It is so so much more complicated than that.
Cancer cells grow and proliferate in a favourable environment and many things affects the pathways that either inhibits cell growth or on the contrary promotes the growth. Sugar gives a more favourable environment for cell growth , so if it doesn't cause the cancer, it still creates a more suitable environment. There are however so many pathways, proteins, compounds that can act at every stage in every direction it is reductionist to say sugar causes or doesn't cause cancer growth.
Carcinogenesis is an extremely complex and multistep process that requires the elimination of several cell-imposed barriers such as anti-proliferative responses, programmed cell death-inducing mechanisms, and cell deterioration. Glucose intervenes at different levels in these processes, by allowing cancer cell to develop and grow faster, inhibiting the immune system responses, and making normal cells at a membrane level more fragile. While it is true that the human body produces glucose even without sugar intake, excessive sugar is recognised as an increased risk for cancer. What is not yet agreed is the amount considered "excessive" .
I translate medical papers and medical books, and to answer the @Catchingbentcoppers research conducted mainly by Valter Longo has proven - and is now being used in several Oncology departments in the world - that fasting 36 hours prior chemo and 24 hours after not only extremely reduces the side effects but also makes chemo more effective as cancer cells become more fragile during starvation. Clinical trials are currently studying a softer version with only 24 hours prior and 12 hours after chemo. The fasting regime described in the research is not a compete fast, you still drink broth and other very low calories food.
It is not recommended for everyone, it can even be dangerous if you are very frail, so talk to your doctors, read the medical papers, and I repeat don't DIY.
It is however worth looking into it.