Effect of image guidance on doses to the rectum and bladder in radiotherapy for cervical cancer

Objective: To analyze the effect of image guidance on the doses to the rectum and bladder in radical external beam radiotherapy for cervical cancer, and to investigate the reasonable application mode of image-guided radiotherapy (IGRT) in the treatment of cervical cancer. Methods: A total of 20 patients with cervical cancer who underwent helical tomotherapy (HT) in PLA Army General Hospital from 2012 to 2016 were enrolled in this study. A megavoltage computed tomography (MVCT) scan was performed before each treatment. The obtained MVCT images were used for dose reconstruction in the adaptive module of HT to obtain the actual dose (Plan-1) and the non-image-guided dose was simulated (Plan-2). Each single dose distribution and the corresponding fused CT image were sent to the software MIM 6.0 to obtain the total radiation dose by dose superposition. The radiation doses and volumes of the rectum and bladder were compared between the two therapeutic plans. Results: The radiation doses to the rectum and bladder in Plan-2 were significantly higher than those in Plan-1. There were significant differences in D_max and V₅₀ of the rectum and V₅₀ of the bladder between Plan-1 and Plan-2 (P = 0.040; P = 0.000; P = 0.047). Compared with Plan-1, there were statistical differences in inter-fractional D_max and V₅₀ during the initial treatment (P = 0.047, 0.037), and V₅₀ of the rectum within the 13^th to 21^st radiotherapy, respectively (P = 0.009, 0.017, 0.028). Besides, differences regarding V_max and V₅₀ in the initial treatment and the 21^st to 23^rd radiotherapy were close to the statistical significance when compared to those in Plan-1, respectively (P = 0.061, 0.053; P = 0.072, 0.058). Conclusions: IGRT can reduce the radiation doses and volumes of the rectum and bladder, especially the rectum. The therapeutic plan should be rescheduled when tumor retraction is evident at half of the total radiation dose (around 13^th fraction) in external beam radiotherapy. If it is difficult to achieve image guidance in each treatment, selective image guidance could be performed to effectively reduce the injuries of the rectum and bladder. (authors)