These cancers lack features common to most of the malignant tumors that are curable by chemotherapy.
Many curable tumors grow rapidly and tend to have high growth fractions.
With the exception of small cell carcinoma of the lung, the less curable tumors have low growth fractions. Agents that are effective against the common cancers are often limited in number or have a low therapeutic index. Most, but not all, of the currently curable tumors occur in young persons, whereas the common cancers that elude cure by chemotherapy typically affect older persons. Drug resistance is believed to be the major impediment to successful treatment of these cancers, but new approaches are emerging that are designed to combat the frequent problems of low growth fraction and drug resistance. Some of the approaches described here are experimental and should not be introduced into clinical practice until their value has been shown in controlled clinical trials.