I am also a massage therapist (specialize in rehab) and she thought your question was more surgery relevant than cancer relevant, so she tossed it to me.
My pre-surgery suggestion is to do some myofascial release on the lats so that there are no adhesions. This may reduce surgical trauma.
For post-surgery mastectomy patients, focus on scar tissue, adhesions, and the rotator cuff muscles. Having the chest wall musculature removed or scraped can be traumatic and often results in frozen shoulder from the patient babying the affect side(s). Passive mobilization is also advised.
I postpone post-surgical therapy until:
• Sutures/staples are removed
• Scars are dry and healing
• Physician has given clearance