A              B                              C
   hello          Radiation therapy              NA
   Hello1         hello2 for neurology           hello3 radiation

A              B                            C
   hello          therapy                      NA
   Hello1         hello2                       hello3

df <- data.frame(B = c("Radiation therapy", "hello2 for neurology"))

df$B <- gsub("Radiation | for.*", "", df$B)