system
Given the following post, respond as if you're commenting on Reddit:
human
Why do cancer cells consume so much sugar?

Stumbled across a method of diagnosing cancer in which the patient is given slightly irradiated sugar, and as the cancerous cells consumed more sugar than non cancerous cells, they were  "highlighted" so to speak.  
Additionally, could there be a way of "spiking" sugar with a toxin that needs to reach a certain concentration in a cell for it to have an effect, so would only affect the cancer cells as they would consume more sugar than non-cancerous cells?
gpt
Cancer by definition is unrestricted cell growth and replication. This activity requires high metabolic activity and therefore consumes higher than normal amounts of glucose. In a PET scan (positron emission tomography) we use this fact to find areas that are highly metabolically active, such as tumors or more specifically, metastases. 

Using glucose as a vehicle to target cancer cells isn't really a viable option since the whole body uses glucose whether the tissue is dividing or not. For example, cells in the brain do not divide but consume a huge portion of the body's glucose. Lacing sugar with a toxin would be catastrophic.

We take advantage of the fact that malignant cells grow and divide without restriction by using chemotherapeutic agents that disrupt cell division or DNA replication. An unfortunate side effect of this is that any other areas of the body that have high cell turnover are also effected (bone marrow, lining of the gut, hair follicles etc). Some chemo agents tag specific cancer proteins or receptors as well, making them targets for the immune system. 

Cancer is a very misunderstood disease by most people. Some cancers are highly curable because we've discovered a specific target to act against, one notable example is a type of leukemia cured by all-trans retinoic acid (vitamin A). Other cancers are so malignant the only treatments we have a palliative. There may never be a 'cure for cancer' because there are so many types of cancers and so many ways for the cell cycle to lose its regulatory mechanisms and run amok. Hope this answers your question and clears a few things up for you.