A lipoblastoma-like tumour of the paratesticular region - male counterpart of lipoblastoma-like tumour of the vulva
Abstract
Lipoblastoma-like tumour of the vulva is a rare mesenchymal neoplasm exhibiting histological features similar to a lipoblastoma, myxoid liposarcoma and spindle-cell lipoma, but lacking PLAG1 or DDIT3 rearrangements, and 13q alterations.1 Lipoblastoma-like tumour has previously only been described in the vulva. Here we describe a tumour arising in the paratesticular region of a 20-year-old male with histological features of a lipoblastoma-like tumour of the vulva.
The patient presented with a large, solid left-sided groin mass which was clinically distinct from the testicular cord. On magnetic resonance imaging, the tumour appeared to arise just beneath the superficial tissues, lying outside the pelvis.
The radiological features were suspicious for sarcoma. Histological examination revealed a tumour showing a lobular architecture comprising spindle-cells with occasional mature adipocytes embedded in a myxoid matrix with a fine capillary network (Figure 1A,B).
Cells with lipoblastic differentiation were identified (Figure 1C). There were no areas of necrosis.
CD99 immunohistochemistry was positive. S100 showed positive staining in the mature adipocytes and lipoblasts, and very occasional nuclear positivity in the spindle-cell component (Figure 1D).
CD34 showed focal positivity. Smooth muscle actin (SMA), desmin, epithelial membrane antigen, anti-cytokeratin antibody (MNF116), CD117, signal transducer and activator of transcription 6 (STAT6), oestrogen receptor and progesterone receptor were negative.
Ki67 stained fewer than 1% of tumour cells. The morphology of the tumour, together with its location, raised the possibility of a well-differentiated liposarcoma and myxoid liposarcoma.
However, fluorescence in-situ hybridisation (FISH) for MDM2 amplification and CHOP (DDIT3) rearrangement were performed, and these were negative. Although the patient was aged 20 years we considered lipoblastoma in the differential diagnosis, as it can rarely occur in this age group.
We also wanted to exclude spindle-cell lipoma and cellular angiofibroma. A next-generation sequencing panel (a customized version of Archer FusionPlex Sarcoma kit) looking for fusions in a range of genes associated with soft tissue tumours - including PLAG1 and NCOA2 - found no abnormalities.
Retinoblastoma-associated protein (RB1) immunohistochemistry showed patchy loss of expression but FISH analysis with forkhead box O1 (FoxO1) probe for 13q14 region showed no deletion. We concluded that the morphological, immunohistochemical and cytogenetic results would be consistent with a lipoblastoma-like tumour of the vulva.
Lipoblastoma-like tumour of the vulva was first described by Lae et al. in 2002.2 In their report of three cases, they described a lobular neoplasm composed of uniform, slender, spindle-shaped cells, together with entrapped clusters of adipocytes, in a rich myxoid stroma with a chicken-wire capillary vascular network. They found the tumours were negative for CD34, SMA and desmin.
Mirkovic and Fletcher further characterised this tumour, analysing eight cases.3 They found a lack of PLAG1 expression, separating this entity from lipoblastoma, and a lack of CHOP (DDIT3) rearrangement, excluding myxoid liposarcoma. MDM2 and CDK4 immunohistochemistry was negative in five of six of the tumours tested, and one showing only focal staining.
Two of five tumours examined showed CD34 expression. They discovered loss of RB1 expression, leading them to postulate that this tumour might be related to spindle-cell lipoma.
A more recent study by Schoomeester et al., however, showed that despite the loss of RB1 expression, unlike spindle-cell lipoma these tumours show no structural abnormality in 13q. Genomic copy number analysis by chromosomal microarray showed a normal diploid profile in 86% of tumours.1 Follow-up of the patients in Lae's and Mirkovic's studies revealed no distant metastases, and only one local tumour recurrence, suggesting benign behaviour.
All the tumours described in these studies occurred in the vulva, with a size between 3.5 and 15 cm. Our case appears to be the first described instance of this entity arising outside the vulva.
This is an important - albeit rare - diagnosis of which pathologists should be aware, considering the similar morphological features to the other soft tissue tumours described above, and the different prognostic implications of each tumour.