A tissue biopsy allows direct microscopic examination of the actual cells in question, which makes it the definitive method for diagnosing cancer and many other diseases. It can identify:

*Whether a lesion is cancerous (malignant) or non-cancerous (benign)
*The specific type and subtype of cancer, based on how the cells look and behave
*The grade of the cancer, which reflects how abnormal the cells appear and how quickly they are likely to grow
*Gene mutations and molecular markers within the tumour, such as changes in EGFR, KRAS, BRAF, or HER2, which guide treatment selection
*Protein expression patterns on the surface of cells, tested through specialized staining techniques
The tissue architecture and how far abnormal cells have spread into surrounding structures, which a blood-based or fluid-based test cannot show

Because the pathologist is examining actual tissue structure, not just circulating fragments, a tissue biopsy also confirms the physical presence and exact location of disease, which is central to establishing an initial diagnosis.