my sister was diagnosed with In situ ductal carcinoma and she had a mastectomy and she was told that they found 5-6 nodes that had invasive cancer and she will be needing chemotherapy. in the beginning we were told that this was only preventative and no reason to worry. Why all of a sudden, chemo is required?
The doctors changed your sister's recommended treatment plan because they got new information when they found the positive lymph nodes. They probably didn't expect to find that when they first diagnosed her. As Angie points out, there are many different factors to consider when making decisions about chemo. I found my treatment plan changed more than once as I went through treatment and unexpected things happened.
Anytime the cancer has spread outside of the breast into a lymoh node chemo is usually recommended to prevent the cancer from spreading to other organs. It's a standard of care, and sometimes it's required even when the cancer has not spread to nodes. Sometimes it has to do with how agressive the cancer is, what type or stage, and even hormone status. Just because the breast was removed does not mean that the likelihood of it returning is nonexistant. I had a mastectomy with no node involvement, had chemo and my cancer still recurred 3 years later in my chest wall. Everyone is different, as is there cancer and treatment - but it's always better to be safe than sorry. Also the doctors follow trials that were done that proved to lower the risk of recurrence. If your sister is anti-chemo then I would recommend asking her doctor to lay out her risk with and without treatment.