Moles usually grow back after removal because the entire mole hasn't been completely removed. This can happen for several reasons including removing inadequate margins during excision, not treating deep enough into the skin during radiofrequency surgery or shave biopsy, and not enough treatments or incorrect settings in laser or IPL treatment. It's not dangerous for a mole to reappear after treatment, although if there's any possibility that the "mole" was really a skin cancer, re-treatment is usually recommended.
In most cases, mole removal by any surgical or non-surgical method results in permanent removal of the mole, as long as the procedure is performed correctly.
Moles sometimes grow back after removal if all the mole cells have not been removed. The remaining mole cells can multiple and lead to reappearance. This happens for several reasons:
- If a mole is surgically excised (cut out), for example in the case of removing moles that are suspected skin cancer, the excision margins may have been too narrow
- If a mole is treated with laser or IPL using incorrect settings, for example if a mole is treated during laser hair removal, the light wavelength or intensity may not be sufficient to destroy all the mole cells
- If a mole or other skin lesion is frozen with cryotherapy, it often requires up to 30 seconds of freezing time for the freezing temperature to penetrate the entire lesion. If the lesion is not treated for this long, there's a higher risk of regrowth.
- If a mole is removed using radiofrequency surgery, the doctor may not be able to see any remaining mole tissue due to colour changes in the treated skin, caused by the procedure. The doctor will normally try to remove or damage the minimum possible amount of skin during the procedure and may decide not to further treat brown or charred tissue because they can't see if there are any remaining mole cells
Inadequate treatment may be the result of the treating practitioner trying to reduce the pain of the procedure (e.g. during cryotherapy), minimise scarring (e.g. during excision or radiofrequency surgery) or because it's technically difficult to remove the lesion with a wide margin (e.g. when performing an excision on the lower legs or some parts of the face).
If a mole or other skin lesion reappears after treatment, it's not dangerous. The treatment does not turn the mole or lesion into a skin cancer. There's usually no medical reason to re-treat the lesion, although if it was removed because it was suspicious for skin cancer then the doctor will normally re-excise it with a wider margin.