Purpose: To describe a fiducial-based geometric and dosimetric method for detecting rectal balloon spacer failure and evaluating hydrogel salvage during prostate stereotactic body radiotherapy.

Methods and Materials: A patient underwent transperineal implantation of a biodegradable rectal balloon spacer and three intraprostatic fiducial markers. The initial planning CT represented an intact balloon (S1). Pretreatment cone-beam CT demonstrated loss of spacer geometry; treatment was withheld, deflation was confirmed, and a repeat CT represented the deflated state (S2). After hydrogel spacer placement, repeat simulation and replanning defined the salvage state (S3). Singular value decomposition point registration quantified fiducial-defined rigid transformations. Plans were compared using target coverage and rectal D2cc.

Results: The S1-to-S2 transformation was predominantly translational (0.50, −0.36, and 0.63 cm along the 3 axes; net displacement, 0.88 cm). With the original plan applied to the deflated-spacer anatomy, planning target volume coverage at 36.25 Gy decreased from 99% to 68%, clinical target volume coverage at 40 Gy decreased from 95% to 56%, and rectal D2cc increased from 12.9 to 29.1 Gy. After hydrogel placement and replanning, target coverage recovered to 98% and 96%, respectively, and rectal D2cc decreased to 22.2 Gy.

Conclusions: Fiducial-based registration converted a qualitative spacer abnormality into a measurable geometric change associated with major dosimetric deterioration. Withholding treatment, confirming device failure, and performing repeat simulation and replanning after hydrogel salvage restored target coverage and rectal sparing in this case. Larger studies are needed before a numerical intervention threshold can be recommended.