Complex sentence: The tumors cause fibrosis of tissues and impede digestion or organ function, and if left untreated, tumors and mucin will fill the abdominal cavity, compressing organs and destroying function of colon, small intestine, stomach, or other organs.
Split into simpler sentences: The tumors cause fibrosis of tissues and impede digestion or organ function, and if left untreated, the tumors and mucin they produce will fill the abdominal cavity. This will result in compression of organs and will destroy the function of colon, small intestine, stomach, or other organs.