Margin evaluation of translational and rotational set-up errors in intensity modulated radiotherapy for cervical cancer.

Xiang Zhang,Ji-ping Liu,Bin-bing Wang,Guo-ping Shan

doi:10.1186/s40064-016-1796-2

Abstract

A clinical target volume (CTV) to planning target volume (PTV) margin recipes was routinely used to ensure dose was actually delivered to target for all (most) patients. Currently used margin recipes were associated with only translational set-up errors in radiotherapy. However, when set-up errors extended to six-degree (6D) scope (three translational and three rotational set-up errors), margin recipe should be re-evaluated. The purpose of this study was to investigate dosimetric changes of targets (both CTV and PTV) coverage when 6D set-up errors were introduced and testify the practicability of currently used margin recipe in radiotherapy. A total number of 105 cone beam computer tomography scans for ten patients with cervical cancer were derived prior to treatment delivery and 6D set-up errors were acquired with image registration tools. Target coverage was evaluated retrospectively for 6D set-up errors introduced plan with 6 mm CTV to PTV margin. Target coverage of PTV showed significant decreases (3.3 %) in set-up errors introduced plans compared with original plans. But CTV coverage was not susceptible to these set-up errors. A tendency of coverage decrease for PTV along with distance away from treatment was testified, from −0.2 to −6.2 %. However, CTV seems changed less, from −0.2 to −0.8 %. The result indicate that a CTV to PTV margin of 6 mm was sufficient to take into account 6D set-up errors for most patients with cervical cancer. Future research suggests a smaller margin to further improve both tumor coverage and organs at risk sparing.

Highlights

External beam radiotherapy (EBRT) followed by brachytherapy is the standard treatment for cervical cancer, especially for advanced disease
The purpose of this study is to evaluate the impact of correct rotational set-up errors in dosimeteric with clinical obtained data from multi-fractions
Translational and rotational set‐up errors Of 105 observations for the 10 patients with cervical cancer, distances from cranial to caudal for clinical target volume (CTV) and planning target volume (PTV) were 19.9 ± 2.3 and 20.9 ± 2.3 cm, respectively

Summary

Introduction

External beam radiotherapy (EBRT) followed by brachytherapy is the standard treatment for cervical cancer, especially for advanced disease. Intensity modulated radiotherapy (IMRT) is increasingly used for EBRT of cervical cancer for its satisfying dose distribution on a plan to ensure target dose and limit normal tissue irradiation (Chen et al.2007; Brixey et al 2002; van de Bunt et al 2006; Ahamad et al 2005). By sort of image registration method, one can estimate patient pose difference in the treatment room from the planning CT. With this advanced imaging techniques, 6D set-up errors (3 translational and 3 rotational, pitch, roll and yaw) can be detected and derived. Rotational set-up errors are not routinely corrected because most couches currently available can only correct one degree of these rotational movements

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