I advise most applicants to not limit, edit, or think about who will be reading this when you first start. In other words, start writing this as you will be the only reading it. Let yourself go so ideas, feelings, etc get on the paper. Then, you will likely get some material to actually mold an essay from. And this will lead, hopefully to having a sincere essay with passion
Under the general idea of "show me" instead of "tell me", I would start about the impact of your Dad's disease, how does a 6-7 year old deal with concepts larger than you can understand at that age. I am speculating trying to understand, to come to terms, with your Dad's death is what started down this path of science. BTW, this just "someone" you knew who died from cancer. This was your Dad, your 7 year old hero and role model perhaps. I cannot imagine the enormity of the impact this had and the impact is has now towards medicine. This will not be corny or trite if you can both get the emotional impact out on paper and the path to understanding science / cancer because of it
Again, I speculate here, but I could see something starting like "It was over a decade ago, but I am still trying to understand it, the 'monster' that had taken my Father."