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. 2018 Feb;91(1083):20170672.
doi: 10.1259/bjr.20170672. Epub 2018 Jan 23.

Efficacy of a Rectal Spacer With Prostate SABR-first UK Experience

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Free PMC article

Efficacy of a Rectal Spacer With Prostate SABR-first UK Experience

Raymond B King et al. Br J Radiol. .
Free PMC article

Abstract

Objective: This study assessed the use of implanted hydrogel rectal spacers for stereotactic ablative radiotherapy-volumetric modulated arc therapy (SABR-VMAT) patients, investigating practicality, dosimetric impact, normal tissue complication probability (NTCP) and early toxicity.

Methods: Data from the first 6 patients treated within a prostate SABR and rectal spacer trial were examined to determine spacer insertion tolerability, resultant changes in treatment planning and dosimetry and early toxicity effects. CT scans acquired prior to spacer insertion were used to generate SABR plans which were compared to post-insertion plans. Plans were evaluated for target coverage, conformity, and organs at risk doses with NTCPs also determined from resultant dose fluences. Early toxicity data were also collected.

Results: All patients had successful spacer insertion under local anaesthetic with maximal Grade 1 toxicity. All plans were highly conformal, with no significant differences in clinical target volume dose coverage between pre- and post-spacer plans. Substantial improvements in rectal dose metrics were observed in post-spacer plans, e.g. rectal volume receiving 36 Gy reduced by ≥42% for all patients. Median NTCP for Grade 2 + rectal bleeding significantly decreased from 4.9 to 0.8% with the use of a rectal spacer (p = 0.031). To date, two episodes of acute Grade 1 proctitis have been reported following treatment.

Conclusion: The spacer resulted in clinically and statistically significant reduction in rectal doses for all patients. Advances in knowledge: This is one of the first studies to investigate the efficacy of a hydrogel spacer in prostate SABR treatments. Observed dose sparing of the rectum is predicted to result in meaningful clinical benefit.

Figures

Figure 1.
Figure 1.
Axial (a–c) and sagittal (d–f) CT and MRI images acquired from patient 6 prior to and after hydrogel spacer insertion. (a, d) CT images acquired prior to spacer insertion. (b, e) CT images acquired following spacer and fiducial marker insertion. (c, f) Corresponding post-spacer MR images. Images include contours for the prostate and seminal vesicles CTV (purple) and PTV (red), bladder (yellow), rectum (blue) and spacer (cyan) structures and dashed lines corresponding to the 100 and 50% isodoses of the PTV prescription dose. CTV, clinical target volume; PTV, planning target volume. (Colour available online only)
Figure 2.
Figure 2.
Dose volume histograms for target structures and rectum for individual patients, for optimized plans generated using the post-spacer (solid lines) or pre-spacer (dashed lines) CT images.
Figure 3.
Figure 3.
Change in Grade 2 + rectal bleeding NTCP, calculated using QUANTEC LKB model parameters, for individual patients as a result of using a hydrogel rectal spacer. LKB, Lyman–Kutcher–Burman; NTCP, normal tissue complication probability.

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