See Section 7.3.
Multiple metastases (in various locations) and the overall tumour load ultimately limit
surgical removal and the effectiveness of anti-cancer drugs. When cancers recur and spread
beyond the initial site or region, systemic treatment is necessary and the goal of this
treatment is no longer curative. Chemotherapy is the most prevalent form of systemic
treatment, because it can reach and destroy cancer cells throughout the body, although the
blood-brain barrier often limits effectiveness in the case of brain metastases. Chemotherapy
may be used alone or in combination with other forms of treatment such as radiation therapy
to specific metastatic sites. Hormone-regulated tumours, such as certain breast and prostate
cancers use the body’s natural hormones to grow, and they are often more responsive to
hormone-based treatments that chemotherapy. As in the case of chemotherapy, tumours can
become increasingly resistant to standard treatments. Certain cancers can be resistant to
systemic treatments at the time of diagnosis. Other cancers become resistant over a period of
months or years. Overall, 30% to 80% of cancers can become refractory