Assessment of anatomical and dosimetric changes by a deformable registration method during the course of intensity-modulated radiotherapy for nasopharyngeal carcinoma

Abstract

The aim of this study was to quantify the anatomic variations and the dosimetric effects accessed by a deformable registration method throughout the entire course of radiotherapy, and to evaluate the necessity of replanning for patients with nasopharyngeal carcinoma (NPC). Plan1(CT1) was based on the original CT, and Plan2(CT2) was generated from the midtreatment CT scan acquired after 25 fractions of IMRT of Plan1. Both sets of CTs, RT structures and RT doses for the two group plans were transferred to a workstation, and then a hybrid IMRT plan, Plan1(CT2), was generated by deforming doses of Plan1 to CT2. Subsequently, the accumulated plan, Plan1 + 2(CT2), was generated to quantify the actual dosimetric effects during the course. The transverse diameter of the neck at the center of the odontoid process was (15.4 ± 1.0) cm and (14.4 ± 1.1) cm in CT1 and CT2, respectively (P < 0.05). Compared with CT1, the mean volumes of the right and left parotid glands were significantly decreased by (24.6 ± 11.9)% and (35.1 ± 20.1)%, respectively. Comparison of Plan1 (CT1) with Plan1 (CT2) indicated that the doses to targets decreased without replanning. With repeated CT and replanning after 25 fractions, the doses to targets would be improved. The doses to normal tissue were increased without replanning. For eight patients out of 12, the doses to the spinal cord and brainstem exceeded the constraints without replanning, while the corresponding values decreased with replanning. During the entire course of IMRT, the volumes of the targets and the parotid glands would be reduced significantly. Midtreatment CT scanning and replanning are recommended to ensure adaptive doses to the targets and critical normal tissues.

Keywords: deformable registration; dosimetry; nasopharyngeal carcinoma; radiotherapy plan.

Fig. 1.

Measurement of transverse diameter at the center of odontoid process.

Fig. 2.

An example of the volumetric changes for the parotid glands between the first CT (CT1) and the second CT (CT2) images. Shown on images are right parotid contour (red) and left parotid contour (blue) for CT1, and right parotid contour (green) and left parotid contour (yellow) for CT2.

Fig. 3.

The dose distribution comparison of treatment planning during the course of IMRT. Plan 1(CT1) is the original plan based on the first CT scan; Plan2 (CT2) is the second IMRT with only the number of fractions used to complete the treatment based on the second CT; Plan1(CT2) is first IMRT plan deformed to the second CT scan; Plan1 + 2(CT2), representing the actual situation in which replanning would have occurred, is the accumulation of Plan1(CT1) and Plan2(CT2).