MBTA General Manager Frank DePaola has been battling cancer and plans to retire June 30, state officials announced on Monday.

"As some of you know, Frank has been battling cancer for months and he has now come to the conclusion that he cannot focus adequately on his health and the treatment he needs while carrying out the heavy responsibilities of the GM's job," Transportation Secretary Stephanie Pollack wrote in a memo Monday.

The move will return the T to a state of transition in a little over a month and midway through a roughly year-long project to overhaul internal processes, repair decades-old equipment and improve performance at an agency that's come under the spotlight for its problems.

The former highway chief who briefly served as transportation secretary at the end of the Patrick administration and beginning of the Baker administration, DePaola took over the troubled transit agency after the surprise departure of Beverly Scott in the late winter of 2015.

On Monday Pollack wrote that she had appointed MBTA Chief Administrator Brian Shortsleeve, a former Marine and business executive, as acting general manager as of July 1, and that MBTA Chief of Operations Jeff Gonneville would take on more responsibility, including oversight of safety and accessibility at that time.

Shortsleeve has handled the financial side of the MBTA, seeking to align expenses and revenues by bringing in more business and cutting down on overtime. Gonneville was promoted from chief mechanical officer to chief operating officer in the first year of the Baker administration.

Beset by managerial quandaries and a more than $7 billion repair backlog, the MBTA has had regular turnover at the top. In July, Shortsleeve will become the seventh leader of the transit system in eight years, according to information supplied by the T.

"I realize that many of you will be concerned because the MBTA is once again in the position of having interim leadership," Pollack wrote. "Both I and the members of the Fiscal and Management Control Board are keenly aware that it has been far too long since the MBTA has had stable leadership for more than a handful of years."

Pollack said she planned to speak with T overseers, staff and "key stakeholders outside the authority" to develop a transition plan for the T.

Shortsleeve recently filled in for DePaola when the general manager took time off for medical reasons. Quick to employ humorous quips, DePaola has also gone out of his way for a joke. Earlier this year DePaola publicly presented Shortsleeve with a Dungeons and Dragons figurine - a reference to a childhood pastime the chief administrator had disclosed to the Boston Globe.

DePaola had worked for the Massachusetts Water Resources Authority, the MBTA and overseen state highways when Pollack tapped him to take over the system of rails, ferries and buses. Scott resigned following devastating snow and cold that ground the T to a halt in many places.

"When I walked into Frank's office to ask if he'd step in as interim GM, he showed me the work he had already begun not just on recovering service but on planning for a springtime push to get a head start on repairs that would be needed before the next winter," Pollack wrote in her memo Monday. "I knew I had the right guy."