Prone versus supine position in radiotherapy (RT) of rectal cancer – which is better for treatment reproducibility?

PURPOSE:Radiotherapy is the standard adjuvant treatment for rectal cancer. However, a standard for patient positioning is still not established. The aim of this study was to evaluate the impact of patient positioning on the reproducibility in treatment of rectal cancer. PATIENTS AND METHODS: 91 consecutive patients with rectal cancer were randomized to be treated in the supine or prone position, respectively. A total dose of 50-54 Gy was given in 25-27 daily fractions of 2 Gy. Reproducibility was defined by deviations between expected and actual position of bony landmarks in the pelvic region. Set-up errors were measured in the three orthogonal directions; anterior-posterior (AP), medio-lateral (ML) and superior-inferior (SI) by electronic portal imaging and the resulting displacement vector was calculated. Imaging and corrections of patient positioning during the RT were performed according to the department´s verification protocol. Average values representing the set-up error, was calculated for each patient in the three directions and for the displacement vector. The vector values was logtransformed in order to obtain normally distributed data for parametric tests. A non-parametric test (Mann Whitney U-test) was also performed for the vector length median values. RESULTS: Data from 81 evaluable patients were reviewed (40 in prone position and 41 in supine position). The differences in mean set-up errors between supine and prone positioning were small (Table 1). Prone position has the largest standard deviation for displacement for all directions and also for the log-vector length compared to supine position. The mean and median value for the vector length is largest for prone positioning. The vector length in prone position is significantly larger (p > 0.05). No statistically significant differences were identified in mean set-up errors in any of the directions (AP, ML and SI) between the two treatment positions. More corrections according to the set up protocol were required with prone positioning. CONCLUSION: This randomized study indicates that prone position seems to be associated with a lower degree of reproducibility than the supine position in rectal cancer. Although the difference in displacement vector length between the patient positions were statistically significant, it may be questioned if this relatively small difference is of clinical relevance.