(CNN) Squeals of delight echo around the room at Vanderbilt's Children's Hospital in Nashville, Tennessee. Nine-year-old cancer survivor Bryce Greenwell turns to his mother, Jenny. "Did you know about this?" he asks, hands cupped to his face in surprise. "Yes," she says with a nod, smiling from ear to ear. "Oh, my gosh, he has missed you guys so much!" pet therapist Michelle Thompson says as she hands Swoosh, her trained toy Pomeranian, over to two eager boys. "Did you miss him? He talks to me about you boys all the time." "I haven't seen him in a long time," says 7-year-old Mitchell Montalbano, who recently graduated from chemotherapy treatment, cancer-free. "Swoosh, what have you been up to?"