Dana-Farber Cancer Institute is exploring a partnership with Rhode Island's Lifespan Health System. Dana-Farber and Lifespan, the parent of Rhode Island Hospital and four other hospitals, signed a letter of intent to negotiate a partnership but are still working out the details of the relationship. The partnership could help boost referrals to Dana-Farber, which is nationally known for its cancer care but still draws about 70% of its patients from Massachusetts. It also would create a larger pool of patients for Dana-Farber to tap for clinical trials of experimental cancer treatments. Lifespan's CEO, Dr. Timothy J. Babineau, said partnering with Dana-Farber will help Lifespan hold on to the 13% of Rhode Island cancer patients who currently leave the state for cancer care.