Exhibit 10.25

THIS NOTE HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR APPLICABLE STATE SECURITIES LAWS. THIS NOTE MAY NOT BE SOLD, OFFERED FOR SALE, PLEDGED OR HYPOTHECATED IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT AS TO THIS NOTE UNDER SAID ACT AND APPLICABLE STATE SECURITIES LAWS OR AN OPINION OF COUNSEL REASONABLY SATISFACTORY TO THE COMPANY THAT SUCH REGISTRATION IS NOT REQUIRED.

This instrument and the rights and obligations evidenced hereby and any security interests or other liens securing such obligations are subordinate in the manner and to the extent set forth in that certain Subordination and Intercreditor Agreement dated as of August 19, 2014 (as amended, restated, superseded, replaced, succeeded, substituted, supplemented or otherwise modified from time to time in accordance with its terms, the “Subordination Agreement”) among Mevion Medical Systems, Inc. (“Borrower”), Life Sciences Alternative Funding LLC (the “Senior Lender”), and the other parties thereto to all obligations owing by Borrower to Senior Lender (including interest, expenses and fees) as described in the Subordination Agreement. Each holder of this instrument, by its acceptance hereof, irrevocably agrees to be bound by the provisions of the Subordination Agreement.

MEVION MEDICAL SYSTEMS, INC.

CONVERTIBLE PROMISSORY NOTE

FOR VALUE RECEIVED, Mevion Medical Systems, Inc., a Delaware corporation (the “Company”), hereby promises to pay to the order of [NAME OF INVESTOR] (the “Investor”) the principal amount of [AMOUNT OF NOTE] Dollars ($AMOUNT), plus interest in arrears from and including the date hereof on the principal balance from time to time outstanding, compounded quarterly, at a rate equal to eight percent (8%) per annum, which shall increase to twelve percent (12%) per annum in connection with an Event of Default. Interest shall accrue daily and be calculated on the basis of actual number of days elapsed over a year of 365 days. Notwithstanding any other provision of this Note, the holder hereof does not intend to charge and the Company shall not be required to pay any interest or other fees or charges in excess of the maximum permitted by applicable law; any payments in excess of such maximum shall be refunded to the Company or credited to reduce principal hereunder.

This Note is one of a series of Notes issued by the Company pursuant to and entitled to the benefits of a certain Note and Warrant Purchase Agreement, dated as of August 19, 2014, among the Company and the persons listed on Schedule I thereto (as the same may be amended or otherwise modified from time to time, hereinafter referred to as the “Purchase Agreement”), and the holder of this Note, by its acceptance hereof, agrees to be bound by the provisions of the

Purchase Agreement, including the amendment and waiver provisions set forth therein. This Note will be registered on the books of the Company or its agent as to principal and interest. Any transfer of this Note will be effected only by surrender of this Note to the Company and reissuance of a new note to the transferee in accordance with the terms herein. Capitalized terms used, but not otherwise defined herein, shall have the meanings ascribed to such terms in the Purchase Agreement.

(a) Mandatory Conversion of Note. If the Company consummates an equity financing pursuant to a single transaction or series of related transactions, which shall include an initial public offering of the Common Stock, yielding gross proceeds to the Company of at least $17,500,000 (including conversion of the Notes and any other convertible debt securities), prior to the repayment of the Notes (a “Next Qualified Financing”), then the principal and all accrued interest under the Notes shall automatically convert in whole upon the closing of such Qualified Financing, without any further action by the Investor, into shares of the security sold to the investors in such Next Qualified Financing (the “New Security”) at a purchase price equal to eighty percent (80%) of the lowest price per share paid by the investors purchasing such New Security (the “Conversion Price”) in the Next Qualified Financing on the same terms and conditions as provided to such investors. In the case of a conversion provided in this Section 2(a), the Company shall ensure that the Investor is entitled to the full benefit of all rights given by the Company to the investors in such Next Qualified Financing, in proportion to, and applicable to, the Investor’s share of the New Security issued in such Next Qualified Financing, including, without limitation, all applicable registration rights, co-sale rights, rights of first refusal, pre-emptive rights and other similar rights and all such New Securities shall be subject to all of the corresponding restrictions and obligations imposed upon other holders of such securities. The Investor may be required to execute and deliver to the Company a customary purchase agreement, stockholder agreement and any and all other documents or agreements evidencing such rights, preferences, restrictions and obligations with respect to such Next Qualified Financing (the “Financing Documents”). The Company shall not be required to issue a stock certificate to the Investor upon conversion of this Note under this Section 2(a) until such Financing Documents are executed by the Investor.

(b) Cash in Lieu of Fractional Shares. No fractional share or interest of a New Security or Series E Preferred Stock (as defined herein), or scrip representing fractional shares or interests, shall be issued upon conversion of this Note. Instead of any fractional shares or interest of a New Security or Series E Preferred Stock, as the case may be, which would otherwise be issuable upon conversion of such Note, the Company shall pay to the holder of such Note a cash adjustment in respect of such fraction in an amount equal to the same fraction of the Conversion Price with respect to New Securities or $16.06 per share for the Series E Preferred Stock, as applicable.

(c) Cancellation of Note. Upon the conversion of the entire principal amount of the Note and the conversion of the accrued interest thereon, it shall be canceled.

For purposes of this Note, “Change of Control” means:

(a) any person or entity (a “Person”) other than the Company and its subsidiaries (a “Third Party”), or group of Third Parties acting in concert, gains direct or indirect Control (as defined below) of the Company and its subsidiaries in one transaction or a series of related transactions;

(b) the Company and its subsidiaries, taken as a whole, merges, is consolidated or otherwise combines with any Third Party;

(c) the Company and its subsidiaries, taken as a whole, Transfers (as defined below) more than fifty percent (50%) of their aggregate assets in one transaction or a series of related transactions;

(d) a liquidation, dissolution or winding up of the Company; or

(e) the purchase or acquisition (whether by way of merger, share exchange, consolidation, business combination or similar transaction or otherwise) by any Third Party or group of Persons that are Third Parties, of any of the majority of the outstanding capital stock of the Company in one transaction or a series of related transactions;

provided, however, that any Person set forth on Exhibit A hereto that owns capital stock of the Company shall be deemed not to be a Third Party for purposes of this definition.

“Control” means, with respect to any Person, if another Person directly or indirectly (i) owns or controls more than fifty percent (50%) of the voting securities of such Person, (ii) has the right to issue more than fifty percent (50%) of the votes in a general meeting of such Person, (iii) has the right or power to elect, appoint and/or dismiss a majority of the board of directors or other governing body of such Person, and/or (iv) possesses the power to direct or cause the direction of management and policies of such Person.

“Transfer” means any direct or indirect sale, lease, conveyance, exchange, assignment, disposition, pledge, contribution, license, sublicense, co-promotion agreement, or other form of transfer, whether voluntary or involuntary, in one transaction or a series of related transactions.

if to the Company, at

Mevion Medical Systems, Inc.

Attn: Chief Executive Officer

300 Foster Street

Littleton, MA 01460

Fax: (978) 540-1501

Email: joe@mevion.com

with a copy (which shall not constitute notice) to:

Mitchell S. Bloom, Esq.

Goodwin Procter LLP

Exchange Place

53 State Street

Boston, MA 02109

Fax: 617-523-1231

Email: mbloom@goodwinprocter.com

if to the holder, at the most recent address provided to the Company by the holder for such purpose; or, in each case, to the most recent address, specified by written notice, given to the sender pursuant to this paragraph.

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IN WITNESS WHEREOF, the undersigned has caused this instrument to be executed by its duly authorized officer as of the date first above written.

[Note – [Investor]]

EXHIBIT A

EXCEPTIONS TO THE DEFINITION OF THIRD PARTIES