Landmark Medical Center has sued Blue Cross & Blue Shield of Rhode Island, alleging that the insurer pays the hospital too little to cover its costs and has contributed to the hospital's insolvency. The suit, filed in Superior Court, Providence, contends that the Woonsocket hospital is powerless against the "monopoly" strength of the state's largest insurer and has been forced to accept reimbursements so inadequate that they "threaten Landmark's very existence." In a strongly worded statement Tuesday morning, Blue Cross denied that it had caused the hospital's financial problems, saying Landmark is "fairly compensated." The insurer said that its payments account for only a quarter of Landmark's income and asserted that Landmark wants Blue Cross to compensate for the hospital's "years of mismanagement."