If treating metastatic cancer is good, then preventing it is even better. And by "better," I mean it'll generate additional sales -- blockbuster sales, in fact.

Amgen's (Nasdaq: AMGN) Xgeva recently gained Food and Drug Administration approval to treat cancer patients that have bone metastasis. Tumors growing on the bone can weaken it, and the drug clearly delays breaks in the brittle bone.

New data, however, is a whole different ball game. According to top-line data released yesterday, Xgeva is able to delay the appearance of those bone metastases in prostate cancer patients. Metastasis-free survival -- the time it takes for the first metastasis or death from cancer to occur -- was 4.2 months longer in patients taking Xgeva, compared with those taking placebo.

The data isn't completely cut and dried though. The trial didn't show that it helped patients live longer -- the ultimate goal of any cancer drug. While delaying painful bone metastasis and the resulting breaks is a good thing, Xgeva is known to cause osteonecrosis of the jaw -- essentially, death of the jawbone.

The FDA and doctors, who could theoretically prescribe Xgeva off-label right now, will have to balance the risk and benefit without overall survival data to justify the use. Ultimately, I think Xgeva will be approved for the expanded use, but expect that there will be a lot of controversy along the way.

As for drugs that treat post-metastasis prostate cancer -- sanofi-aventis' (NYSE: SNY) Taxotere and Dendreon's (Nasdaq: DNDN) Provenge -- the use of Xgeva shouldn't affect sales much. In the worst case, it just pushes the patients back by four months. Xgeva is no cure, and Sanofi and Dendreon will eventually get their hands on the patients.

Xgeva is also being tested in breast-cancer patients for its ability to delay the formation of bone metastasis in that cancer type. The prostate-cancer data should give investors high hopes. But Zometa, a similar bone drug from Novartis (NYSE: NVS), only helped a subset of older women in its breast cancer trial. Then again, Xgeva beat Zometa in head-to-head trials for preventing breaks in cancer patients, so maybe it'll have an edge here, too.

The delay waiting for the breast cancer results will be a lot longer than 4.2 months, though; Amgen says it won't be available until about 2016.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool has a disclosure policy.