Background and purpose: Intensity-modulated radiation therapy (IMRT) is a widely used radiotherapy technique in whole breast irradiation. Therefore

planning target volume (PTV)

with appropriate margins from clinical target volume (CTV)

would adequately account for the setup uncertainty. However

it is still controversial regarding whether the protocol-required margin around the CTV adequately accounts for setup inaccuracies. The aim of this study was to determine the impacts of patient characteristics on the magnitude of geometric setup errors in patients with breast cancer and to obtain individualized PTV margins. Methods: A total of 97 patients treated with IMRT after breast-conserving surgery in Fudan University Shanghai Cancer Center from January 2019 to December 2019 were reviewed. Cone-beam computed tomography (CBCT) scan records were used to analyze setup errors. The impacts of patient characteristics on setup errors were also analyzed

including age

tumor location

body mass index (BMI)

chest circumference and breast volume. Population in the two groups was compared in terms of distribution of the median patient characteristics

and individualized PTV margins were obtained. Results: The BMI

chest circumference and breast volume showed moderate positive correlation with superior-inferior (Pearson=0.40

P0.01)

superior-inferior (Pearson=0.43

P0.01)

and right- left directions (Pearson=0.42

P0.01)

respectively. Meanwhile

there were significant differences in setup errors of superior-inferior direction (t=-3.923

P=0.001; t=-5.182

P=0.001; t=-2.712

P=0.008) among the three groups

and significant differences were  also observed in setup errors of left-right direction between the chest circumference and breast volume groups (t=-2.124

P=0.036; t=-3.550

P=0.001). When dividing patients into two groups on the basis of median value of BMI

chest circumference

breast volume and the margin in left-right

superior-inferior and anterior-posterior directions were 3.1-4.5

4.6-8.0 and 4.9-5.3 mm

respectively. Conclusion: Patient characteristics such as BMI

chest circumference and breast volume were significantly correlated with geometric setup errors. The individualized PTV margins were recommended to adequately compensate for the setup variation detected according to patient characteristics.