You must report the entire lump-sum in the year you receive it. Generally, you use your current-year income to figure the taxable part of the total benefits received in that year. However, you may be able to figure the taxable part of a lump-sum payment for an earlier year separately, using your income for the earlier year. You can elect this method if it lowers your taxable benefits.
Under the lump-sum election method, you refigure the taxable part of all your benefits for the earlier year (including the lump-sum payment) using that year's income. Then you subtract any taxable benefits for that year that you previously reported. The remainder is the taxable part of the lump-sum payment. Add it to the taxable part of your benefits for the current year (figured without the lump-sum payment for the earlier year). See IRS Publication 915 for work sheets and more information on this method.