Is this something we should be concerned about or is it just a myth?"
There is no evidence to support the claim that deodorants or antiperspirants cause cancer. This link was first suggested in an email hoax, and rumours have circulated ever since. Typically, scare stories suggest that aluminium in antiperspirants prevents us from getting rid of toxins in our sweat, which clog up lymph nodes and lead to breast cancer. But the details of this are wrong.
First, sweating does help your body get rid of toxins. Second, breast cancer starts in the breast and spreads to the lymph nodes, not the other way around. Third, there is no evidence that aluminium can lead to cancer. Some reports occasionally claim to have found aluminium, or other deodorant chemicals, in samples taken from breast tumours. But they usually involve a very small number of women, and they never compare levels of aluminium in the tumours to levels in other parts of the body, or to women who don’t have breast cancer. Without this information, these reports tell us nothing about deodorants and breast cancer risk. On the other hand, one study looked at 1600 women and found that those who use deodorant are no more likely to develop cancer than women who don’t.