An incision is made and larger pieces of tissue are give to the pathologist. Any place the biopsy incision touches, contaminates those tissues with tumor cells. An excisional biopsy is where the surgeon removes the tumor completely before a diagnosis is established. The problem with this approach is that if the tumor ends up being cancerous and especially if it is larger than a few centimeters, the cancer is very difficult to treat after such an excision. The entire surgical incision is contaminated with tumor cells and the cells may be more prone to spread.
Excisional biopsies are best limited to the diagnosis of small superficial lesions of soft tissues.