I really don't understand

My husband had synovial sarcoma in his foot and had it amputated. He had clear PET and CT scans for close to 3 years. January of this year his sides started hurting he thought he pulled something. He went to the doctor January 20th and they found nothing. 2 days after he turned 42 on February 24 he went to the ER because he was so sick and they told him that on top of having pneumonia that his sarcoma was back but in his lungs. He was told by his oncologist that chemo would be the only option with the tumors he has in his lungs. He had them on both sides. He has had started chemo and they have him anywhere from 12 to 24 months to live. I don’t understand how it came back so fast for him either when he was doing everything right.