Abstract
BackgroundThe SpaceOAR hydrogel is employed to limit rectal radiation dose during prostate radiotherapy. We identified a novel parameter – the product of angle θ and hydrogel volume – to quantify hydrogel placement. This parameter predicted rectum dosimetry and acute rectal toxicity in prostate cancer patients treated with stereotactic body radiotherapy to 36.25 Gy in 5 fractions.MethodsTwenty men with low- and intermediate-risk prostate cancer underwent hydrogel placement from 2015 to 2017. Hydrogel symmetry was assessed on the CT simulation scan in 3 axial slices (midgland, 1 cm above midgland, 1 cm below midgland). Two novel parameters quantifying hydrogel placement – hydrogel volume and angle θ formed by the prostate, hydrogel, and rectum – were measured, and the normalized product of θ and hydrogel volume calculated. These were then correlated with perirectal distance, rectum maximum 1–3 cc point doses (rDmax 1–3 cc), and rectum volumes receiving 80–95% of the prescription dose (rV80–95%). Acute rectal toxicity was recorded per RTOG criteria.ResultsIn 50% of patients, hydrogel placement was symmetric bilaterally to within 1 cm of midline in all three CT simulation scan axial slices. Lateral hydrogel asymmetry < 2 cm in any one axial slice did not affect rectum dosimetry, but absence of hydrogel in the inferior axial slice resulted in a mean increase of 171 cGy in the rDmax 1 cc (p < 0.005). The perirectal distance measured at prostate midgland, midline (mean 9.1 ± 4.3 mm) correlated strongly with rV95 (R2 0.6, p < 0.001). The mean hydrogel volume and θ were 10.3 ± 4.5 cc and 70 ± 49°, respectively. Perirectal distance, rV95 and rDmax 1 cc correlated with hydrogel angle θ (p < 0.01), and yet more strongly with the novel metric θ*hydrogel volume (p < 0.001). With a median follow up of 14 months, no rectal toxicity >grade 2 was observed. Low grade rectal toxicity was observed in a third of men and resolved within 1 month of SBRT. Men who had these symptoms had higher rDmax 1 cc and smaller θ*hydrogel volume measurements.ConclusionsOptimal hydrogel placement occurs at prostate midgland, midline. The novel parameter θ*hydrogel volume describes a large proportion of rectum dosimetric benefit derived from hydrogel placement, and can be used to assess the learning curve phenomenon for hydrogel placement.
Highlights
The SpaceOAR hydrogel is employed to limit rectal radiation dose during prostate radiotherapy
SpaceOAR hydrogel was present in 51 axial slices and symmetric to within 1 cm of anatomic midline in 45 axial slices
Clinical relevance Our institutional experience describes the early stages of SpaceOAR hydrogel implementation in high-dose per fraction Stereotactic body radiotherapy (SBRT) setting, where the benefits of a well-placed spacer on rectum dosimetry quickly become clinically evident upon examination
Summary
The SpaceOAR hydrogel is employed to limit rectal radiation dose during prostate radiotherapy. We identified a novel parameter – the product of angle θ and hydrogel volume – to quantify hydrogel placement This parameter predicted rectum dosimetry and acute rectal toxicity in prostate cancer patients treated with stereotactic body radiotherapy to 36.25 Gy in 5 fractions. The Augmenix SpaceOAR hydrogel, which received FDA approval following a 2014 phase III clinical trial, showed statistically significant reduction in grades 1 and 2 acute rectal toxicity in men receiving conventionally fractionated, dose escalated prostate radiotherapy [8]. Late grade 1 rectal toxicity at 3 years was significantly lower in the SpaceOAR arm (42% v 17%, p = 0.04) [9] This toxicity improvement is attributable to both reduced intrafraction motion, where the SpaceOAR hydrogel and daily rectal balloon usage are believed to be comparable immobilization tools [10], and improved rectal dosimetry based on posterior, rather than anterior, displacement of the anterior rectal wall with hydrogel instead of a rectal balloon [11]. A hydrogel spacer has the additional benefit of allowing larger planning margins and higher target coverage due to improved rectal sparing [12, 13]
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
