What Causes Repeated Strikes To The Chest During Martial Arts?

Hello. I'm a 42 year old male. Through training for martial arts, I sometimes take repeated strikes to the breast/ chest area. Also, there are some advanced techniques which involve jabbing fingers under the armpits and attacking muscles/nerve points in places such as the armpits. I am worried that although this is not a regular occurance and is certainly not part of weekly training, when this does happen, if I'm puttiong myself at risk of causing cancer to either the breasts or lymph nodes under the armpits?