Looking to raise $69 million in an initial public offering, Mevion Medical Systems has filed to go public on the NASDAQ under the symbol MEVI. The company is a developer of proton therapy systems; proton therapy is a recently approved, targeted radiotherapy for certain cancers. Proceeds from the IPO will be used to expand the manufacturing and distribution of Mevion's systems and to further develop its proton therapy technology. The company booked $7.6 million in revenue in the three-month period ended June 30, 2014, and as of that date had signed 21 purchase agreements for its systems. Mevion has raised over $131 million in VC funding, growing its valuation from $13.6 million at its 2005 Series A to over $215 million in its most recent funding in June 2013. Caxton Associates currently holds a 35.3% stake, while ProQuest Investments holds 14.1% and Venrock 8.4%.