In May, 2004, MassHealth3 denied a request for authorization of a surgical procedure for the plaintiff, Ashley Shaw (Ashley), stating that the requested procedure was not covered. The procedure was, however, deemed medically necessary by her physician. Ashley, through her mother, Elizabeth Shaw (Shaw), pursued an administrative appeal of MassHealth’s decision, but while the appeal was pending, Shaw, believing the procedure to be medically necessary, assumed financial responsibility for the procedure in the event Ashley lost her appeal. Accordingly, Shaw instructed Ashley’s physician to proceed with the surgery. MassHealth then denied Ashley’s appeal on the ground that the procedure had been performed without authorization.4 Ashley sought judicial review in the Superior Court under G. L. c. 30A, § 14. The judge denied her motion for judgment on the pleadings and entered judgment for the defendants. This appeal followed. We vacate the judgment of the Superior Court.

Background. As a side effect of medications used in treating Ashley’s life-long HIV-AIDS, she developed an abnormal fat *220pad on her neck and shoulder area, known medically as lipodistrophy, or more commonly as a “buffalo hump.” By February, 2004, it was of significant size for a child who then was about fifteen years old, five feet, two inches tall, and weighing 115 pounds. It measured fifteen by fifteen by five centimeters. In addition to being disfiguring, it caused Ashley to have abnormal posture, difficulty in swallowing, back and neck pain, headaches, and an inability to sleep without medication.

In May, 2004, Dr. Elof Eriksson requested authorization from MassHealth for a surgical procedure to remove the deformity. He submitted a form dated May 6, 2004, to MassHealth requesting review for procedure codes 15877 and 15876. On the form submitted, in a box requesting the “reason for request/medical justification,” there were two handwritten notes: “Please see attached clinical note w/photos”; and “[pjlease note this request for clinical review is on behalf of a minor child under the age of 18 . . . with extenuating medical circumstances.”

In a May 11, 2004, letter to MassHealth, Dr. Sandra Burchett, Ashley’s attending physician at Children’s Hospital Medical Center, stated that removal of the growth by liposuction was a medical necessity.5 6 In a May 13, 2004, letter, Dr. Eriksson provided additional clinical information to MassHealth, and further stated the surgery was scheduled for May 27, 2004.®

At the hearing before a DMA hearing officer on November 1, 2004, Dr. Gail LoPreste7 testified for MassHealth that she reviewed the request for procedure code 15876, which she identi*221fled as liposuction.8 She stated that the request initially was denied because it did not “meet medical necessity criteria,” and was not a “covered procedure.” However, she stated that “[subsequently, in the course of trying to obtain further information ... to support the evidence that Ashley’s lipodystrophy was causing the headaches and other pain that she was having . . . , [a member of the staff] at Children’s Hospital . . . told me that the procedure had, in fact, already been done.” She stated that the request “would have to be denied because it’s a [Retroactive [Request.” She referred, without further explanation, to the regulation “433.408A.”

Ashley’s counsel questioned Dr. LoPreste on her evaluation of the materials submitted by Dr. Erikkson and Dr. Burchett, but her answers merely elaborated on the additional information she previously stated she sought. She did not offer any explanation of the basis for the denial letter sent to Shaw, nor did she indicate whether she played any role in that communication.

Medical necessity. Although counsel for Ashley summarized and argued the evidence supporting the request for prior authorization, the hearing officer made no findings on the medical necessity of the request, because the procedure occurred without prior authorization and hence in his view the claim was properly denied. This was error.

We reject DMA’s view that the review of Ashley’s claim may be terminated because the procedure had been performed without authorization. There was no timely and reasonable alternative available when the request for authorization was denied. Without knowing the reason for the denial, Shaw requested a hearing on Ashley’s behalf, and MassHealth’s unfavorable ruling, on the ground that the procedure had not received prior authorization rather than on the merits of medical necessity, required her to appeal to the Superior Court and this court, a process that has taken over three and one-half years. To treat prior authorization as overriding all other considerations is not consistent with the regulation’s purpose.

While we give deference to an agency’s interpretation of its *222own regulations, “courts will not hesitate to overrule agency interpretations when those interpretations are arbitrary, unreasonable, or inconsistent with the plain terms of the regulation itself.” Warcewicz v. Department of Envtl. Protection, 410 Mass. 548, 550 (1991). We reject MassHealth’s interpretation of the regulation as it was applied to terminate the review.

Reading the plain language of 130 Code Mass. Regs. § 433.408 (A)(1) & (2) as a whole, see note 4, supra, it is apparent that it principally is concerned with the medical necessity of a request as the controlling prerequisite for payment of services for certain procedures not otherwise covered by MassHealth. Recognizing that the request in this case was filed prior to the provision of the services, and still is pending, it cannot be considered a “retroactive request.” A later decision, if favorable to the plaintiff on appeal, although not an authorization prior to the services, nonetheless meets the overarching requirement of the regulation that the determination of medical necessity be a prerequisite for payment. This interpretation harmonizes the regulation’s requirements with its principal purpose.9

Since there was no hearing or decision on the merits by MassHealth as to the medical decision, the matter is remanded to MassHealth for a review of the medical necessity of the procedure. Such review is to be conducted according to defined criteria with an adequate record of the proceedings.

Conclusion. The judgment of the Superior Court is vacated. An order is to enter remanding the case to MassHealth for further proceedings consistent with this opinion.

So ordered.