SAN DIEGO - Nationals vice chairman and principal owner Mark Lerner, the most visible member of the franchise’s family ownership group, revealed tonight why he has been absent from public view for much of the season: Lerner has been battling cancer in his left leg for months and recently needed to have the limb amputated above the knee.

“Thanks very much for your concern and good wishes,” Lerner wrote in a letter provided to MASNsports.com through a club spokeswoman. “I know you recognize that only something really challenging would have kept me from my favorite seat at the ballpark these past months. In early January, they discovered Spindle Cell Sarcoma in my left leg above the knee. Radiation was completed in March and I had surgery in April to successfully remove the cancer. The radiation treatment eventually caused the wound not to heal properly.

“With my doctors and medical team, we decided that amputation of that leg was my best choice to maintain the active and busy lifestyle that I have always enjoyed. The limb was removed in early August and I’m healing well, cancer-free, and looking forward to my eventual new prosthetic.”

Though managing principal owner Ted Lerner, 91, has been the club’s highest-ranking official since his family purchased the team from Major League Baseball in 2006, Mark Lerner - his only son - has been the public face of the Lerner family for the last decade-plus.

Whether from his front row seats next to the first base dugout, in the clubhouse or around the batting cage, Lerner, 63, has been a constant presence around the Nationals, friendly with players, reporters, stadium workers and fans alike. He was present in West Palm Beach, Fla., through most of his team’s first spring training in its new complex, even though it’s now known he was already receiving his cancer treatment by then.

Lerner had not been seen at Nationals Park nearly as much during the season, and when he didn’t attend last month’s press conference promoting next summer’s All-Star Game - an event he had been vigorously trying to bring to Washington for years - close observers began to worry he was dealing with a serious ailment.

Word began circulating around the park over the weekend that Lerner had needed to have his leg amputated, though those who had been in contact with him said he remained upbeat throughout the ordeal and marveled at his ability to find positives and take action to get the problem fixed and make plans to resume a normal lifestyle.

“I’ve been very blessed with my wonderful wife Judy, who has never left my side, our great kids, amazing family and close friends,” Lerner said. “I really appreciate everyone respecting our family’s privacy as we’ve gone through this. I’m not sure of the timeline yet, but you know I’ll be at Nationals Park as soon as I possibly can.”

Though some of the Nationals’ uniformed personnel had known about Lerner’s situation throughout the process, most did not officially learn about until general manager Mike Rizzo called a brief team meeting before Thursday night’s game against the Padres.

“I’ve talked with the family the last couple months, so I kind of knew something was going on,” said first baseman Ryan Zimmerman, the only player remaining in the organization who pre-dates the Lerners’ ownership. “But anytime you hear that about anyone, its tough, especially someone like Mark. He’s around all the time. He’s not only an owner but a huge fan of D.C. and baseball. I know it’s probably killing him more than anyone to not be able to be around. But I think the news is good news, for the most part. It’s obviously going to be tough for him, but we’ll be here to support him and help him with anything he needs. But I think the important thing is, it’s relatively good news, which is all we really worried about. We’ll look forward to seeing him out here at some point this year. I’m sure he’ll work as hard as he can to make it back. We miss him, and we’re thinking about him.”

“It came as a shock to all of us,” manager Dusty Baker said. “Mike Rizzo came in and told me he was going to have a team meeting, and told me what the meeting was about. I remember I saw Mark about a month ago. He just said he was going to be alright, and we didn’t have an idea what he was going through, because we just thought he had that infection in his knee. I’ve got a couple friends who have the same thing. But Mark has a great attitude about life, and we wish him well. And the family well. And we certainly would like to win this for him.”