Mevion Medical Systems, Inc., has announced it has raised $55 million primarily from existing equity investors.

Mevion, based in Littleton, Mass., is a developer of single-room proton therapy systems for the treatment of cancer.

The funds were raised from existing equity investors, including Caxton Heath Life Sciences, ProQuest Investments, Venrock and CHL Medical Partners, as well as from debt financing provided by Life Sciences Alternative Funding LLC.

The funds will be used to accelerate the deployment of Mevion’s MEVION S250 proton therapy system.