Cancer is a disease that occurs on the cellular level - precisely where we differ from animals. Treatments for cancers, like the breast cancer shown at right, have come from serendipity, clinical observation, computer and mathematical modelling and other non animal-based research modalities.
If not with animals, how can we ever hope to cure cancer?
Cancer has now overtaken heart disease as the number one killer in the UK. One major reason we have not yet stemmed mortality from cancer is this: animal cancer is not the same as human cancer.
Cancer is not one disease. It is many. In humans, there are over 200 different forms of cancer afflicting different organs, tissues, and cells. Though comparable animal organs, tissues, and cells may become cancerous, the cancers are never identical to human carcinomas.
Given substances are not necessarily carcinogenic to all species. Studies show that 46% of chemicals found to be carcinogenic in rats were not carcinogenic in mice.  If species as closely related as mice to rats do not even contract cancer similarly, it's not surprising that 19 out of 20 compounds that are safe for humans caused cancer in animals. 
The US National Cancer Institute treated mice growing 48 different "human" cancers with a dozen different drugs proven successful in humans, and in 30 of the cases, the drugs were useless in mice. Almost two-thirds of the mouse models were wrong. Animal experimentation is not scientific because it is not predictive.
The US National Cancer Institute also undertook a 25 year screening programme, testing 40,000 plant species on animals for anti-tumour activity. Out of the outrageously expensive research, many positive results surfaced in animal models, but not a single benefit emerged for humans. As a result, the NCI now uses human cancer cells for cytotoxic screening.
Dr. Richard Klausner, as director of the US National Cancer Institute, plainly states:
"The history of cancer research has been a history of curing cancer in the mouse... We have cured mice of cancer for decades - and it simply didn't work in humans."