Evaluation of setup errors in conformal radiotherapy for pelvic tumours: Case of the Regional Center of Oncology, Agadir

Abstract

Context: The errors made during repetitive patient placements affect significantly the accuracy of treatment and the results of radiotherapy. Objectives: To determine the total, systematic and random error in order to estimate the margin between the Clinical Target Volume (CTV) and the Planning Target Volume (PTV) in the pelvic region. Materials and methods: Set up errors was estimated by superimposing a digitally reconstructed radiograph (DRR) as a reference image with an electronic portal image device (EPID). The total errors in the Medio-Lateral (ML), Cranio-Caudal (CC) and Antero-Posterior (AP) directions were compared by t-test. For systematic and random errors, the ratio of variance (F statistic) was used. Margins were calculated using Van Herk formalis. Results: 208 images (80 DRRs and 128 EPIDs) were assessed. The systematic error ranged from 1.93 to 2.01 mm, 1.26 to 1.39 mm, and 1.20 to 2.94 mm in the x, y and z-axis, respectively. The random error ranged from 2.33 to 2.90 mm, 1.25 to 1.66 mm, and 1.04 to 1.28 mm. The PTV margin according to the Van Herk equation in the x, y and z directions was estimated to be 7.11, 4.64 and 3.90 mm for the cervix uteri and a 6.47, 4.03, and 3.70 mm for the rectum. Conclusion: The use of weekly EPID/DRR allows estimating the setup of the planning target volume (PTV) expansion according to our configuration. In this study, the evaluated set up margin was approximately 8 ​mm in the X-axis, 5 ​mm in Y and Z axis for pelvic conformal radiotherapy.

Keywords

• Setup errors
• PTV margin
• Conformal radiotherapy
• Pelvic cancers