EXCLUSIVE

Bushwick Bill's ongoing battle with pancreatic cancer is prompting one last hurrah for the Geto Boys ... he's joining forces with Scarface and Willie D for the legendary rap group's farewell tour.

We've learned the Geto Boys are gearing up to announce their final round of shows ... and we're told Bushwick's cancer diagnosis was the catalyst for the reunion.

The Geto Boys are calling their last tour "The Beginning of a Long Goodbye, The Final Farewell" ... and they're donating a portion of the proceeds to pancreatic cancer awareness.

Gigs in Michigan, Ohio, D.C. and NYC are scheduled for the end of this month ... and we're told more shows are in the works.

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TMZ broke the story ... Bushwick Bill is fighting stage 4 pancreatic cancer and is undergoing intensive chemotherapy treatments. He was diagnosed back in February, and told us the news came as a complete shock.

His manager, Jakk Swiss, gave us an update on Bill's condition ... telling us he's spending lots of time in the studio, working long nights as he puts the finishing touches on a new single.

Scarface says he's looking forward to performing alongside Willie D and Bushwick for one last go round ... but admits it's gonna be a tearjerker.