Purpose: Imaging for treatment planning shortly after hydrogel injection is optimal for practical purposes, reducing the number of appointments. The aim was to evaluate the actual difference between early and late imaging.
Patients and methods: Treatment planning computed tomography (CT) was performed shortly after injection of 10 ml hydrogel (CT1) and 1-2 weeks later (CT2) for 3 patients. The hydrogel was injected via the transperineal approach after dissecting the space between the prostate and rectum with a saline/lidocaine solution of at least 20-ml. Hydrogel volume and distances between the prostate and rectal wall were compared. Intensity-modulated radiotherapy (IMRT) plans up to a dose of 78 Gy were generated (rectum V70 < 20 %, rectum V50 < 50 %; with the rectum including hydrogel volume for planning).
Results: A mean planning treatment volume of 104 cm(3) resulted for a prostate volume of 37 cm(3). Hydrogel volumes of 30 and 10 cm(3) were determined in CT1 and CT2, respectively. Distances between the prostate and rectal wall at the levels of the base, middle, and apex were 1.7 cm, 1.6 cm, 1.5 cm in CT1 and 1.3 cm, 1.2 cm, 0.8 cm in CT2, respectively, corresponding to a mean decrease of 24, 25, and 47 %. A small overlap between the PTV and the rectum was found only in 1 patient in CT2 (0.2 cm(3)). The resulting mean rectum (without hydrogel) V75, V70, V60, V50 increased from 0 %, 0 %, 0.6 %, 10 % in CT1 to 0.1 %, 1.2 %, 6 %, 20 % in CT2, respectively.
Conclusion: Treatment planning based on imaging shortly after hydrogel injection overestimates the actual hydrogel volume during the treatment as a result of not-yet-absorbed saline solution and air bubbles.