I've been asked this question many times by women who visit BreastFree.org. Some wonder whether lumps and bumps left after surgery will smooth out. Others worry about concavity. For most, the answer seems to be that while some small changes may occur over time, it's unlikely your chest will undergo a dramatic transformation.
If you're slim, your ribcage may become visible after mastectomy surgery. Because you had little fat to begin with, once your breast tissue has been removed there may not be much padding between your skin and your rib cage. Usually, it's only after post-operative swelling resolves that you may notice the outline of your ribcage beneath your skin. Unfortunately, your chest is unlikely to plump up over time. As my surgeon explained it to me, fat cells don't migrate from other parts of the body to fill in the spaces. Fortunately, not all slim women have this issue, but some do.
If you're on the heavier side, you may find that you have too much tissue under your arm at the end of your incision. This phenomenon is known as a "dog ear." A dog ear can also occur at the other end of your incision, next to your sternum, though this is much less common. Slimmer women can have dog ears as well, but they're usually smaller and less bothersome. If you merely have little puckers at the ends of your incision line, these will likely flatten out over time, but larger dog ears generally won't resolve on their own.
I haven't been able to track down the origin of the term "dog ear," but basically a dog ear occurs because the elliptical incision used to remove a breast is closed in a linear fashion. This causes tissue to bunch up at the ends, especially under the arm. If your breast surgeon knows you don't want reconstruction, it should be possible to avoid or minimize dog ears, though your incision line would likely have to extend further under your arm.
Even if you wind up with dog ears, the good news is that they can be fixed. However, you will need minor surgery, preferably by a plastic surgeon. The procedure is normally done on an outpatient basis, during which the dog ear is excised. This usually results in a longer incision line, but a smooth result. Lumpiness caused by less-than-perfect surgical technique can also often be revised by a skilled plastic surgeon.
Another unwelcome surprise after your mastectomy swelling recedes can be some degree of chest concavity. This can vary with the type of mastectomy performed, the amount of body fat left on your chest after surgery, and even your underlying ribcage anatomy. Further, if you've had implant reconstruction and opted to have your implants removed, there's likely to be more pronounced concavity, as implants put pressure on the ribs and tend to compress them. This is especially true if you've had the your implants in for a long time.
The small amount of concavity that may occur after a simple mastectomy probably won't be noticeable in clothes even if you choose to go flat. If you wear breast forms, the concavity is easily corrected and should have no impact on your appearance in clothes.
Bottom line, while problems like lumpiness and dog ears probably won't disappear on their own, they may improve a bit over time and, if you're still unhappy, an outpatient revision by a plastic surgeon can leave you with a smooth, flat chest. Prominent ribs and concavity unfortunately won't diminish over time, but can be easily camouflaged. And for those of you who find yourselves with a smooth, flat chest after your post-op swelling goes down—you're fortunate, because it's likely to stay that way.
Note: I'm not a medical professional and the comments above should not be construed as medical advice.
To see some very moving images of women post-mastectomy, with and without reconstruction, visit The Scar Project.