OSU Department of Pathology

Anatomical Pathology Interesting Case Conference

Microscopic Description:

  • CD99 - Positive
  • SALL4 - Focal Positive
  • AE1/AE3 - Focal Positive
  • CD30 - Negative
  • OCT 3/4 - Negative
  • GFAP - Negative
  • SOX11 - Negative
Microscopic image possibly detecting cancerous areas in human tissue
Discussion:

A rare somatic-type malignancy arising from a teratoma, showing primitive embryonic/neuroectodermal differentiation and historically termed primitive neuroectodermal tumor (PNET).

In the WHO 5th edition (2022), the historical term PNET was replaced by embryonic-type neuroectodermal tumor (ENET) to avoid confusion with Ewing sarcoma; ENET lacks the characteristic Ewing sarcoma EWSR1::FLI1 fusion.

Somatic-type malignancy develops in approximately 3–6% of testicular germ-cell tumors; ENET/PNET is an uncommon subtype, and metastatic involvement is particularly important clinically.

Histologic features: Typically composed of primitive small round/embryonal cells, often with neuroectodermal differentiation such as rosette-like or pseudorosette structures and, in some cases, neuropil-like material.

Immunostains: Immunophenotype is variable; CD99 may be positive, while other neural/germ-cell markers may be variable. In this case: AE1/AE3 focal+, CD99+, SALL4 focal+ CD30−, OCT3/4−, GFAP−, SOX11−.

The combination of known teratoma + metastatic retroperitoneal tumor + primitive neuroectodermal morphology + compatible IHC supports ENET as a somatic-type malignancy arising in metastatic teratoma.

Clinical importance: ENET is important because somatic-type malignancies in metastatic teratoma can have an aggressive course and respond poorly to conventional platinum-based GCT chemotherapy with complete surgical resection.

References:

Primitive Neuroectodermal Tumor Arising in a Testicular Teratoma with Retroperitoneal Metastasis: Report of an Interesting Case with Review of Literature. https://www.sciencedirect.com/science/article/abs/pii/S0090429507019073

A Case Series of Transformation of Teratoma to Primitive Neuroectodermal Tumor: Evolving Management of a Rare Malignancy. https://pmc.ncbi.nlm.nih.gov/articles/PMC3977172/

An introduction to the WHO 5th edition 2022 classification of testicular tumours. https://pmc.ncbi.nlm.nih.gov/articles/PMC9544657/

Transformed Testicular Teratoma to Embryonic-Type Neuroectodermal Tumor With Metastasis to Mediastinum. https://pmc.ncbi.nlm.nih.gov/articles/PMC10725136/

Outcome analysis of patients with transformed teratoma to primitive neuroectodermal tumor. https://www.sciencedirect.com/science/article/pii/S0923753419400641

A neuroglial neoplasm in retroperitoneal lymph nodes: metastasis from a testicular teratoma with embryonic-type neuroectodermal tumour. https://doi.org/10.1016/j.mpdhp.2025.05.005

Metastatic malignant transformation of teratoma to primitive neuroectodermal tumor (PNET): results with PNET-based chemotherapy. https://pubmed.ncbi.nlm.nih.gov/23799289/

Case Images