1.   But the result, as has been known to surgeons for a century, is that a primary tumor often keeps lesser, metastatic tumors in check.

2.   Folkman has shown that primary tumors can secrete antiangiogenic substances that keep secondary cancer growths from spreading.

3.   If surgeons remove the primary tumor, in some cases the metastases grow more aggressively.

4.   Presumably the balance of rival factors favors angiogenesis in the primary tumor and repression of angiogenesis elsewhere.

5.   When the primary tumor is removed, the metastatic tumors flourish.

6.   They were injected with a radioactive substance near the site of the primary tumor on the day before surgery.