Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of medical-device developer Delcath Systems (Nasdaq: DCTH) plummeted today, falling as much as 45% on heavy volume.

So what: The Food and Drug Administration handed Delcath a "refusal to file" letter for its liver cancer treatment. The company filed the application back in December and there were hopes for a quick turnaround and an approval by June. No such luck.

Now what: The refusal to file letter doesn't mean the cancer treatment is less likely to be approved. Rather, the FDA wanted more information about the manufacturing plant and other safety information. What the letter does mean, though, is that the timeline for getting the treatment approved has gotten stretched out, and time is money when you're waiting on your first source of revenue. In the volatile world of upstart medical companies, investors have to be ready for both the boom and the bust of the stocks' wild swings. Delcath's longer-term prospects may still be good, but for today investors are getting a heavy dose of the bust side of those swings.

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Fool contributor Matt Koppenheffer does not own shares of any of the companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or on his RSS feed. The Fool's disclosure policy prefers dividends over a sharp stick in the eye.