Minimum Required Interval Between Hydrogel Spacer Injection and Treatment Planning for Stereotactic Body Radiation Therapy for Prostate Cancer.

Abstract

The present study evaluated the short-term characteristics (<3 days) of a hydrogel spacer from the time of injection during stereotactic body radiation therapy (SBRT) for prostate cancer. Fifteen patients treated with SBRT via the CyberKnife system (36.25 Gy/5 fractions) were enrolled in this retrospective study. Two magnetic resonance (MR) images were obtained with a hydrogel spacer: one on a computed tomography (CT) simulation day (MR pretreatment [MR_pre]) and the other on the last treatment day (MR posttreatment [MR_post]). Two medical physicists contoured the hydrogel spacer on each MR image. The changes of the shapes and the volume for the hydrogel spacer between 2 MR images were evaluated. The median period between hydrogel spacer injection and CT simulation was 1 day (range, 1-9 days). The median period between CT simulation and the last treatment was 17 days (range, 14-25 days). Regarding the volume change of the hydrogel spacer, the 2 observers observed significant differences between the volumes of the hydrogel spacer on the MR_pre and MR_post. However, the average volume difference between them was less than 1 cm³. The average dice similarity coefficient between the MR_pre and MR_post to compare the shape was more than 0.83. In addition, no clear correlation was confirmed between the volume change and the period from hydrogel spacer injection to CT simulation. A single day is an acceptable interval between hydrogel spacer injection and treatment planning for SBRT for prostate cancer.