Vitamin C benefit in cancer?

A recent study published by Kuiper et al (Cancer Res; 70(14) July 15, 2010) has generated a lot of interest as it links low levels of vitamin C in endometrial (womb) cancer cells with more aggressive tumours.  It was a very well conducted study and is unusual for a medical paper in that it is also a good read.  Sadly, this study does not show that vitamin C supplementation helps or prevents cancer.  Rather, it shows that in some tumours, lower levels of vitamin C in the tumour cells are associated with more aggressive tumours.  This was only in the tumours, and vitamin C levels were not measured in the patients so we do not know whether patients who had higher levels of vitamin C intake had milder cancers.
To date, there is evidence that intravenous vitamin C can produce concentrations in the body that inhibit the growth of some rat tumours in the lab, but no evidence at all that this helps humans; nor is there any logical reason to believe that any inhibitory effect would last long enough to make any difference.  Oral vitamin C has been proven to be ineffective in treating tumours.  It does not work for colds either.