Cancer is a problem of global importance, since the incidence is increasing
worldwide and therapeutic options are generally limited. Thus, it becomes imperative to find
new therapeutic targets as well as new molecules with therapeutic potential for tumors. Flavonoids
are polyphenolic compounds that may be potential therapeutic agents. Several studies have shown
that these compounds have a higher anticancer potential. Among the flavonoids in the human diet, quercetin
is one of the most important. In the last decades, several anticancer properties of quercetin have been described,
such as cell signaling, pro-apoptotic, anti-proliferative and anti-oxidant effects, growth suppression.
In fact, it is now well known that quercetin has diverse biological effects, inhibiting multiple enzymes involved
in cell proliferation, as well as, in signal transduction pathways. On the other hand, there are also studies
reporting potential synergistic effects when combined quercetin with chemotherapeutic agents or radiotherapy.
In fact, several studies which aim to explore the anticancer potential of these combined treatments
have already been published, the majority with promising results. Actually it is well known that quercetin can
act on the chemosensitization and radiosensitization but also as chemoprotective and radioprotective, protecting
normal cells of the side effects that results from chemotherapy and radiotherapy, which obviously provides
notable advantages in their use in anticancer treatment. Thus, all these data indicate that quercetin may
have a key role in anticancer treatment. In this context, this review is focused on the relationship between flavonoids
and cancer, with special emphasis on the role of quercetin.