Mass. Gen. Laws ch. 32 § 83
In every city, except Boston, which has accepted this section or corresponding provisions of earlier laws, the appropriate retirement board, established under section twenty, shall, at his own request or at the request of the chief or superintendent of police if, in the judgment of said board, he is disabled for useful service in said department, retire from active service and place upon a pension roll any member of the police department of such city whom the city physician of such city certifies in writing to be permanently disabled, mentally or physically, by injuries sustained through no fault of his own in the actual performance of duty, from further performing duty as such member, or any member of said department who has performed faithful service therein for not less than twenty years continuously if, in the judgment of said board, such member is disabled for useful service in the department; provided, that any permanent member of said department who has performed faithful service therein for twenty-five years as aforesaid shall, at any time after attaining the age of sixty and before attaining the age of seventy, be retired at his request and shall, on attaining the age of seventy, be retired without any request on his part, and no other permanent member of said department shall remain in service after he has attained or shall attain the age of seventy. Every member so retired shall annually receive as a pension one half the amount of the highest compensation paid since May first, nineteen hundred and thirty-one for the grade held by him at his retirement, such amount to be paid by the city, which shall appropriate money therefor.
The board of police, or the mayor in cities having no such board, may in an emergency call upon any person so pensioned for such temporary service in the department as he may be fitted to perform, and during such service he shall be entitled to full pay.
Mass. Gen. Laws ch. 32, § 83