Set-up verification of cervix cancer patients treated with long treatment fields; implications of a non-rigid bony anatomy

Background and purpose: For cervix cancer patients, treatment fields may ex tend up to vertebra L1. In clinical practice, set-up verification is based on measured displacements of the pelvic rim as visible in the caudal part o f the treatment fields. The implications of this procedure for the position s of bony structures in the cranial part of the fields were investigated. Materials and methods: Twelve patients had four repeat simulator sessions. Both during treatment simulation (the reference) and the repeat sessions, a nterior radiographs were acquired covering the whole treatment field. The f ilms were used to investigate differences between the cranial and the cauda l parts of the treatment field in day-to-day bony anatomy displacements. Results: Both in the transversal and the longitudinal directions, these dif ferences were significant (3.5 mm, 1 SD). Indications were found that large differences in the cranio-caudal direction may be correlated with (non-rig id) internal pelvic rim rotations around a lateral axis. In the longitudina l direction, the position of LI correlated much better with the position of vertebra S1 than with the position of the pelvic rim, which is usually use d for set-up verification. Conclusions: Due to the non-rigid bony anatomy of the studied patients, the usual set-up verification and correction procedure can result in set-up er rors of 10 mm and more for structures in the cranial part of the treatment field, even in the case of a perfect set-up of the pelvic rim. Possibly, ot her patient set-up and immobilization procedures may result in a better day -to-day reproducibility of the 3D bony anatomy shape. (Remaining) Differenc es in anatomy position changes between the caudal and cranial field ends ma y be accounted for by using non-uniform clinical target volume-to-planning target volume margins, or by an adapted patient set-up verification and cor rection protocol. (C) 2001 Elsevier Science Ireland Ltd. All rights reserve d.