Data Sheet 1_Case Report: Acute prostatic swelling following hydrogel spacer insertion managed by MR-guided online adaptive radiotherapy.docx

While generally safe, acute anatomical changes in the prostate gland itself, specifically significant swelling, are poorly described and can compromise the precision of radiotherapy.</p>Case presentation<p>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.</p>Conclusions<p>We report a rare instance of acute prostate swelling following hydrogel spacer insertion.