metastatic lung cancer to sacral bone.

Hello all..I am the daughter of a lung cancer survivor. My dad was  diagnosed with non small cell lung cancer-squamous  cell in may of 2011. had  chemo/radiation. In 2012, he was told he had a 8mm lesion on L5-S1 sacral bone. He did have 10 radiation treatments to that spot.My question is. He had a  Pet scan on 4/11/13.It indicated that this  area had a  higher suv rating .Non the less, no other areas  of metastasis. How in 2 yrs does a person only have 1 spot of mets  if its cancer?

He had a biopsy yesterday. We are waiting on results. the size of the "spot" is 2.4 x 1.5.now.Is that common to have 1 area.He is 79 yrs old.

Thanks,Kado.