Application of a Radiopaque Viscous Hydrogel Spacer for Prostate Cancer Radiotherapy - A Prospective Multi-Institutional Phase II Study.

PURPOSE/OBJECTIVE(S): Rectal dose can be significantly decreased after spacer application. An initially fluid spacer (FS) spreads in a predetermined space, whereas the positioning of a viscous spacer is determined by needle guidance. The aim of this study was to evaluate the application a radiopaque viscous spacer (RVS) for prostate cancer radiotherapy (RT).

MATERIALS/METHODS: In 30 patients including three centers, 10ml (median) of a RVS was injected between the prostate and anterior rectal wall. The resulting distance and distribution were determined in different planes and compared with a control group of 30 patients with a FS. A cone beam computed tomography (CBCT) was performed at least at the end of RT, a magnetic resonance imaging (MRI) 3 months and 12 months after RT. Injection and treatment tolerability were analyzed.

RESULTS: With a median prostate volume of 40cm3 , no overlap was found between planning target volume and rectal volume in most cases (median = 0cm3 , interquartile range, IQR 0-0.6cm3 ). The median contoured RVS before, at the end and 3 months after RT was 8.5cm3 , 8.7cm3 und 6.4cm3 . Signs of hydrogel migration were not found in any case. Hydrogel visibility in CBCT was rated with "4" on a scale between 1 (poor) and 5 (excellent). Small residues were detected in the MRI 12 months after RT in 3 cases (10%). The median distance between prostate and rectum at the base / midplane / apex was 14mm / 12mm / 11mm. The median rectal volume percentage within the 90% and 80% isodoses was 3.0% (IQR 1.5-4.5%) bzw. 5.9% (IQR 3.0-10.6%). On a scale between 1 (difficult) and 5 (very easy), ease of application was rated with "3-4". Procedure- or device related adverse events have not been observed in any case. Acute and late gastrointestinal toxicities were found in 17% and 3%, respectively (all grade 1). The control of gel distribution of RVS relative to FS was rated with "3-5" on a scale between 1 (much worse) and 5 (much better), the ease of application relative to FS with "2-3" on a scale between 1 (much harder) and 5 (much easier). With a median contoured FS volume of 9.9cm3 , the prostate-rectum distance at the base / midplane / apex was 12mm / 10mm / 10mm (significantly smaller in comparison to RVS). However, the lateral gel extension of RVS was smaller in all planes, statistically significant for all comparisons (base: 28mm vs. 34mm, midplane: 28mm vs. 32mm, apex: 21mm vs. 26mm). The gel symmetry (right vs. left) was comparable.

CONCLUSION: The application of a radiopaque viscous hydrogel spacer is harder in comparison to the initially fluid spacer. However, it can be better controlled and is achievable without any complications. Visibility in CBCT is good. The resulting prostate-rectum distance is >10mm in most cases and slightly larger at the base in comparison to FS. The resulting rectum volume within the high dose region and radiotherapy toxicity are very low.