It's unusual for cancer to spread/increase during treatment, but It has happened to me, and was going to say it could be an inflammatory response, as I was told in my case, and was closely monitored. Radiation should shrink a tumor a certain percentage, and if that doesn't seem to occur, doctors can make certain decisions such watch and wait, change treatment or to do surgery. Also, radiation keeps working a number of weeks after treatment stops. One of mine was still enlarged, had two cancerous, that we waited 5 months after treatment to do a PET scan, which came back clear, at least that scan did. Others doctors may scan after 6 weeks when the usual wait time is 3 months or 12 weeks, so they can effectively deal with any reminiscent cancer, and have heard of scanning during treatment, but not sure if that was part of a clinical trial or reason the reason for scanning, such as one trial checking for tumor hypoxia by PET scan, and another type of infusion.