Skin nonmelanocytic tumor
Neural tumors
TNM staging of Merkel cell carcinoma (AJCC 8th edition)

Topic Completed: 5 March 2019

Revised: 11 April 2019

Copyright: 2002-2019,, Inc.

PubMed Search: Merkel cell carcinoma [title] staging[title]

Robert E. LeBlanc, M.D.
Page views in 2019: 1,002
Page views in 2020 to date: 88
Cite this page: LeBlanc RE. TNM staging of Merkel cell carcinoma. website. Accessed January 26th, 2020.
Pathologic TNM staging of Merkel cell carcinoma, AJCC 8th edition
Definition / general
  • All primary cutaneous neuroendocrine carcinomas are covered by this staging system
  • Metastatic extracutaneous neuroendocrine carcinomas are not covered
Essential features
  • AJCC 7th Edition staging was sunset on December 31, 2017; as of January 1, 2018, use of the 8th Edition is mandatory
ICD coding
  • ICD-10: C4A - Merkel cell carcinoma
  • ICD-10: C7B.1 - Metastatic Merkel cell carcinoma or nodal presentation without known primary
Primary tumor (pT)
  • pTX: primary tumor cannot be assessed (e.g. curetted)
  • pT0: no evidence of primary tumor
  • pTis: in situ primary tumor
  • pT1: maximum clinical tumor diameter ≤ 2 cm
  • pT2: maximum clinical tumor diameter > 2 cm but ≤ 5 cm
  • pT3: maximum clinical tumor diameter > 5 cm
  • pT4: primary tumor invades fascia, muscle, cartilage or bone

  • Tumor diameter: measured as the largest diameter of the primary tumor; this is a clinical measurement
  • Gross or microscopic measurement should only be used when the clinical measurement is unavailable since tumor shrinkage during formalin fixation will result in underestimation of diameter
Regional lymph nodes (pN)
  • pNX: cannot be assessed (staging procedure not performed or previously removed)
  • pN0: no regional metastasis detected on pathological evaluation
  • pN1a(sn): microscopic lymph node metastasis detected by sentinel node biopsy, clinically occult
  • pN1a: microscopic regional lymph node metastasis following dissection, clinically occult
  • pN1b: microscopic confirmation of clinically detected regional lymph node metastasis
  • pN2: in transit metastasis without lymph node metastasis
  • pN3: in transit metastasis with lymph node metastases

  • Regional lymph nodes / sentinel lymph nodes: sentinel node(s) receive direct lymphatic drainage from the primary tumor site
  • Clinically occult: nodal metastasis detectable only by microscopic evaluation
  • Clinically detected: lymph node enlarged or abnormal by physical or radiologic examination
  • Isolated tumor cells: counted as metastases whether identified on H&E or by IHC
  • In transit metastasis: intralymphatic tumor deposits discontinuous from the primary lesion and located either between the primary lesion and the draining regional lymph nodes or distal to the primary lesion
Distant metastasis (pM)
  • pM0: no evidence of distant metastasis by clinical or radiological examination
  • pM1a: microscopic confirmation of metastases to skin, distant subcutaneous tissue or distant lymph nodes
  • pM1b: microscopic confirmation of metastases to lung
  • pM1c: microscopic confirmation of metastases to all other distant sites
AJCC pathological prognostic stage groups (pTNM)
    Stage group 0: Tis N0 M0
    Stage group I: T1 N0 M0
    Stage group IIA: T2 - T3 N0 M0
    Stage group IIB: T4 N0 M0
    Stage group IIIA: T1 - T4 N1a(sn) or N1a M0
    T0 N1b M0
    Stage group IIIB: T1 - T4 N1b - N3 M0
    Stage group IV: T0 - 4 N0 - N3 M1

  • Unknown primary tumor: patients presenting with metastatic Merkel cell carcinoma to a lymph node in the absence of a primary tumor have a significantly more favorable prognosis than patients with a primary tumor and a clinically detected nodal metastasis
  • Survival rates are similar to those of patients with a primary tumor and occult nodal metastases detected by sentinel lymph node biopsy (J Am Acad Dermatol 2013;68:433, Am J Surg 2013;206:752, J Am Acad Dermatol 2012;67:395)
Registry data collection variables
  • Largest tumor diameter (clinical measurement in millimeters)
  • Regional node status (clinically detected, pathologically detected or neither)
  • Unknown primary status (yes or no)
  • Tumor thickness (reported in millimeters)
  • Excision margin status (tumor base transected or not transected)
  • Profound immunosuppression (no conditions, HIV / AIDS, solid organ transplant recipient, CLL / SLL, other non-Hodgkin lymphoma, multiple conditions or condition NOS)
  • Lymphovascular invasion (present or absent)
  • Merkel cell polyomavirus positive staining by IHC (yes, no or not applicable)
  • p63 positive staining by IHC (yes or no)
  • Tumor infiltrating lymphocytes in primary tumor (not present, present nonbrisk or brisk)
  • Growth pattern of primary tumor (circumscribed, nodular or infiltrative)
  • Extranodal extension in regional lymph nodes (yes or no)
  • Tumor nest size in regional lymph nodes (greatest dimension of largest aggregate in millimeters)
  • Isolated tumor cells in regional lymph node (yes or no)
  • Eyelid tumor (no, yes upper lid, yes lower lid or yes involving both lids)
  • Eyelid tumor involving eyelid margin, defined as the juncture of eyelid skin and tarsal plate at the lash line (no, yes non full thickness or yes full thickness)

  • Tumor thickness: measure from the surface of the epidermal granular layer to the point of maximum tumor thickness at a right angle to adjacent epidermis
  • Tumor infiltrating lymphocytes: considered brisk when the inflammation diffusely infiltrates the tumor or is present around the entire base of the tumor
Histologic grade (G)
  • Not used in Merkel cell carcinoma staging
Histopathologic type
  • Not used in Merkel cell carcinoma staging
Board review question #1
    Which of the following Merkel cell carcinoma is associated with the worst prognosis?

  1. Primary tumor involves dermis and there are clinically detected regional lymph node metastases
  2. Primary tumor involves fat and there are no regional lymph node metastases
  3. Primary tumor involves muscle and there are no regional lymph node metastases
  4. Metastatic Merkel cell carcinoma is present in a lymph node but no primary tumor is identified
Board review answer #1
A. The presence of nodal metastases upstages the carcinoma regardless of primary tumor attributes. Patients with clinically detected nodal metastases in the absence of an identifiable primary Merkel cell carcinoma (Stage IIIA) fare significantly better than patients with a known primary tumor and clinically detected regional node metastases (Stage IIIB).

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