Analysis of set-up errors during radiotherapy of prostate cancer

LIU Yueping; JIN Jing; WANG Weihu; WANG Shulian; SONG Yongwen; LI Yexiong

Abstract：Objective To analyze the set-up errors in the left-right( LR, X axis) , superoinferior ( SI, Y axis) and antero-posterior (AP, Z axis) directions and the rotations during radiotherapy of prostate cancer in the supine position. Methods Twenty-five prostate cancer patients who received radiotherapy from October 2011 to June 2013 were analyzed. All the patients were treated with intensity-modulated radiotherapy ( IMRT) in the supine position after external immobilization using thermoplastic shell. Measurements for set-up errors and rotations in X, Y, Z directions were collected from daily image-guided radiotherapy( IGRT, bony match) pre-treatment. The set-up errors and rotations were analyzed. Results Each patient received 9 fractions of measurement with a total of 225 fractions. Mean set-up errors were (0. 19±0. 18)cm, (0. 36±0. 30)cm, and(0. 21±0. 16)cm in the X, Y and Z axes, respectively. The set-up errors of≥5 mm was 5. 8%, 24. 3%, and 8. 0% of all measured fractions in the X, Y and Z axes, respectively. Mean rota-tions were (1. 07±1. 03)°, (0. 82±0. 66)°, and (0. 79±0. 68)°in the axis, vertical, and lateral directions, respectively. After com-paring the first 5 and the following 4 set-up and rotation errors, there were no significant differences( P>0. 05) . Conclusion The set-up errors is larger in the Y than in the X and Z axes during radiotherapy of prostate cancer in the supine position. The rotation errors are much smaller and can be omitted.