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Spacer insertion prior to prostate radiotherapy can improve patient toxicity outcomes through improved sparing of organs at risk without PTV compromise. At our centre, hyaluronic acid spacers are inserted under transrectal ultrasound (TRUS) intra-operatively prior to the insertion of interstitial needles and HDR treatment planning and delivery whilst the patient is under a general anaesthetic. Prior to spacer use at our HDR brachytherapy centre, the CTV would be expanded by 3mm in all but the posterior direction due to the proximity of the rectum to create the PTV (posterior having a 0mm margin). Due to the additional space created by the spacer, we have extended the posterior PTV margin to 3mm to create a 3mm isometric expansion in hope to achieve fewer local recurrences. We present the results of a direct dosimetric comparison of the differences between a no-spacer plan with 0mm posterior PTV margin versus a spacer plan with 3mm isometric PTV margin.

This is a single-centre retrospective review of 10 patients with prostate cancer undergoing an intra-operative hyaluronic acid spacer insertion followed by interstitial needle insertion and TRUS guided HDR brachytherapy boost treatment planning and delivery. For each patient two plans were created to allow dosimetric comparison. The spacer plan (with a 3mm isotropic PTV margin) versus a no-spacer plan (with 0mm posterior PTV margin). The spacer plan was the used plan in each patient’s treatment.

10 cases from June 2021 to November 2021 were analysed. Patients had intermediate to high risk prostate cancer and age range 60-77. The average prostate gland size was 47cc. Through spacer insertion, the average prostate-rectal space was 9.1mm when measured at mid-gland in the axial plane on TRUS. TRUS images of pre- and post-spacer insertions were comparable and the presence of a hyaluronic acid spacer did not impede anatomical visualization in any of our 10 cases. We compared dosimetry from the actual plan used (spacer plan with 3mm posterior PTV margin) and non-spacer plan (0mm posterior PTV margin) and found the dosimetry to be as expected. The mean PTV D90% in the spacer plan was 15.83Gy versus 15.47Gy in the no-spacer plan. Likewise, the rectal D2cc was 9.87Gy versus 10.45Gy, the urethral D10% was 16.94Gy versus 16.92Gy and the urethral D30% was 16.4Gy versus 16.4Gy in the spacer versus no-spacer plans.

In this small review, we have shown that increasing the prostate posterior PTV margin by 3mm in the presence of a spacer is safe and provides similar dosimetry to traditional treatment techniques. We hope this will lead to fewer local relapses in time. We also demonstrate safe and effective intra-operative insertion of hyaluronic acid spacers immediately prior to HDR prostate brachytherapy boost treatment planning and delivery. This enables our patients to have only one invasive procedure allows our department to use its resources more efficiently.