Mass. Gen. Laws ch. 32 § 90A
Any city or town or the Massachusetts Port Authority which accepts the provisions of this section in the manner hereinafter provided may, in the case of a city by two thirds vote of the city council and with the approval of the mayor, or, in the case of a town, by two thirds vote at the annual town meeting or, in the case of the Massachusetts Port Authority, by vote of a majority of its members, increase the retirement allowance of any former employee thereof who has been retired under any provision of this chapter or similar provision of earlier law on account of injuries sustained or of hazard undergone in the performance of his duty, to an amount not exceeding one half the rate of regular compensation payable to employees of such city or town holding similar positions, at the time of increasing such allowance, in the same grade or classification occupied by such former employee at the time of his retirement.
Any metropolitan district police officer who has been retired under any provision of this chapter or similar provision of earlier law on account of injuries sustained or of hazard undergone in the performance of his duty shall have his retirement allowance increased to an amount not exceeding one half the rate of regular compensation payable to state police officers holding similar positions, at the time of increasing such allowance, in the comparable grade or classification, as determined by the personnel administrator occupied by such former officer at the time of his retirement.
Mass. Gen. Laws ch. 32, § 90A