I am sorry for your loss, my friend. Lung cancer has metastasized in the advanced stage. I guess your father is dying. There are many kinds of dreams, and there are indeed dreams that predict the future. But your dream is logical. Because you subconsciously love your father and are afraid of losing him, you have psychological factors such as self-blame, fear and cowardice, so you will dream that your father left.
These days, you should spend more time with your father, take good care of him, and don't entertain foolish ideas. It is human nature to be born and die. As long as you serve filial piety well in his lifetime and let him leave safely, you will free yourself and your soul will be comforted.