What does a Ki-67 stain indicate in tumor analysis?

A Ki-67 stain is crucial in tumor analysis because it marks the proliferation index of tumor cells. Ki-67 is a nuclear protein expressed in cells during the active phases of the cell cycle, specifically in G1, S, G2, and mitosis, but absent in the resting phase (G0). The presence and intensity of Ki-67 staining provide insights into how many cells within a tumor are actively dividing, which correlates with the aggressiveness of the tumor. A higher Ki-67 index indicates a higher proportion of proliferating cells, suggesting a more aggressive tumor behavior and potentially informing treatment decisions.

Other options, while related to various aspects of tumor pathology, do not specifically pertain to what Ki-67 staining reveals. Cell differentiation status, for instance, assesses how much tumor cells resemble normal cells but does not indicate the growth rate. Anaplasia refers to the loss of differentiation and is characterized by features such as pleomorphism and abnormal mitotic figures, which are distinct from what Ki-67 reflects. Lastly, tumor grade is a classification based on differentiation and other histopathological features, but the Ki-67 index specifically contributes to understanding proliferation versus the overall grading system of the tumor.