You are not required to use MARSSIM. However, it is a technical best practices document. Additionally, it has multi-agency consensus; the Department of Defense (DOD), the Department of Energy (DOE), the Nuclear Regulatory Commission (U.S.NRC), and the EPA all endorse the document. Many state regulatory programs have also adopted the use of MARSSIM for demonstrating dose-based, or risk-based, compliance.
For more information about MARSSIM, visit MARSSIM on EPA.gov.