[en] Background and purpose: The intention of this prospective study is to assess the influence of different patient positionings and the use of belly boards on the coverage of the uterus by standard radiation fields. Material and methods: In 21 women with carcinoma of the uterine cervix magnetic resonance imaging (MRI) scans in prone patient position with and without belly board and computed tomography (CT) scans in supine position were analysed after superimposing standard pelvic box fields. Further, all patients underwent a second MRI field control in prone position with belly board to detect intraindividual variations in the uterus position during treatment. Results: Standard portals did not completely cover the uterus in supine position in 7/21 (33%), in prone position with belly board in 7/21 (33%) and without belly board in 5/21 (24%). Insufficient uterine coverage was found only in the anteroposterior direction. The mean distance (± standard deviation) between the field borders of the lateral portals and the uterus was in supine position anteriorly 3.4 cm (±2.2 cm) and posteriorly 1.8 cm (±1.3 cm), in prone position with belly board anteriorly 2.2 cm (±2.7 cm) and posteriorly 2.6 cm (±1.6 cm), prone without belly board anteriorly 3.3 cm (±2.4 cm) and posteriorly 1.9 cm (±1.1 cm). The difference was statistically significant between supine and prone position with belly board and between prone position with and without belly board. Repeated MRI controls during therapy showed no significant changes compared to the MRIs at the beginning of therapy. Conclusions: The use of standard radiation fields results in a high percentage of geographical misfits. Three-dimensional treatment planning is a prerequisite for adequate uterus coverage