The present invention relates to biomarkers to monitor the activity of the compound 3-Z-[1-(4-(N-((4-methyl-piperazin-1-yl)-methylcarbonyl)-N-methyl-amino)-anilino)-1-phenyl-methylene]-6-methoxycarbonyl-2-indolinone or a pharmaceutically acceptable salt thereof, and especially its monoethanesulphonate salt form, when used alone or optionally in combination with further pharmaceutically active ingredients and/or further treatments, such as for example radiotherapy.