March 18 (Reuters) - Swiss drugmaker Roche Holding AG's U.S. unit Genentech said on Monday its immunotherapy Tecentriq won approval for a tough-to-treat type of lung cancer, the latest win for the drug whose sales trail medicines from Merck & Co and Bristol-Myers Squibb.

The U.S. Food and Drug Administration (FDA) approved Tecentriq plus chemotherapy for untreated extensive-stage small cell lung cancer (SCLC), after a study showed patients getting the drug cocktail lived a median 12.3 months, compared to 10.3 months for those getting chemotherapy alone, Genentech said in a statement.

About 10 to 15 percent of lung cancers are small cell, or oat cell cancer, where tumours tend to spread very early. Advances in SCLC treatment have been slow in coming, and Roche has hailed Tecentriq as the first potential new first-line option in two decades.

The FDA's decision on Monday follows approval of Tecentriq combinations in recent weeks for triple negative breast cancer and non-small cell lung cancer.

Tecentriq's 2018 revenue were 772 million Swiss francs ($766 million), compared with $7.2 billion for Merck's Keytruda immunotherapy and $6.7 billion for Bristol's Opdivo.

The two rival drugs that harness the body's own immune system to fight disease had a roughly two-year head start on Roche's drug, which along with mounting data underscoring their effectiveness for some patients with different cancers has helped them gain a big lead in sales.

Opdivo is already approved in small cell lung cancer in cases where patients fail at least two other treatments, while Keytruda is now under priority FDA review, also for third-line treatment of SCLC. (Reporting by John Miller in Zurich and Mekhla Raina in Bengaluru; Editing by Rashmi Aich)