Last year I reported on an early study looking at the effects of ellagic acid which is found in raspberries, strawberries and walnuts on cancer cells.[original article] In a culture of cervical cancer cells low doses of ellagic extract stopped cell growth in 48 hours and caused cancer cell destruction in 72 hours. The concentrations used can be achieved by a moderate daily ration of raspberries.
So what's new. An in vivo study using rats was published in August, 2001. Esophageal cancer was chemically induced in the rats. Feeding the rats freeze dried black raspberries prior to administering the carcinogenic chemicals reduced the tumors by about 45% depending on the amount of raspberries consumed. Feeding the rats raspberries three times a week (at a rate of 0.25 mg/kg) inhibited tumor progression. At 25 weeks after carcinogen admnistration the raspberry rats had half the tumor incidence and half the number of tumors as non-raspberry eating control rats 
Another study caught my eye, this one compared quercetin with ellagic acid in chemically induced lung cancer in mice. Treating the mice with ellagic acid reduced tumor incidence to 20% from the control value of 72%. Quercetin caused the tumor incidence to decrease from 76% to 44%.
Yes, these are animal not human studies. If we tried the same experiment with people raspberries might not be as protective. On the other hand they might work better. Raspberries, strawberries and walnuts are our best sources of ellagic acid. Frozen fruit should work as well as fresh. The best inexpensive source I've found so far is Costco which sells large bags of blueberry, raspberry and blackberry mix. The package says it's good for smoothies, which is true. It also makes quite an adequate pie.