Dr. H. Gilbert Welch has little time for conventional wisdom on medical testing. Mammograms can be harmful, he says; prostate cancer screenings can lead to unnecessary surgeries, risking complications for no medical benefit. Welch, at the Dartmouth Institute for Health Policy and Clinical Practice, has devoted years of research to proving that a modern fixation on finding cancer early - more screenings, more testing, more biopsies and thyroidectomies - has actually harmed patients. His ideas have sparked pushback among doctors who support more frequent screening and patients worried by changing guidelines, said Dr. James Stewart, Chief of Hematology and Oncology at Baystate Medical Center. Welch will present those ideas Friday evening in a Springfield Public Forum talk at the UMass Center in Springfield at 5:30 p.m., which is open to the public with free registration. Space is limited to 150 attendees, and registration is available by contacting firstname.lastname@example.org.