It depends.

The IRS gives many definitions of what is and is not a medical expense.  I think clothing items that were required due to the mastectomy (mastectomy bras, etc) would fall into the category below:

• Special supplies, equipment, devices and services relating to a medical condition or impairment. The cost of eyeglasses and contact lenses, hearing aids, dentures and false teeth, artificial limbs, prosthetic devices, crutches, orthopedic shoes, wheelchairs, diabetic testing materials, elastic stockings, oxygen and oxygen equipment, an air conditioner, humidifier or air cleaner to benefit a sick person, special telephone equipment and television close-captioning equipment for the hearing-impaired, guide dogs or other animals aiding the blind, deaf and disabled, X-rays, blood and other diagnostic tests, etc. are deductible. The cost of maintenance and upkeep of these items, such as hearing aid batteries and repairs and contact lens solutions, is also deductible.

Items that were not medically necessary would most likely not be deductible.  

The IRS allows you to deduct qualified medical expenses that exceed 7.5% of your adjusted gross income for 2017 and 2018.  You can add medical expenses under "deductions and credit."