For an insured member who is receiving an active course of medical treatment from a health care provider for a serious disease, that if disrupted in the course of medical treatment would pose an undue hardship to the patient and began this active course of treatment before being enrolled in a reduced selective network plan where the provider is not part of the reduced or selective network or began this active course of treatment before being enrolled in a tiered network plan where the provider is in the highest cost-sharing tier, the carrier shall provide coverage for those medically necessary and covered services that are part of that active course of treatment provided by that health care provider. See definition section Mass. Gen. Laws ch. 176J, § 1.