UPDATE 1-Biogen gets Avonex use patent through 2026

* Patent covers Avonex for MS, cancers, other ailments

* Biogen shares unchanged in after-hours trade

LOS ANGELES, Sept 15 (Reuters) - Biogen Idec (BIIB.O) has been issued a U.S. patent for the use of beta interferon for treating viral conditions and diseases, cancers or tumors, the company said in a regulatory filing on Tuesday.

The method of use patent, which will expire in September 2026, also covers the treatment of multiple sclerosis with Avonex, Biogen's brand of recombinant beta interferon and the top-selling treatment for MS.

Other patents on the drug, the last of which will expire in 2013, cover the protein and manufacture of the protein, said Biogen spokeswoman Amy Reilly.

Multiple sclerosis is an autoimmune disease in which the body mistakenly attacks the fatty myelin coating surrounding nerve cells.

Biogen is conducting trials of a long-acting version of Avonex, which is currently given as a weekly injection.

The company's shares were unchanged in after-hours trade after ending up 1.1 percent at $51.41 in regular trading on Nasdaq. (Reporting by Deena Beasley; Editing by Tim Dobbyn)