Rotation of the sacrum during bellyboard pelvic radiotherapy

Patients with cervical, uterine, and rectal carcinomas are usually treated in the prone position using the bellyboard positioning device. Specific and uncomfortable prone position gives rise to uncertainties in the daily set-up of patients during the treatment. During investigation of translational movements, rotational movements of the pelvis are observed and investigated. The film portal imaging is used to discover patient positioning errors during treatment. We defined the rotational set-up errors by angle deviations of the sacrum. Thirty-six patients were included in the study; 15 patients were followed during the whole treatment and 21 during the first 5 consecutive treatment days. The image acquisitions were completed in 84%. Systematic and random positioning errors were analyzed in 725 images. Approximately half of the patients had adjusted to the bellyboard in the first few fractions, with sacrum angles remaining the same for the rest of the treatment. The other half had drifts of the sacrum angle during the whole treatment. The rotation of the sacrum during treatment ranged up to 14 degrees , causing the usual set-up verification and correction procedure to result in errors up to 15 mm. Rotational movements of the patient pelvis during bellyboard pelvis radiotherapy can introduce considerable patient position error.