Case Report: Acute prostatic swelling following hydrogel spacer insertion managed by MR-guided online adaptive radiotherapy.

BACKGROUND

Hydrogel spacers are widely used in radiotherapy for localized prostate cancer to reduce the rectal dose. While generally safe, acute anatomical changes in the prostate gland itself, specifically significant swelling, are poorly described and can compromise the precision of radiotherapy.

CASE PRESENTATION: We report a case of a 62-year-old man with high-risk prostate cancer (PSA 15.9 ng/mL, Gleason score 4 + 4, cT2cN0M0) who underwent hydrogel spacer placement for stereotactic body radiotherapy (SBRT) (40 Gy in 5 fractions). Although magnetic resonance imaging (MRI) for radiation planning on day 8 postinsertion revealed a normal prostate volume (22.0 cc), pretreatment MRI for MR-guided online adaptive radiotherapy (MRgOART) on day 23 (at the second fraction) revealed a peak volume of 31.4 cc, a 42.7% increase. This swelling compromised the dosimetry of the initial plan, risking insufficient coverage of the clinical target volume (CTV). Using MRgOART, the plan was successfully reoptimized to prevent target underdosing.

CONCLUSIONS

We report a rare instance of acute prostate swelling following hydrogel spacer insertion. Although the precise etiology remains unconfirmed, such an anatomical change, if unrecognized, could degrade treatment accuracy. This case demonstrates that online adaptive radiotherapy via an MR-Linac is essential for detecting and managing unexpected anatomical variations during prostate SBRT.