What's the accuracy of histology in diagnosing malignancies? how often do you have to go back for another sample?

Depends on tissue: Histology is the study of tissues under the microscope. Most cancer biopsies are reviewed under the microscope after staining and have characteristic signs of malignancy. If the biopsy sample is inadequate, does not include normal tissue borders, or involves tissues that are difficult to diagnose by histology alone then further samples or other tests may be required.