Sounds like it could be just like treatments for parasites and fungi...
Introduce just enough poison to kill the bad stuff without killing the patient... Once the bad stuff has died, the patient heals and recovers and has basically survived/outlasted the same poison the bad stuff was not able to survive. The factor that defines a poison is the dosage at which it becomes lethal. If this new drug can shrink a tumor, there is a good chance the bulk of the tumor can be removed via surgery and the 2 treatments would work together to rid the patient of the cancer.