The reason why cancer is so hard to fight is that the disease so often reoccurs and now a research team has come up with a new explanation – it’s the treatment.

The common theory of cancer reoccurrence is that the cancer cell develops "internal resistance to treatment," and overrides the toxic effects of the drug. This is the case even with newer, less toxic, targeted treatments.

But an Israeli team of scientists has come up with another explanation– they believe that it is the body’s reaction to the treatment that results in making the tumor even more aggressive and likely to spread.

In this specific study, they treated mice with multiple myeloma -- a malignant disease of the plasma cells produced in bone marrow and spread throughout the body via the circulatory system -- with the selective anti-cancer drug Velcade (bortezomib).

The researchers found that treatment with Velcade led to a physiological reaction that actually reinforced the intensity of the myeloma in the mice. A

They say the drug caused inflammatory cells (macrophages) in the bone marrow to enhance the aggressiveness of the disease and provide the cancer cells with resistance to treatment.

"It is important to clarify that treatment with Velcade is essential and necessary," says Shaked, "but its disadvantage is that along with the benefit there is damage."

This creates the effect of a double-edged sword: although chemotherapy kills cancer cells, it also causes the secretion of substances that confer resistance to the tumor, Shaked says.

But when the researchers inhibited the secreted factor related to the activity of inflammatory cells, they observed a decrease in the proliferation of cancer cells, he noted, adding that now the team is working on various ways to inhibit the body’s response.

The researchers hope their findings will add to the knowledge on how cancer spreads – and what can be done to defeat it.

The study appears in Journal of Pathology.