By Robert Bazell, NBC News correspondent

Tonight on "NBC Nightly News" we heard the story of Zach Witt, a vivacious 6-year-old who was close to death from a rare form of lymphoma.  But he was brought back to health by a drug that has been on the market as a treatment for a form of lung cancer.  It truly is a heartening tale.

The backstory to this achievement shows how progress is being made against certain cancers.  In some ways it is very encouraging. But viewed in other ways, the progress is far slower than many would have predicted.

Before the late 1970s and early 1980s scientists had no idea what happened inside cells to make them cancerous.  Then a series of discoveries revealed that the same genes that control cell growth and division as a fertilized egg becomes a human being can also cause cancer when the growth control genes become mutated.

After these mutations were discovered the great and obvious hope was that there would be drugs to target them and stop the cancer from growing.  There have been several targeted therapies, but far fewer than anyone expected.  The gene mutation that drives Zach’s tumor, known as ALK, was discovered 25 years ago.  But companies had little interest in developing a drug for that type of cancer –- anaplastic lymphoma -- because it affects only a few hundred children a year in the United States.

Eventually scientists discovered that ALK is also a driver of about 10 percent of lung cancers.  Lung cancer is so prevalent that even 10 percent makes a substantial market. After extensive testing Pfizer won approval to market Xalkori to treat lung cancer with ALK – at a cost of about $100,000 a year for each patient. The drug works by binding with and inhibiting the action of the enzyme that is produced by the mutated gene.

And in a study out Wednesday, doctors at the Children’s Hospital of Philadelphia have shown that in eight children Xalkori can very effectively treat Zach’s type of lymphoma. In each child, evidence of cancer disappeared. The research will be presented at the annual meeting of the American Society of Clinical Oncology in June.

Scientists are decoding the genes of many cancers now, looking for situations like this where, because of a similar gene mutation, a drug already out there -- or combinations of them -- might help other cancers.  But it is going slowly.  The biology is not as simple as one gene mutation causes one cancer. In fact, there can be hundreds of gene mutations contributing to one type of cancer. Many scientists caution against expecting too many spectacular results.

As Dr. Yael Mosse, Zach’s doctor, put it, “Our goals have shifted. Now we feel that it is more impactful to make a big difference for a small group of patients rather than a small difference for a big group of patients.”