A study of changes in volume and location of target areas and organs at risk in intensity-modulated radiotherapy for cervical cancer

Abstract

Objective To investigate the changes in volume and spatial location of target areas and normal tissues before and during intensity-modulated radiotherapy(IMRT)for cervical cancer by quantitative means. Methods Forty patients with cervical cancer who were treated with IMRT were enrolled as subjects. Computed tomography(CT)was performed before IMRT and during IMRT when a dose of 27 Gy(15 fractions)was reached. Clinicians delineated the target areas and organs at risk in the two groups of CT images. The target areas and organs at risk in one group of images were mapped to the other group of CT image by image registration using the Pinnacle treatment planning system. Volume changes in target areas and organs at risk were analyzed, and changes in the spatial location were evaluated by volume difference method and Dice similarity method. Comparison was made by paired t-test. Results There were significant differences in gross target volumes of primary tumor lesions(GTV-T)and pelvic metastatic lymph nodes(GTV-N)before and during IMRT(P= 0.000; P= 0.000). According to the evaluation by volume difference method, the average rate of change in GTV-T was(38.64±19. 50)% with a range between 3. 16% and 86. 49%, while the average rate of change in GTV-N was(42.49± 25. 68)% with a range between 2. 79% and 87. 42%. In the organs at risk, the bladder had the maximum rate of volume change, the average of which was(55.13±33. 40)% with a range between 3. 25% and 116. 01%. According to the evaluation by Dice similarity method, the average Dice similarity coefficient for GTV-T was 0.50± 0. 18 with a range between 0. 10 and 0. 85, while the average Dice similarity coefficient for GTV-N was 0.31±0. 20 with a range between 0.00 and 0.71. The rectum had the minimum Dice coefficient in the organs at risk, the average of which was 0.57± 0. 14 with a range between 0. 18 and 0. 76. Conclusions For patients with cervical cancer to receive IMRT, since there are substantial changes in volume and spatial location of target areas and normal tissues before and during treatment, it is quite necessary to modify the treatment regimen in time in order to provide adequate doses for target areas and avoid overdose for organs at risk. Key words: Cervical neoplasms/intensity-modulated radiotherapy; DICE parameter; Volume change rate