Grading of carcinoma is most often done after a treating physician and/or surgeon obtains a sample of suspected tumor tissue using surgical resection, needle or surgical biopsy, direct washing or brushing of tumor tissue, sputum cytopathology, etc. A pathologist then examines the tumor and its stroma, perhaps utilizing staining, immunohistochemistry, flow cytometry, or other methods. Finally, the pathologist classifies the tumor semi-quantitatively into one of three or four grades, including:

Grade 1, or well differentiated: there is a close, or very close, resemblance to the normal parent tissue, and the tumor cells are easily identified and classified as a particular malignant histological entity;
Grade 2, or moderately differentiated: there is considerable resemblance to the parent cells and tissues, but abnormalities can commonly be seen and the more complex features are not particularly well-formed;
Grade 3, or poorly differentiated: there is very little resemblance between the malignant tissue and the normal parent tissue, abnormalities are evident, and the more complex architectural features are usually rudimentary or primitive;
Grade 4, or undifferentiated carcinoma: these carcinomas bear no significant resemblance to the corresponding parent cells and tissues, with no visible formation of glands, ducts, bridges, stratified layers, keratin pearls, or other notable characteristics consistent with a more highly differentiated neoplasm.Although there is definite and convincing statistical correlation between carcinoma grade and tumor prognosis for some tumor types and sites of origin, the strength of this association can be highly variable. It may be stated generally, however, that the higher the grade of the lesion, the worse is its prognosis.
Carcinoma of unknown primary site
The term carcinoma has also come to encompass malignant tumors composed of transformed cells whose origin or developmental lineage is unknown (see cancer of unknown primary origin; CUP), but that possess certain specific molecular, cellular, and histological characteristics typical of epithelial cells. This may include the production of one or more forms of cytokeratin or other intermediate filaments, intercellular bridge structures, keratin pearls, and/or tissue architectural motifs such as stratification or pseudo-stratification. ICD-10 code
(8010-8045) Epithelial neoplasms, NOS
(8050-8080) Squamous cell neoplasms
(M8070/3) Squamous cell carcinoma, NOS
(8090-8110) Basal cell neoplasms
(M8090/3) Basal cell carcinoma, NOS
(8120-8130) Transitional cell carcinomas
(8140-8380) Adenocarcinomas
(M8140/3) Adenocarcinoma, NOS
(M8142/3) Linitis plastica
(M8155/3) Vipoma
(M8160/3) Cholangiocarcinoma
(M8170/3) Hepatocellular carcinoma, NOS
(M8200/3) Adenoid cystic carcinoma
(M8312/3) Renal cell carcinoma
(M8312/3) Grawitz tumor
(8390-8420) Adnexal and Skin appendage Neoplasms
(8430-8439) Mucoepidermoid Neoplasms
(8440-8490) Cystic, Mucinous and Serous Neoplasms
(8500-8540) Ductal, Lobular and Medullary Neoplasms
(8550-8559) Acinar cell neoplasms
(8560-8580) Complex epithelial neoplasms

Carcinoma In situ
The term carcinoma in situ (or CIS) is a term for cells that are significantly abnormal but not cancer. They are thus not typically carcinomas. Pathogenesis
Cancer occurs when a single progenitor cell accumulates mutations and other changes in the DNA, histones, and other biochemical compounds that make up the cells genome. The cell genome controls the structure of the cells biochemical components, the biochemical reactions that occur within the cell, and the biological interactions of that cell with other cells.