Mass. Gen. Laws ch. 156A, § 11

No proxy for shares of a professional corporation shall be valid unless it shall be given to a qualified person, or unless it is limited to voting for one or more of the actions described in subsection (a) of section thirteen under the circumstances described therein. A voting trust with respect to shares of a professional corporation shall not be valid unless all the trustees and beneficiaries thereof are qualified persons, except that such voting trust may be validly continued for a period of not more than six months after the death of a trustee or beneficiary or for a period of not more than six months after a trustee or beneficiary has become a disqualified person.