This is a controversial category that occupies the gap between the well differentiated follicular and papillary thyroid carcinomas and the undifferentiated or anaplastic carcinomas.
The epidemiology is not well understood as the tumour is frequently not reported. It is thought to account for about 5% of thyroid malignancies.
The aetiology is unknown; some tumours seem to arise from well differentiated tumours whereas others are thought to arise independently.
Tumours frequently invade extensively and are more locally advanced than follicular or papillary carcinomas at diagnosis. Lymph node involvement is common. Distant metastases frequently occur, most commonly to liver or bone.
++ Clinical Features
Patients may present with a rapidly growing lump in the thyroid (occasionally from a previously stable mass). Presentation with symptoms related to metastatic disease is not uncommon. Thyroid hormone imaging shows a cold thyroid nodule. Bone scan or CT/MRI may provide evidence of distant metastases.
These tumours are usually larger than their well-differentiated cousins. They are usually white to grey and may contain necrotic areas.
The diagnosis is made microscopically, based on:
- Architecture – either an insular, trabecular or solid growth pattern
- Infiltrative growth pattern
- Lymphovascular invasion
Mitotic figures are common. Nuclei are cytologically abnormal but regular in size; some may show features of papillary carcinoma.
TTF1 and thyroglobulin staining is usually positive, but less evident than well differentiated malignancies. TP53 and Ki-67 are usually positive. E-cadherin expression is negative.
Major chromosomal abnormalities are more frequent in poorly differentiated carcinoma than follicular/papillary carcinomas, but less frequent than undifferentiated carcinoma. TP53 and the RAS genes are mutated in about 50% of cases.
Staging and Outcomes
Staging is conventional and not adjusted for age.
5 year survival is 50%, midway between the well-differentiated and undifferentiated carcinomas.