Updated in 2026 to expand the treatment section. What are desmoid tumors? Desmoid tumors, also called aggressive fibromatosis, occur when connective tissue cells called myofibroblasts begin to multiply out of control, creating a non-malignant but locally invasive tumor. Find our full video library only on Osmosis: http://osms.it/more. Join millions of current and future clinicians who learn by