Though there is no timetable for Westmoreland's return, the Red Sox were heartened by Tuesday's development.
"It sounds like very encouraging news, so we're obviously thrilled about that," manager Terry Francona said. "We're thankful."
The Red Sox had announced on Saturday that Westmoreland, a 19-year old Rhode Island native viewed as one of the best position players in the team's farm system, would require surgery to repair the potentially life-threatening condition. Westmoreland left Sox camp on March 4 after experiencing headaches and numbness, and was diagnosed the next day at Massachusetts General Hospital.
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A cavernous malformation is a vascular issue which, according to an audio report on the Mayo Clinic Web site, consists of a group of "abnormal, thin-walled blood vessels." In most cases, such malformations do not cause symptoms. They are often discovered only if doctors are looking for something else via an MRI exam.
If the malformation bleeds, it can cause stroke-like symptoms, seizures, numbness, vision changes or other neurological problems.
"Typically, a stroke might be more dramatic, while symptoms from a cavernous malformation come on more gradually," Mayo Clinic neurosurgeon Dr. David Piepgras said in the audio report. "Most people who have cavernous malformations, we can't tell them why they occur."
"The entire Red Sox organization stands in support of Ryan as he courageously deals with this issue," general manager Theo Epstein said in a statement on Saturday night.
Westmoreland previously underwent surgery to repair a torn labrum in his throwing shoulder in November 2008, and had another operation after running into an outfield wall last summer and fracturing his collarbone.
A fifth-round Draft pick out of Portsmouth (R.I.) High School, Westmoreland hit .296 with seven home runs, 19 stolen bases and a .401 on-base percentage in 60 games for Class A Lowell last season, his first as a professional. He ranked 27th on MLB.com's Top Prospect list for this season, and Baseball America named him the organization's top overall prospect, its best hitter for average and its best athlete.
Westmoreland's rare combination of power, defense and speed prompted farm director Mike Hazen to call him "sort of the classic five-tool player" this past offseason.
Fellow prospect Anthony Rizzo, who underwent chemotherapy for Hodgkin's disease in 2008, considers Westmoreland a close friend. Rizzo said that his own medical troubles paled in comparison with those of Westmoreland.
"Before my first chemotherapy, I was more scared than I've ever been in my life," Rizzo said. "I was being strong for my family, but I couldn't imagine what he was going through with the surgery and all the things that could happen."
Rizzo learned of Westmoreland's successful surgery on the bus ride to Port Charlotte on Tuesday afternoon.
"The Red Sox, the last couple of years with Lester and myself, they got us through with the best doctors in the world, basically," Rizzo said. "Ryan's a tough, strong kid and we're all pulling for him."
Boston pitcher Jon Lester was diagnosed with a rare but treatable form of lymphoma in 2006. He made it back to the Majors in less than a year.