What is the difference between typical and atypical carcinoid?

Difference in Grade: These are two terms used by pathologists to make a distinction between well differentiated carcinoid(called typical) and less than well differentiated or high grade carcinoid(called Atypical carcinoid) which is more common in bronchial carcinoids. One reason for making this distinction is for chemotherapy which is not useful in typical but tends to be more effective in Atypical carcinoids