Scan after treatment of SCLC

Hi all, 

My dad was diagnosed with small cell lung cancer in August (10 cm mass) after a biopsy of a mass found in a CAT scan. He quickly started chemo because his doctor felt it was the most important. After the first round of chemo we finally did a PET scan and found that the cancer had responded and was down to about 8cm already...so they recommended that he start radiation. He did 30 days of high dose radiation of his lung while doing chemo weekly at a significantly reduced amount. After radiation he had two more full rounds of chemo (three days a week every three weeks) and we were told treatment was over. About three weeks after the last round of chemo and a month and a half after radiation ended they did a CAT scan of his lung and it still reports a 4.8 cm mass. This seemed big and was a huge bummer...but the radiation oncologist says it is likely dead cancer and scar tissue that the body needs to expel and heal and will take a long time. I asked how we know for sure and he said there isn't any way to know for sure. They want to just wait and see if it grows. This is making me very uneasy. Has anyone had the experience of a mass still showing on the scan after treatment and told the only way to tell if it's cancer is to wait and see if it grows? My dad loves his doctors but I'm worried we shouldn't be seeking a second opinion. Thank you!