If you’re expecting Joan Rivers: A Piece of Work to showcase her famously blue stand-up routines – a testament, perhaps, to the undiminished ferocity she still brings to the stage at 76 – you’ve got another think coming.

That’s not to say Ricki Stern and Anne Sundberg’s illuminating documentary, which closes the 53rd San Francisco International Film Festival tonight at the Castro Theatre, isn’t funny. But the story it tells, of a comedy icon (and acknowledged workaholic) struggling to stay at the top of her profession, is both edgy and bittersweet.

“It’s not about whether the talent is good,” says agent Larry Thompson. “It’s about whether they’re hot.” Rivers isn’t cold, exactly – she works steadily, writing plays, jokes and even a 2009 murder mystery inspired by her experience as a red-carpet Oscar-telecast host. But for a woman whose need to be busy seems almost pathological, and who still takes every perceived slight as a personal rejection, steady doesn’t cut it. “Good luck to the next queen of comedy, because she’s not abdicating,” says her manager Billy Sammeth, who thinks bookers see her as a “plastic-surgery freak” past her prime. “There will be nail marks on that red carpet before she abdicates.”

Joan Rivers: A Piece of Work plays tonight at the Castro at 7 p.m. For tickets, click here.