To assess the subjective variability in gross tumor volume(GTV) delineation by radiation oncologists

six physicians from Departments of Radiotherapy and Radio-diagnosis were invited to delineate the GTV on sequential CT slices twice with one-month interval.The CT images were from 12 cancer patients with different tumor location in their body.The variability of GTV volume value and its difference from inter-speciality and inter-interval were calculated.A high variability of GTV volume was found

the ratio of the largest to the smallest volume was less than 2 in 9 patients

while it was larger than 2 in all the others.As for the change range of GTV volume compared with its mean value in each patient

there were 16% volume values larger than ±25%.With the well-controlled objective in-fluences

there were statistical differences of GTV volume of 3 patients in the inter-speciality variation.The variation within the radiation oncologist was larger than that within the radio-diagnosticians.A large subjective variability in GTV delineation was demonstrated.A better training and cooperation of radiation oncologist

and standardization of the QA and QC approach with guidelines will result in a reduction in this variation of radiotherapy planning.