure 2

Morphology of SCL. SCL mainly composed of spindle cells, rope-like collagen fibers and mature adipocytes (A, H&E × 200). Most of the tumors were surrounded by clear fibrous capsule (B, H&E × 10). Case 5 showed that a small amount of spindle cells are set in a predominantly collagenous stoma, called fibrous subtype (C, H&E × 100). Case 15 showed obvious myxoid stromal changes, with compact and deeply stained spindle-shaped nucleus, which was called myxoid subtype (D, H&E × 400). Case 18 contained numerous branching dilated fissures formed by flattened spindle cells, without visible red blood cells, resembling vascular channels, called pseudoangiomatous subtype (E, H&E × 10). Case 14 showed the apparent adipose tissue, only with scattered spindle cells and rope-like collagen fibers, called fat-rich type (F, H&E × 200), while case 17 showed a large number of spindle cells, with a small proportion of fat components (less than 5%) (G, H&E × 400), and case 6 was free of adipose tissue, called low-fat subtype (H, H&E × 200).

Figure 3

Immunohistochemical findings of SCL. Immunohistochemical staining revealed that CD34 (A, IHC × 100) and vimentin (B, IHC × 200) were strongly positive in 40 cases of SCL. Non-fat spindle cells expressing S-100 protein with a cytoplasmic and nuclear pattern in 6 cases, of which case 18 was strongly positive for S-100 (C, IHC × 200), while case 20 was partly positive for S-100 (D, IHC × 200).

Figure 4

FISH analysis of SCL. FISH with the RB1 deletion probe for case 1: a nucleus with heterozygous deletion of the RB1 gene (one red signal) (A). FISH with the MDM2 (12q15) gene amplification in ten cases were all negative (red single/green single < 2) (B).