A 35-year-old woman from Delhi presents with a 6-month history of a rapidly enlarging thyroid mass associated with dysphagia and dyspnea. On examination, she has a hard, fixed thyroid mass with ipsilateral cervical lymphadenopathy. TSH is normal. Core needle biopsy shows sheets of atypical cells with high mitotic activity, tumor necrosis, and loss of follicular architecture. Immunohistochemistry is positive for p53 and negative for thyroglobulin. What is the most likely diagnosis?

Explanation

## Diagnosis: Anaplastic Carcinoma of Thyroid ### Clinical Presentation The rapid growth, dysphagia, dyspnea, and fixed mass with cervical lymphadenopathy are hallmark features of anaplastic carcinoma. This is the most aggressive thyroid malignancy, with a median survival of 3–6 months if untreated. **High-Yield:** Anaplastic carcinoma accounts for only 1–2% of thyroid cancers but is responsible for up to 50% of thyroid cancer deaths due to its aggressive behavior. ### Pathological Features | Feature | Anaplastic Carcinoma | |---------|---------------------| | Differentiation | Undifferentiated (loss of follicular architecture) | | Cell type | Spindle cells, giant cells, or pleomorphic cells | | Mitotic activity | Very high (brisk) | | Necrosis | Prominent tumor necrosis | | Capsular invasion | Present (often with extrathyroidal extension) | | Lymph node involvement | Common | | Distant metastases | Frequent (lung, bone, brain) | ### Immunohistochemistry **Key Point:** Anaplastic carcinoma is **negative for thyroglobulin and thyroid peroxidase (TPO)** because it has lost differentiation. The positive p53 indicates loss of p53 tumor suppressor function, a hallmark of anaplastic transformation. **Clinical Pearl:** Anaplastic carcinoma often arises from pre-existing differentiated thyroid cancer (papillary or follicular) through a process of dedifferentiation. This explains why some patients have a prior history of thyroid cancer. ### Diagnostic Criteria 1. **Undifferentiated histology:** Loss of follicular pattern, sheets of atypical cells 2. **High mitotic rate:** Brisk mitotic activity with abnormal mitoses 3. **Tumor necrosis:** Extensive areas of necrosis 4. **Negative for thyroglobulin:** Loss of thyroid differentiation markers 5. **Positive for p53, TP53 mutations:** Indicates malignant transformation ### TNM Staging **All anaplastic carcinomas are classified as Stage IV:** - **Stage IVA:** Intrathyroidal disease (surgically resectable) - **Stage IVB:** Gross extrathyroidal extension - **Stage IVC:** Distant metastases ### Management Algorithm ```mermaid flowchart TD A[Anaplastic Carcinoma Diagnosed]:::outcome --> B{Resectable?}:::decision B -->|Yes, Stage IVA| C[Total thyroidectomy + lymphadenectomy]:::action B -->|No, Stage IVB/IVC| D[Unresectable]:::urgent C --> E[Adjuvant therapy]:::action D --> E E --> F[Chemotherapy + External beam RT]:::action E --> G[Targeted therapy if BRAF/RAS mutation]:::action F --> H[Palliative care]:::outcome G --> H ``` **Key Point:** Multimodal therapy (surgery + chemotherapy + radiation) offers the best chance of survival, though prognosis remains poor. ### Molecular Pathology - **TP53 mutations:** ~70% of cases - **BRAF V600E mutations:** ~25–40% of cases - **RAS mutations:** ~20–25% of cases - **PTEN loss:** ~20% of cases These mutations can guide targeted therapy (BRAF inhibitors, MEK inhibitors). [cite:Robbins 10e Ch 24]