Assessment of intact cervix motion using implanted fiducials in patients treated with helical tomotherapy with daily MVCT positioning

Abstract

Objectives This study assesses the relative motion of the cervix during the course of treatment in patients with intact cervical cancers treated with pelvic intensity modulated radiation therapy (IMRT). Methods Nineteen patients with intact cervical cancer were treated definitively with tomotherapy using daily megavoltage (MV) CT positioning to pelvic bony anatomy. Textured gold fiducials were placed in three of four quadrants of the cervix prior to CT simulation. Daily MVCTs were registered to the planning CT based on bony anatomy. The center of mass (COM) of the fiducials was used to assess interfractional cervical motion by comparing the relative positions of the COMs. Tumor regression was indirectly assessed by calculating the contraction in the relative distance between fiducial pairs. Results Fourteen patients were evaluable. Average vector motion of the COM was 7.0 mm (SD=5.7 mm; range of 0 to 34 mm). Ninety-five percent of fiducial excursions were encompassed by 8 mm right–left, 12 mm anterior–posterior, and 11 mm superior–inferior. Ninety-five percent of all measured COM motions were ≤18 mm. Contraction was observed in 9 out of 12 patients that were analyzable. In those nine patients, the average distances between the fiducial pairs were 23 mm (range, 5.8 to 48 mm) and 18 mm (range, 3.8 to 41 mm) intheplanningCTand inthe lastdailyMVCT, respectively. On average, fiducials moved 23 % closer together. Conclusion Quantification of the excursions of the intact cervix using daily imaging of implanted fiducials was feasible and useful to define appropriate clinical target volume–planning target volume (CTV–PTV) margins for IMRT. Nonuniform CTV–PTV margins may be appropriate based on differential directional motion.