The TNM classification calculator for Merkel cell carcinoma evaluates the cancer stage based on specific characteristics of the tumor, lymph node involvement, and presence of metastasis. Users provide information by selecting from predefined options across three categories:

1. Tumor Classification (T):
   - Tx: Primary tumor cannot be assessed
   - T0: No evidence of primary tumor
   - Tis: In situ tumor
   - T1: Tumor less than or equal to 2 cm in greatest dimension
   - T2: Tumor greater than 2 cm but less than or equal to 5 cm in greatest dimension
   - T3: Tumor greater than 5 cm in maximum dimension
   - T4: Tumor invading bone, muscle, fascia, or cartilage

2. Lymph Node Classification (N):
   - Nx: Regional lymph nodes cannot be assessed
   - N0: No regional lymph node metastasis
   - N1: Regional lymph node metastases
   - N2: In transit metastasis without lymph node metastasis
   - N3: In transit metastasis with lymph node metastasis

3. Metastasis Classification (M):
   - M0: No distant metastasis
   - M1: Distant metastasis present

The calculation process involves matching user selections to specific staging criteria:

- Stage 0 is identified when there is an in situ tumor (Tis) with no regional lymph node metastasis (N0) and no distant metastasis (M0).
- Stage I is determined by a small tumor (T1) with no nodal involvement (N0) and no distant spread (M0).
- Stage IIA includes scenarios where a medium-sized or large tumor (T2 or T3) is present without nodal involvement (N0) and no distant metastasis (M0).
- Stage IIB is characterized by a tumor invading deeper tissues (T4) but with no nodal involvement (N0) and no distant metastasis (M0).
- Stage III is assigned when there is any recognized tumor presence (T0 through T4) with regional nodal involvement (N1, N2, or N3) and no distant metastasis (M0).
- Stage IV is concluded if there is any evidence of tumor (T0 through T4), any level of nodal classification, and distant metastasis is present (M1).

This classification system provides an insight into the tumor's progression, helping to gauge the severity and guiding treatment decisions. Each stage corresponds to different levels of severity, affecting prognosis and therapeutic strategy.

The TNM classification for Merkel cell carcinoma is a clinical staging system developed by the American Joint Committee on Cancer (AJCC). It is designed for the stratification of patients with Merkel cell carcinoma, a rare and aggressive skin cancer, based on the extent of the disease. This classification system aids in guiding treatment decisions and providing prognostic information.

The primary components of the TNM classification for Merkel cell carcinoma include the size and extent of the primary tumor (T), the absence or presence and number of regional lymph nodes involved (N), and the absence or presence of distant metastasis (M). Each of these components is assigned a numerical value, with higher numbers indicating a more advanced disease state. 

The T category is determined by the size of the primary tumor and its level of invasion into surrounding tissues. The N category is determined by the number and location of regional lymph nodes involved, while the M category is determined by the presence or absence of distant metastasis. 

The final stage of the disease, ranging from stage I (early disease) to stage IV (advanced disease), is determined by combining the T, N, and M categories. This comprehensive staging system allows for a standardized comparison of patient populations and facilitates clinical research in Merkel cell carcinoma.