HENDERSON, Nev. ( TheStreet) -- Spectrum Pharmaceuticals ( SPPI) reported $41 million in sales of its colon cancer drug Fusilev in the September quarter, a 21% sequential jump boosted by supply shortages of a generic competitor that don't appear to be abating. It's time to admit my error. In April I penned a bearish column on Spectrum arguing that Fusilev sales would fall off this year once supplies of the cheaper generic form of the drug known as leucovorin were re-introduced to the market. I assumed that the three generic drug makers of leucovorin, including Teva, would get their act together. Once leucovorin supplies normalized, doctors would abandon the higher cost Fusilev. Well, it's almost November and leucovorin is still in short supply with little evidence to suggest a dramatic improvement in the shortage situation any time soon. In fact, as the weeks pass, the likelihood of a permanent leucovorin shortage grows. Doctors, meantime, need drug to treat their colon cancer patients so they are embracing Fusilev, which is now on a $160 million annual run rate and growing. Fusilev is likely established well enough today to assume that a good portion of doctors may not switch back to leucovorin even if the shortage ended. As long as insurers continue to reimburse for the drug, some doctors will prefer the reliability of Fusilev supply over risking the chance that leucovorin supplies may run dry again. This doesn't mean Fusilev sales wouldn't retrench at all if leucovorin bounces back, but the days of Fusilev generating a paltry $6 million per quarter aren't returning either. That's the story Spectrum CEO Raj Shrotriya pitched to investors earlier this year, which I didn't believe. Today, he looks to be right. I was wrong. --Written by Adam Feuerstein in Boston. >To contact the writer of this article, click here: Adam Feuerstein. >To follow the writer on Twitter, go to http://twitter.com/adamfeuerstein. >To submit a news tip, send an email to: firstname.lastname@example.org. Follow TheStreet on Twitter and become a fan on Facebook.