Rectal Dose Reduction Using Perirectal Hydrogel Spacers in Men with Prostate Cancer Treated with Stereotactic Ablative Radiation Therapy (SABR)

Abstract

The use of perirectal hydrogel spacers has been shown to reduce the rectal dose in prostate cancer (PCa) patients treated with intensity modulated radiation therapy and brachytherapy. This retrospective single institution study sought to evaluate the rectal dosimetric effects of perirectal hydrogel spacers in PCa patients treated with SABR and fiducial tracking. From October 2016 through January 2018, 77 men (38 with perirectal hydrogen spacers and 39 without) with low or intermediate PCa were treated with definitive SABR monotherapy and fiducial tracking to 36.25 Gy in 5 fractions using a homogeneous dose distribution. The perirectal hydrogel spacers were implanted under ultrasound guidance using an aseptic transperineal technique at the time of fiducial marker placement a median 8 days prior to the treatment planning simulation. The perirectal space was defined as the distance between anterior rectal wall and the posterior prostatic capsule at mid-gland on axial slices of T2-weighted MRIs. The planning target volume (PTV) included the prostate and seminal vesicle with a 3-5 mm expansion. Dose volume histogram (DVH) data were generated using treatment planning software. The evaluated target rectal dose constraints were V18.125 <50%, V29 <20%, V32.625 <10%, V36.25 <5%, and V36 <1cc. A Student’s T-test was performed to determine any significant differences between the rectal DVH data. The median age and prostate volume of patients with and without perirectal hydrogel spacers were 66 and 70 years and 51 and 54 cc, respectively. The mean perirectal distance in the spacer group was 12.4 ± 2.3 mm. Between the non-spacer and spacer group, the mean rectal V18.125 was significantly reduced from 35.4% to 30% (p=0.014), V29 from 11% to 6.8% (p<0.001), V32.625 from 5.4% to 2.8% (p<0.001), V36.25 from 0.72% to 0.37% (p<0.001), and V36 from 0.69 cc to 0.43 cc (p<0.001). The mean prostate V40 significantly improved from 93.3% to 98.4% (p=0.046); there was no difference in the PTV V36.25. In this single institution retrospective dosimetric study of PCa patients treated with SABR and fiducial tracking, the use of perirectal hydrogel spacers resulted in meaningful rectal dose reductions and increased prostate coverage. Quality of life studies with a substantial sample size and extended follow up are necessary to determine the clinical impact on acute and late rectal toxicity.