These researches show the synergic effect of combining Quercetin (+ Vitamin C) with free radicals:

Induction of cancer-specific cytotoxicity towards human prostate and skin cells using quercetin and ultrasound

Here they combined Quercetin with a source of free radicals (ultrasound). It reduced the IC50 of Quercetin to 1.2 micromol (normally 98 micromol). This is 0.4 microgram/ml. Cisplatin has an IC50 of 1.8 microgram/ml and carboplatin has an IC50 of 1.7 microgram/ml. This means that this mix is much more potent then these chemotherapeutic drugs.

Flavonoids as Enhancers of X-Ray-induced Cell Damage in Hepatoma Cells

Here genistein (related to Quercetin) made x-rays (free radical source) much more potent.

The effects of quercetin vs cisplatin on proliferation and the apoptotic process in A549 and SW1271 cell lines in in vitro conditions.

Quercetin greatly improved therapeutic index of doxorubicin against 4T1 breast cancer by its opposing effects on HIF-1(alpha) in tumor and normal cells

Here are some other interesting researches:

Ascorbic acid-enhanced antiproliferative effect of flavonoids on squaous cell carcinoma in vitro