On May 4, 2014, about 1740 eastern daylight time, a BK-117 A-4, N504TJ, was substantially damaged while in cruise flight near Hamburg, New York, after a cabin door separated from the helicopter and struck the main rotor blades. The commercial pilot and the two passengers were not injured. The helicopter was registered to and operated by Mercy Flight Incorporated under the provisions of 14 Code of Federal Regulations Part 91 as a positioning flight. Visual meteorological conditions prevailed and no flight plan was filed. The flight departed Women and Children's Hospital Heliport (7NK9), Buffalo, New York, at 1730. According to the pilot, the helicopter was in cruise flight at an altitude of 3,000 feet mean sea level (msl). The flight nurse alerted the pilot and flight paramedic that the left sliding door had opened several inches. The flight nurse and the flight paramedic then attempted to close the door, but as the pilot reduced the forward airspeed the lower portion of the door dislodged from the lower track assembly and pivoted up, striking the main rotor system. Upon contact with the main rotor system the door departed the airframe. The pilot initiated a forced landing to a baseball field, and the door was recovered for further examination. The pilot reported that the flight paramedic completed the walk-around and inspected doors for closure prior to sitting in the co-pilot seat and securing that door. The door caution lights were extinguished prior to takeoff and did not illuminated until the left sliding door opened at cruise altitude. A cursory examination of the door assembly by the operator revealed that the door was broken into several pieces. The airframe latch assembly did not reveal any anomalies. The door latch and spring assembly was examined and it was questioned by the operator that it may have failed. The operator also stated that they did not have engineers on staff to examine the door assembly latch. The left cabin door and the main rotor blades were examined at Airbus Helicopters Incorporated under supervision of the Federal Aviation Administration. The examination revealed that the main rotor blades were substantially damaged and the doors upper sliders exhibited excessive wear. Both lower door bracket guide assemblies were damaged from impact, and partially separated from the composite door structure. Both guide brackets on the door assembly were worn. The aft lower catcher and guide assemblies revealed impact damage, and were partially separated from the door structure. The catcher and guide assemblies revealed a 45 degree worn away section at the forward catch. Although the door latch was impact damaged, it appeared to operate properly. In accordance with Chapter 21, page 16B, of the Maintenance Manual MBB-BK117, the caution sections states, "if the minimum measure of 1.0 + 0.5 mm is not reached on one of the two sliders, both sliders must be replaced with new ones." The sliders were at the minimum tolerance level of 0.5 mm. A review of the logbook excerpts revealed that the door assembly was inspected 127 hours prior to the accident in accordance with Alert Service Bulletin MBB-BK117 No. ASB-MBB-BK117-20-111. No anomalies were noted with the door assembly at the time of that inspection.