Interfractional change of high-risk CTV D90 during image-guided brachytherapy for uterine cervical cancer

Abstract

The purpose of this study was to evaluate interfractional changes of the minimum dose delivered to 90% of the high-risk clinical target volume (HR-CTV D90) and D2cc of the bladder and rectum during brachytherapy for uterine cervical cancer patients. A total of 52 patients received external beam radiotherapy and high-dose-rate intracavitary brachytherapy (ICBT). For each of four ICBT applications, a pelvic CT scan was performed and the HR-CTV was delineated. Retrospectively, these patients were divided into two groups: (i) the standard dose group with 6 Gy to point A in each ICBT, and (ii) the adaptive dose group with a modified dose to point A to cover the HR-CTV with the 6-Gy isodose line as much as possible. The HR-CTV D90 was assessed in every session, and analyzed as interfractional changes. In the standard dose group, the interfractional changes of the HR-CTV D90 showed a linear increase from the first to the third of the four ICBT (average 6.1, 6.6, 7.0 and 7.1 Gy, respectively). In contrast, those of the adaptive dose group remained almost constant (average 7.2, 7.2, 7.3 and 7.4 Gy, respectively). Especially, in the case of a large HR-CTV volume (≥35 cm(3)) at first ICBT, the total HR-CTV D90 of the adaptive dose group with brachytherapy was significantly higher than that of the standard dose group. There were no significant differences in total D2cc in bladder and rectum between the two groups. Image-guided adaptive brachytherapy based on interfractional tumor volume change improves the dose to the HR-CTV while keeping rectal and bladder doses within acceptable levels.

Keywords: 3D image-based planning; dose-volume histogram analysis; high-dose-rate brachytherapy; radiotherapy; uterine cervical cancer.

Figures

Fig. 1.

Axial computed tomography image of an example case of ICBT. (A) A dose of 6 Gy was administered to point A. The high-risk clinical target volume (HR-CTV) was not covered with the 6-Gy isodose line. (B) A dose of 7 Gy was prescribed to point A. The 6-Gy isodose line almost encompasses the HR-CTV.

Fig. 2.

Changes during treatment time-course of prescribed point A dose (A), and the HR-CTV D90 (B) of first to fourth HDR-ICBT. In the standard dose group, the first and second HR-CTV D90 were significantly lower than those of the adaptive dose group. Circles and solid lines show the averages for the standard dose group; boxes and dotted lines show the averages for the adaptive dose group. Bars indicate standard deviations. Asterisks indicate P < 0.05 by Mann–Whitney U test.

Fig. 3.

Changes in the HR-CTV D90 and volume during treatment time-course in differentiated pretreatment tumor size groups. A small group (≤4 cm) is shown in (A) and (D), a medium group (4–6 cm) in (B) and (E), and a large group (≥6 cm) in (C) and (F). At first ICBT, the HR-CTV D90 in the standard dose group was significantly lower than in the adaptive dose group for the medium and large tumor groups. Circles and solid lines show averages for the standard dose group; boxes and dotted lines show averages for the adaptive dose group. Bars indicate standard deviations. Asterisks indicate P < 0.05 by Mann-Whitney U test.

Fig. 4.

Changes in the HR-CTV D90 and volume during treatment time-course in differentiated HR-CTV volumes at first ICBT. The ‘small’ group (3) is shown in (A) (C) and the ‘large’ group (≥35 cm3) in (B) (D). At first, second and third ICBT, the HR-CTV D90 in the standard dose group was significantly lower than in the adaptive dose group in the large tumor groups. In the adaptive dose group, the HR-CTV volume at second ICBT diminished more steeply in the large tumor groups. Circles and solid lines show averages for the standard dose group; boxes and dotted lines show averages for the adaptive dose group. Bars indicate standard deviations. Asterisks indicate P < 0.05 by Mann–Whitney U test.

Fig. 5.

Changes in D2cc of bladder and rectum during treatment time-course in differentiated HR-CTV volumes at first ICBT. A small group (3) is shown in (A) and (C) and a large group (≥35 cm3) in (B) and (D). Circles and solid lines show averages for the standard dose group; boxes and dotted lines show averages for the adaptive dose group. Bars indicate standard deviations. Asterisks indicate P < 0.05 by Mann–Whitney U test.