Mont. Code § 33-22-134
Each group and individual disability policy, certificate of insurance, or membership contract that is delivered, issued for delivery, renewed, extended, or modified in this state must provide coverage for hospital inpatient care for a period of time as is determined by the attending physician and, in the case of a health maintenance organization, also the primary care physician, in consultation with the patient, to be medically necessary following a mastectomy, a lumpectomy, or a lymph node dissection for the treatment of breast cancer. This section also applies to any employee group insurance program of a city, town, county, school district, or other political subdivision of the state, and any self-funded multiple employer welfare arrangement that is not regulated by the Employee Retirement Income Security Act of 1974.
§ 33-22-134, MCA