Path Quiz Case- 18

                             Case history and images:

 

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Alveolar soft part sarcoma is a slow growing but, highly malignant tumour.

Histogenesis of this tumour has not been established. Some have supported the idea that the tumour represents a distinct variant of rhabdomyosarcoma. However, subsequent studies have failed to demonstrate rhabdomyoblastic differentiation.

Age- 

Adolescents and young adults and rarely in children.

Site-

The tumour presents as a deeply located mass in the lower limbs or limb girdle. Rarely these are  located in the mediastinum and female genital tract. In children the tumour is usually located in the head and neck region.

Gross- 

Poorly circumscribed soft and friable tumour. Cut section reveals yellowish white to greyish red areas. Necrosis and haemorrhagic areas are usually present.

Microscopic features- Image Link1 ; Image Link2 ; Image Link3 .

- Dense fibrous trabeculae divide the tumour into compartments of irregular sizes.
- These are further divided into nests or islands of tumour cells. These islands are separated by thin walled vascular channels.
- The tumour cells are large, round  to oval shaped, with eosinophilic granular cytoplasm ,eccentric rounded nuclei and prominent nucleoli.
- The cellular aggregates show central degeneration and loss of cohesion, resulting in pseudoalveolar pattern.
- Presence of PAS positive and diastase resistant intracytoplasmic granules and crystalline rods. Image Link4 .
- Pleomorphism may be present focally and mitotic figures are scarce.
- Dilated veins are noted at the margin. Vascular invasion is common.
- Rarely paraganglioma-like or pseudoglandular patterns and psammomatous calcification may be present.
- In children the tumour is characterized by uniform sheets of large granular cells without  nest-like arrangement. This variant has a better prognosis.    

Immunohistochemistry:

Some cases  stain positively with desmin, muscle specific actin, sarcomeric actin, S100 protein and neuron specific enolase.

There is inconsistent demonstration of MyoD1.

PAS positive diastase resistant cytoplasmic granules are strongly reactive to monocarboxylate transporter1(MCT1) and CD147.                                

Cytogenetics:  Rearrangement of 17q 25                                                       

Indicator of poor prognosis:

(i) Older age  (ii)Tumour size greater than 10cms.

Metastasis: 

The tumour usually metastasizes to the lungs and brain.

Brain metastasis is a common feature of Stage IV alveolar soft part sarcoma and routine intracranial imaging has been recommended as part of the staging evaluation in all patients with ASPS.

The tumour may even metastasize 30 years after excision of the primary tumour.

Differential diagnosis-

-Metastatic renal cell carcinoma
-Metastatic melanoma
-Malignant granular cell tumour
(Appropriate immunohistochemical examination helps in confirming the diagnosis)

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