How can I fix sagging skin on my back after a mastectomy?

Sometimes the transition between a healed mastectomy and surrounding tissue can be associated with bunching and irregularity of the skin whether reconstruction was done or not. It’s quite possible that a scar revision or surgical contouring of the surrounding tissue can be done. Laser is unlikely to be effective.

It is not uncommon to have excess skin and subcutaneous tissue lateral to the breast after mastectomy.  Sometimes this lateral excess is mainly skin, as you seem to be describing, due to removal of the adjacent breast tissue and preservation of nearby skin.  In that case, the loose skin can be directly excised with a remaining straight-line scar in its place.  
Sometimes there is also a copious amount of subcutaneous tissue in the lateral excess, which was present previous to the mastectomy but after removal of the breast volume appears to be more prominent because it is now more visible.  This occurrence is especially true when no reconstruction was performed.  In the case of subcutaneous tissue excess, the subcutaneous tissue can be liposuctioned and/or directly excised with the excess skin.  I would recommend discussing your concerns with your breast surgeon and have he or she recommend a plastic surgeon for consultation.
Ziyad Hammoudeh, MD

It is not uncommon to have excess skin and soft tissue lateral to the breast after mastectomy. This tissue is not breast tissue but joins the breast and generally looks fine before mastectomy. Once the breast is removed- especially without reconstruction, this tissue becomes more prominent. The tissue can be reomoved bysurgical excision (will result in scar),  liposuction, or cool sculpting and I recommend you consult with a breast surgeon or plastic specialist. You deserve to be happy with your body. Good luck and take care.