(Reuters) - Merck & Co Inc said on Tuesday its blockbuster cancer drug Keytruda won approval from the U.S. Food and Drug Administration to treat a type of head and neck cancer.

The drug was approved for use as a monotherapy, as well as in combination with a common chemotherapy regimen, to treat previously untreated patients with head and neck squamous cell carcinoma, Merck said.

The approval is based on results from a late-stage trial, where Keytruda showed a significant improvement in overall survival in cancer patients, Merck said.

Keytruda, a type of immunotherapy called a PD-1 inhibitor, is already an approved treatment for several forms of cancer, including lung and skin cancers.

Head and neck cancer includes tumors in the mouth, tongue, nose, sinuses, throat and lymph nodes in the neck.

Merck estimates that there will be more than 65,000 new cases of head and neck cancer diagnosed in 2019 in the United States.

Keytruda works by increasing the ability of patients’ immune system to help detect and fight tumor cells.

The drug has been amassing approvals as a standalone therapy and in combination with other drugs to treat several forms of cancer. It is the leading immunotherapy for treating lung cancer, ahead of rival drugs from Bristol-Myers Squibb, Roche and AstraZeneca.

Keytruda, first approved for advanced melanoma in 2014, is Merck’s most important growth driver. It has overtaken Bristol’s Opdivo as the industry’s immuno-oncology leader with sales expected to top $10 billion this year and $20 billion in 2024, according to IBES data from Refinitiv.

Keytruda brought in revenue of $7.17 billion for Merck in 2018, while Bristol’s Opdivo earned $6.74 billion.