def save_userLUT(voidef, out_lut_file):
    """
    Save look-up table for user-defined VOI.

    Format for outputted table:
        |user VOI No.|user VOI name|

    """
    df = voidef[["user VOI No.", "user VOI name"]]
    df2 = df.drop_duplicates(["user VOI name"])

    df2.to_csv(out_lut_file, index=False, sep=" ", header=False)

    return None