Dolly Parton Secretly Traveled to Germany for Experimental Cancer Treatment

Dolly Parton secretly traveled to Germany for an experimental cancer treatment about a year before her death ... TMZ has learned.

Sources familiar with the situation tell TMZ ... Dolly privately battled cancer during the final year of her life and sought treatment overseas.

We're told the music icon flew to Frankfurt on a private jet to undergo the treatment. It's unclear exactly what the procedure involved ... but our sources say Dolly remained in Germany for about a week before returning home.

We're told Dolly believed the treatment helped restore her strength and felt it had been effective.