When Mark Karan was battling throat cancer in the summer of 2007, an acupuncturist told him the Chinese word for crisis was the same as opportunity.

When Mark Karan was battling throat cancer in the summer of 2007, an acupuncturist told him the Chinese word for crisis was the same as opportunity. That seemed to open a new world to the RatDog guitarist.

"It allowed me not to be swept away by fear or anger, but see things from almost a witness place," he says. "But what was I supposed to take out of this? What the hell am I supposed to come up with?"

The answer was acceptance.

"A lot of us can get wrapped up in thinking we've been done wrong," he says. "If you do that you're carrying around a whole lot of dark stuff. For me it became about being OK with whatever it was. Letting go and finding a way to move forward."

Karan, 54, rejoined RatDog last year and will be playing lead guitar when the band, fronted by longtime Grateful Dead rhythm guitarist Bob Weir, visits Britt on Friday, Aug. 21. Jackie Greene will open at 7 p.m.

In a phone interview, Karan says that acceptance of cancer and its lessons did not come easy but seemed to bring rewards when it came.

"Two ex-girlfriends reached out, and we had healing," he says. "Phil (Grateful Dead bassist Lesh) even showed up."

Illness even prompted Karan to get off the dime and release a CD in his own name ("Walking Through the Fire," Quacktone Records) after years of playing with the likes of the Dead, Dave Mason, Delaney Bramlett, Jesse Colin Young and many others. He says it had been too easy to put off the work involved in an album.

"The guys on the album are all touring with Phil (Lesh and Friends) and Bobby (Weir) and Bruce (Hornsby) through the year," he says. "It's pretty rare to have the same free time.

"I got hit with the cancer and realized I don't know how much time I get on the planet. I'm not interested in waiting anymore. Stuff that requires waiting, I'm gonna take another path."

Karan grew up in the San Francisco Bay Area, where he says he "cut a lot of high school classes, dropped a lot of acid, played in a lot of garage bands." He listened to Neil Young, The Allman Brothers and the Sons of Champlin, but his knowledge of the Dead dropped off after 1974's "Mars Hotel."

In 1998 Weir asked him to join RatDog, a band that both critics and fans say has evolved radically over the years. It began life in 1995 as a laid-back blues outfit and Weir project in the wake of Dead lead guitarist Jerry Garcia's death. It's long since become a snarling rock band with jazzy chops.

RatDog started with Weir, the Grateful Dead's rhythm guitarist for 30 years, and bassist Rob Wasserman, and added Bay Area drummer Jay Lane, formerly of the Freaky Executives. Lane introduced Weir to rock/jazz/blues keyboardist Jeff Chimenti, and Karan joined in 1998 after a turn with The Other Ones, an early post-Garcia project of surviving Dead members.

Jazz saxophonist and New England Conservatory graduate Kenny Brooks and British-born bassist Robin Sylvester round out the band. RatDog plays most of the Dead Songbook, including not just "Bobby songs" such as "Sugar Magnolia" but "Jerry songs" such as "Touch of Gray" and "Standing on the Moon," not to mention a slow-growing roster of post-Dead tunes. They use many of the songs as platforms for jazzy improvisations.

"We're not trying to be a Dead cover band," Karan says, "just play the book the way we wanna play it.

He believes two things account for RatDog's progress to a place where it can "make some magic." One was the departure of long-time Weir collaborator Rob Wasserman.

"A stand-up, acoustic bassist wasn't what the doctor ordered for this songbook," he says. "He's fun to play with, but he helped shift the focus of the band."

He thinks the other thing was Weir getting more comfortable.

"Maybe it had to do with grief," he says. "Or being in a secondary role all those years. He didn't wanna do the Jerry tunes back then, just ballads and blues and swing. Now, there's a freedom of expression and a looseness in the way Bob goes about it. On our East Coast tour he was like a madman, totally on top of his game."

Yet another thing occurs to Karan about RatDog circa 2009.

"The membership has stayed long enough we can communicate," he says. "We know each other well."

Which is what you need for the magic to happen.