Orbital mass in a young adult
This is a low power image. The tumour is solid (as opposed to eg cystic or nested). There are prominent vascular spaces with an irregular “staghorn” profile.
In the annotated image, I’ve outlined a few of the irregular vessels in green.
In this higher power field, we see the irregular blood vessels. There are trabeculae of tumour cells with oval nuclei and little cytoplasm. In the spaces between the trabeculae, there is blood.
In the annotated image, I’ve again outlined a couple of vessels in green, and marked on top of a few trabeculae in blue. Note that the blood vessels are lined by (vascular) endothelium, and that the blood-filled channels between the trabecular are not lined by endothelium.
In a different field, we see collagen deposition. Tumour cells are more spindled in this field, with haphazard distribution (“patternless pattern”) and elongated nuclei. The appearance isn’t as compact as in the previous image, and there is plenty of cytoplasm.
Again, I’ve outlined a couple of blood vessels in green. I’ve marked on top of the collagen deposits in blue.
In the diagnostic process, I’d start off by broadly viewing this as a spindle cell tumour. In our department, these are relatively rare in the orbit (much rarer than epithelial tumours or lymphomas). In adults, the differential diagnosis includes:
solitary fibrous tumour – probably the commmonest orbital spindle cell tumour that we see. However, we only get one every 2-3 months on average
neural tumours – schwannoma and neurofibroma. We get maybe 2-3 of these per year (in the orbit). Malignant peripheral nerve sheath tumour is incredibly rare. I’ve never diagnosed one although we may have a case in our departmental archives
meningioma – we get a couple of these per year. Although the appearance is typically nested, it can have a solid spindle cell appearance
spindle cell lipoma – we don’t even get one a year
melanoma – even though this case doesn’t look like one on the H&E slides, I would always keep it in mind
In this case, my favoured diagnosis on H&E would be solitary fibrous tumour. This is because of the collagen deposition, compact haemangiopericytoma-type pattern (in the second image) and lack of other specific features. I requested immunohistochemistry as below.
This is immunohistochemistry for CD34. CD34 is expressed in a number of normal cells, including vascular endothelium, haematopoietic stem cells and corneal stroma. It can also be expressed in various tumours, including soft tissue tumours such as spindle cell lipoma and solitary fibrous tumour.
Caution! Immunohistochemistry must be interpreted in the context of clinical information, morphology and immunohistochemistry pattern: a single immunostain doesn’t magically make the diagnosis for you.
This is immunohistochemistry for STAT6, which is a relatively new antibody compared to CD34. It is stated to be expressed in nuclei of solitary fibrous tumour. In this field, we see nuclear positivity although it’s not all that strong.
The CD34 and STAT6 support my initial impression of solitary fibrous tumour. This case does not show histologically worrying features (eg anaplasia, frequent mitoses). But since it is incompletely excised, it is likely to recur.
Here are a few open access articles on clinical and radiological findings, histopathology and immunohistochemistry of solitary fibrous tumour.
Solitary Fibrous Tumor of the Orbit: A Case Report and Review of the Literature by Genc et al (2015). This is a PDF article
Role of Immunohistochemistry in the Diagnosis of Solitary Fibrous Tumor, a Review by Geramizadeh et al (2016)
Solitary fibrous tumor of the orbit: Computed tomography and histopathological findings by Sayit et al (2019)