A novel model to correlate hydrogel spacer placement, perirectal space creation, and rectum dosimetry in prostate stereotactic body radiotherapy

• Published in: Radiation oncology (London, England) Vol. 13; no. 1; p. 192
• Main Authors: Hwang, Mark E, Black, Paul J, Elliston, Carl D, Wolthuis, Brian A, Smith, Deborah R, Wu, Cheng-Chia, Wenske, Sven, Deutsch, Israel
• Format: Journal Article
• Language: English
• Published: England BioMed Central Ltd 01.10.2018; BioMed Central; BMC
• Subjects: Asymmetry; Background radiation; Cancer; Cancer therapies; Care and treatment; CAT scans; Clinical trials; Computed tomography; Computer simulation; Correlation; Dosimeters; Dosimetry; FDA approval; Health aspects; Health risks; Hydrogels; Learning curves; Men; Methods; Parameter identification; Patients; Placement; Prostate cancer; Radiation; Radiation dosage; Radiation therapy; Radiotherapy; Rectal toxicity; Rectum; SpaceOAR hydrogel; Stereotactic body radiotherapy; Symmetry; Toxicity; Urology; United States; Stereotactic body radiotherapy; Dosimetry; Rectal toxicity; SpaceOAR hydrogel; Prostate cancer
• ISSN: 1748-717X; 1748-717X
• DOI: 10.1186/s13014-018-1135-6

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. Twenty 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 (rD 1-3 cc), and rectum volumes receiving 80-95% of the prescription dose (rV80-95%). Acute rectal toxicity was recorded per RTOG criteria. In 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 rD 1 cc (p < 0.005). The perirectal distance measured at prostate midgland, midline (mean 9.1 ± 4.3 mm) correlated strongly with rV95 (R 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 rD 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 rD 1 cc and smaller θ*hydrogel volume measurements. Optimal 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.