Hi JT. I'm sorry that you find yourself here, but welcome.
Perhaps you could ask your GP for a referral to another breast surgeon/oncologist for a second opinion. There's no harm in hearing another point of view and it might clarify things for you.
Is there a history of breast cancer in your family? Has your specialist spoken to you about genetic testing? These things can sometimes influence the doctors and ourselves when it comes to clinical decisions. Your age can also be an influencing factor, as can the grade of the DCIS.
How do you feel about having a mastectomy? What is your gut telling you?