临床医生最常接触的是肿瘤的分期（staging of cancer），而病理报告中常常会出现肿瘤病理分级（grading of cancer）的报告。肿瘤的分级指的是在显微镜下看到的肿瘤细胞与正常细胞、肿瘤结构与正常结构相比所表现出来的不正常（abnormal）的程度。可以反应肿瘤的生长和侵袭速度。常见的反应这种不正常程度的描述性的词语有如“分化良好”、“未分化”、“分化差”等，体现分级的结论性用于常为“低级别”、“高级别”等。
肿瘤的病理分级有病理科医生通过镜下检查活检组织获得。和肿瘤分期类似，不同的肿瘤也会具有不同的分级策略。但是大体而言，根据肿瘤与正常组织和细胞的差异程度，分为“1、2、3、4”四个等级，如无特指，常用如下的分级系统：
- GX：Grade cannot be assessed (undetermined grade)
- G1: Well differentiated (low grade)
- G2: Moderately differentiated (intermediate grade)
- G3: Poorly differentiated (high grade)
- G4: Undifferentiated (high grade)
一些具有特殊的分级系统的肿瘤举例：
- 乳腺癌分级系统
Doctors most often use the Nottingham grading system (also called the Elston-Ellis modification of the Scarff-Bloom-Richardson grading system) for breast cancer. This system grades breast tumors based on the following features:
- Tubule formation: how much of the tumor tissue has normal breast (milk) duct structures
- Nuclear grade: an evaluation of the size and shape of the nucleus in the tumor cells
- Mitotic rate: how many dividing cells are present, which is a measure of how fast the tumor cells are growing and dividing
Each of the categories gets a score between 1 and 3; a score of “1” means the cells and tumor tissue look the most like normal cells and tissue, and a score of “3” means the cells and tissue look the most abnormal. The scores for the three categories are then added, yielding a total score of 3 to 9. Three grades are possible:
- Total score = 3–5: G1 (Low grade or well differentiated)
- Total score = 6–7: G2 (Intermediate grade or moderately differentiated)
- Total score = 8–9: G3 (High grade or poorly differentiated)
- 前列腺癌分级系统
The Gleason scoring system is used to grade prostate cancer. The Gleason score is based on biopsy samples taken from the prostate. The pathologist checks the samples to see how similar the tumor tissue looks to normal prostate tissue. Both a primary and a secondary pattern of tissue organization are identified. The primary pattern represents the most common tissue pattern seen in the tumor, and the secondary pattern represents the next most common pattern. Each pattern is given a grade from 1 to 5, with 1 looking the most like normal prostate tissue and 5 looking the most abnormal. The two grades are then added to give a Gleason score. The American Joint Committee on Cancer recommends grouping Gleason scores into the following categories:
- Gleason X: Gleason score cannot be determined
- Gleason 2–6: The tumor tissue is well differentiated
- Gleason 7: The tumor tissue is moderately differentiated
- Gleason 8–10: The tumor tissue is poorly differentiated or undifferentiated
参考资料：American Joint Committee on Cancer. AJCC Cancer Staging Manual. 7th ed. New York, NY: Springer; 2010.