Fat-forming variant of solitary fibrous tumour of the orbit: the entity previously known as lipomatous haemangiopericytoma

Sir,

The fat-forming variant of solitary fibrous tumour (SFT) previously known as lipomatous haemangiopericytoma is a distinctive soft tissue tumour composed of haemangiopericytoma-like areas interspersed with mature adipose tissue. Around 40 cases have been reported in the literature.¹ These tumours usually occur in deep soft tissue and only three orbital cases have been described.^{1, 2, 3}

Case report

A 49-year-old woman presented with acute left eye pain, epiphora, and proptosis. There was a history of thyroid eye disease and bilateral proptosis that had required orbital decompression and low-dose radiotherapy 6 years previously.

On examination, there was proptosis of the left eye (displaced 10 mm anterior to the right). The visual acuity was 6/6 and colour vision was normal. There was no relative afferent pupil defect and visual fields were full.

A computerised tomography scan showed a 3.5 cm intraconal soft tissue mass with some heterogeneous attenuation displacing the globe anteromedially. The patient underwent excisional biopsy via a lateral orbitotomy.

The specimen consisted of a firm ovoid mass that on cut section displayed distinct yellow areas (Figure 1a). Microscopy showed a tumour composed of spindle-shaped cells with a prominent branching vascular pattern with lipomatous areas throughout the tumour (Figure 1b and c). Immunohistochemical staining was positive for CD34, CD99, and Bcl2 (Figure 1d). These features are characteristic of fat-forming variant of SFT. Postoperatively her proptosis resolved and her visual acuity is maintained.

Figure 1

(a) The orbital mass was well circumscribed with pink and yellow areas. (b) Histology of the tumour displaying cellular areas with staghorn branching blood vessels (v) and variable amounts of fat (f). (H&E, original magnification × 40). (c) On higher magnification, the tumour is composed of bland spindle-shaped cells with interspersed adipocytes (arrows). (H&E, original magnification × 400). (d) On immunohistochemical staining, the tumour cells are positive for CD34 (CD34, original magnification × 40).

Comment

Fat-forming variant of SFT is a rare tumour usually occurring in middle age. Almost all cases reported to date have behaved in a benign manner and no cases have recurred despite microscopic evidence of positive margins.^{1, 2} However, it is important to be aware of this entity, which may be mistaken for well-differentiated liposarcoma, particularly in small biopsies.⁴

This tumour was recognised as a variant of SFT in a recent update of the World Health Organization (WHO) classification of soft tissues.⁵ This group of tumours is one of the more common orbital soft tissue tumours that show morphological similarities and an identical immunoprofile. Previously considered separate entities hamangiopericytoma and giant cell angiofibroma are now also included in this group and have been named cellular and giant cell-rich variants of solitary SFT, respectively.⁶