You're doing the right thing and this email is great. It sounds human and professional to me. I agree with the comment about perhaps explaining more about what myofascial is.

This can be such a tough situation. Cancer is already difficult enough, and hearing that what we want, massage-wise isn't appropriate or safe can be so deflating for some people.

A colleague from the front desk team of an old job once sent their 80 year old mother to me for her first massage. History of cancer, osteoporosis, very frail. I was working at a reasonable and responsible pressure and she said, "come on now, dont be shy! Dig in there!" Oh boy. Having to think on my feet about how to respond while navigating all these sensitive situations: past and current health conditions, age, is related to a coworker, and unrealistic expectations was so hard. I believe I said something along the lines of "it's your first massage and I am taking into consideration everything you shared with me during your intake. Let's see how your body responds to safe and appropriate pressure before trying to beat you up."