Investigation of the changes in the prostate, bladder, and rectal wall sizes during external beam radiotherapy

The change in the prostate size for radiotherapy has not yet been elucidated. The coverage of radiation dose is affected by changes in the prostate size. We evaluated the changes in the prostate, rectum, and bladder wall sizes during IMRT of fraction 2 Gy/day using MRI.

Twenty-four patients with prostate cancer were enrolled in this study. MRI was performed at three time points. While the initial MRI was performed before the start of radiotherapy (RT), the second MRI was performed at 38 Gy (range: 36-40 Gy), which represented the halfway point of the RT course. The last MRI was performed on the day of completion of the RT course (76 Gy; range: 74-78 Gy). We estimated the prostate, rectum, and bladder wall sizes at three time points.

We observed no significant difference between the estimated sizes of the prostate during RT in all three phases. In addition, the volume of the rectal wall remained unchanged in all phases. However, the volume of the bladder wall significantly decreased from the initial to the last time points. Furthermore, the standard deviation (SD) obtained by subtracting the final size from the initial one was large (mean, 30.1; SD, 10.1).

The volume of the bladder wall decreased during IMRT. The range of subtraction of the volume of the bladder wall was extensive. Thus, the estimation of the bladder wall may be useful to reduce the inter-fraction variation.