His right foot

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My father was diagnosed with stage 4 cancer and the cancer cells have already spread through out his body. He can't stand up anymore and he has difficulty moving his body especially his right foot. However, we worry about his right foot which looks like its about to burst. Its bigger than his left and when I touched it, it felt a bit hard. We can't move his right foot much because he says that it really hurts. What do you think we should do? We asked one of his doctors and he just said that HOPEFULLY its a delayed reaction of the drug he was given(forgot the name). Did anyone experience this? Please help. :(