Although virus did not, directly, cause a tumor (it is a by product of the infection), only its removal can cure the cancer. Since tumor and metastases evolve as one system as a protection measure, host resits to therapy. Primary tumor generates anti-angiogenic and keeps the metastases dormant. If perturbs, like in surgery, a metastatic development breakout may take place. http://annonc.oxfordjournals.org/content/19/11/1821.full
"The effects of surgery on tumor growth: a century of investigations
A few clinical investigations suggest that while primary Breast Cancer surgical removal favorably modifies the natural history for some patients, it may also hasten the metastatic development for others. ..."