Chemotherapy was a type of cancer treatment that was used on Earth in hte 20th century. It was often evaluated against other methods of cancer treatment, among which including image therapy.
Doctor Leonard McCoy, traveling back in time to 1986 was once asked what his views were regarding chemotherapy after he remarked that it was "unbelievable" patients had to endure such treatment. McCoy's response was: "Sounds like the goddamn Spanish Inquisition if you ask me!" (Star Trek IV: The Voyage Home)