Analysis of the placing position error of radiotherapy for rectal cancer patients with different body mass index

Abstract

Objective To measure the placing position error of different body position by analog positioning machine in patients with different body mass index (BMI) during radiotherapy for rectal cancer, then calculate the clinical target volume (CTV) to plan target volume (PTV) margins (Mptv). Methods Thirty-six patients with rectal cancer were selected, and the patients were divided into vacuum bag group (18 cases) and routine belly board group (18 cases) according to method of placing position. The BMI was calculated. Each patient was treated with positive and lateral position X-ray film before treatment and once a week during the radiotherapy. The data were contrasted with digitally rendered radiographs (DRR) of three-dimensional conformal radiotherapy plan system, then the placing position error on X,Y and Z axis was calculated, and the Mptv on X,Y and Z axis of patients with different BMI was calculated. Results The placing position error on X,Y and Z axis gradually increased with the increase of BMI in vacuum bag group and routine belly board group, and there were statistical differences (P <0.01 or <0.05). The Mptv on X,Y and Z axis in patients with normal BMI were 4.60, 5.51 and 5.29 mm respectively. The Mptv on X,Y and Z axis in patients with overweight were 5.48, 6.81 and 6.16 mm respectively. The Mptv on X,Y and Z axis in patients with obesity were 8.92, 8.59 and 7.02 mm respectively. Conclusions The rectal cancer patients with overweight and obesity have more placing position error. The influence of BMI should be considered when the patient's Mptv is determined. Key words: Rectal neoplasms; Radiotherapy, computer-assisted; Body mass index; Pendular position errors