Can radiation therapy increase cell growth?

No and Yes: Radiation therapy slows or stops cell growth. The goal is of course to kill cancer cells and they don't grow and die. However if you look up the four r's of radiation biology. The last one is repoplulation in which during fractionated radiation cells increase growth to repopulate. Luckily we see this more often in the good cells but unfortunately in some cases resistant cancer cells.

No: Radiation acts on a cellular level to break single or double strands of dna, and most cells have repair enzymes to repair the damage, but double strand breaks are more lethal. Cell cycle is controlled by complex molecular paths, but cells are most susceptiple to dying as the chromosomes thicken during metaphase, and most resistant during synthesis/transciption. No way for increased cell growth.