Non-surgical. Active surveillance usually means following a cancer patient on a regular basis with medical visits to be examined, blood/urine tests, radiologic (x-ray +/or ultrasound) studies + possibly biopsies. Used in urology for instance to hopefully avoid radical prostatectomy in patients with low grade prostate cancer, or to hopefully avoid major abdominal surgery (node dissection) with testicular cancer.
See below. It is the concept of aggressively monitoring a disease state for a ceratin outcome or threshold at which point there is intent to intervene.