Duke University has done some work on prostate cancer and flaxseed.  Here
are the results:

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The flaxseed study was aimed at fighting prostate cancer, not treating a
side effect. The edible seed has been used for hundreds of years in cereals
and breads and is high in beneficial omega-3 fatty acids and in lignan, a
substance that can affect hormone levels and perhaps squelch their
cancer-promoting effects.

Four groups of about 40 men who were scheduled to have their prostates
removed three weeks later were assigned to get either 30 grams of powdered
flaxseed, a low-fat diet, both or neither until their surgery.

After the men's prostates were removed, researchers found that tumors had
been growing 30 to 40 percent slower in the two groups taking flaxseed,
based on how quickly cells were multiplying. Low-fat diets had no effect on
this, said Wendy Demark-Wahnefried of Duke University Medical Center, who
led the study.

''Our findings are compelling but they're preliminary,'' she cautioned.

But several doctors said flaxseed is nutritious and seems to have little
downside other than a sawdust-like consistency, since it must be used ground
or powdered because it has an inedible hull or coating.

Scientists plans to study flaxseed on men with prostate cancer that comes
back after initial treatment, and Canadian scientists also are testing it
for breast cancer, she said.