Merkel cell cancer classification
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Classification
Histologically, MCC has been classified into three distinct subtypes:
- Trabecular: classic pattern, large-cell type, high density or granules on ultrasound examination.
- Intermediate: solid pattern (most common).
- Small cell: diffuse, few high density granules on ultrasound examination (second most common).
Staging
- According to the [TNM staging system], there are four stages of Merkel cell carcinoma based on the tumor size, presence of lymph nodes and metastasis. Each stage is assigned a letter/number.
- The stages of Merkel cell carcinoma are shown in the table below:
| Stage | Primary Tumor | Lymph Node | Metastasis |
| 0 | In situ primary tumor | No regional lymph node metastasis | No distant metastasis |
| IA | Less than or equal to 2 cm maximum tumor dimension | Nodes negative by pathologic exam | No distant metastasis |
| IB | Less than or equal to 2 cm maximum tumor dimension | Nodes negative by clinical exam* (no pathologic node exam performed) | No distant metastasis |
| IIA | Greater than 2 cm tumor dimension | Nodes negative by pathologic exam | No distant metastasis |
| IIB | Greater than 2 cm tumor dimension | Nodes negative by clinical exam* (no pathologic node exam performed) | No distant metastasis |
| IIC | Primary tumor invades bone, muscle, fascia, or cartilage | No regional lymph node metastasis | No distant metastasis |
| IIIA | Any size tumor (includes invading tumors) | Micrometastasis (diagnosed after sentinel or elective lymphadenectom) | No distant metastasis |
| IIIB | Any size tumor (includes invading tumors) | "Macrometastasis (defined as clinically detectable nodal metastases confirmed by therapeutic lymphadenectomy or needle biopsy) OR In transit metastasis (a tumor distinct from the primary lesion and located either between the primary lesion and the draining regional lymph nodes or distal to the primary lesion) | No distant metastasis |
| IV | Any size tumor (includes invading tumors) | Any lymph node metastasis | Metastasis beyond regional lymph nodes |
Adapted from AJCC's Cancer Staging Manual 2009