Reine, every circumstance is different and fact specific. However, the duty to withdraw is on the books. You can throw a punch when cornered, however once you have your opponent off of you and you can withdraw, you must withdraw. If you decide to duke it out when you could have left, your going down for battery. The model jury instructions for self-defense in Massachusetts states
2. Duty to retreat
A person cannot lawfully act in self-defense unless he or she has exhausted all other reasonable alternatives before resorting to force. A person may use physical force in self-defense only if he (she) could not get out of the situation in some other way that was available and reasonable at the time. The Commonwealth may prove the defendant did not act in self-defense by proving beyond a reasonable doubt that the defendant resorted to force without using avenues of escape that were reasonably available and which would not have exposed the defendant to further danger.
You may consider any evidence about where the incident took place, whether or not the defendant might have been able to escape by walking away or otherwise getting to safety or by summoning help if that could be done in time, or by holding the attacker at bay if the means were available, or by some other method. You may consider whether the use of force reasonably seemed to be the only means of protection in the circumstances. You may take into account that a person who is attacked may have to decide what to do quickly and while under emotional strain.