A Pilot Study: The Role of Radio-Opaque Hydrogel Tissue Marker in the Treatment of Postprostatectomy Intensity Modulated Radiation Therapy

Abstract

It is well documented that the prostate bed is highly susceptible to inter-fraction motion leading to larger treatment planning margins to account for daily treatment set up uncertainties when matching to bony anatomy. The use of fiducial markers in the prostate bed has significantly improved accuracy of treatment delivery. This pilot study aims to investigate the role and benefits of radio-opaque hydrogel tissue marker in the treatment of post prostatectomy intensity modulated radiation therapy (IMRT). Twenty patients treated with IMRT to a dose of 70.2Gy in 39 fractions between Jan 2016 to Jan 2017 were included in this study. All patients underwent transperineal injection of hydrogel tissue marker at the level of vesico-urethral anastomosis and cystoscopically into the posterior bladder wall. Instructions for bladder and bowel preparation to be followed during CT simulation and daily treatments were provided to the patients. Daily on-line cone beam CT matching to the hydrogel tissue marker was performed prior to each treatment and off-sets relative to skin tattoos were recorded in 3 directions: left-right (LR); superior-inferior (SI) and anterio-posterior (AP). Daily image registration using bony matching was then assessed off-line and off-sets relative to skin tattoos were again recorded. The mean and standard deviation of the differences between the hydrogel tissue markers and bony off-sets were calculated for all fractions for each patient; then the overall mean and systematic (Σ) and random errors (σ) for the entire cohort were calculated. The planning target volume (PTV) margin required for bony matching was estimated using van Herk's formula of 2.5Σ + 0.7σ, which is designed to ensure a minimum dose of 95% of prescribed dose to the clinical target volume (CTV) in 90% of patients. These margins are relative to the hydrogel tissue markers which are assumed to be in the target tissue. A total of 1520 images were analysed. The overall mean shift differences between the hydrogel tissue markers and bones for the entire cohort were LR: 0.13mm (Σ 0.38mm, σ 1.35mm); SI: 0.92mm (Σ 1.40mm, σ 2.50mm) and AP: 0.40mm (Σ 1.98mm, σ 3.18mm). The calculated PTV margins for bony matching relative to hydrogel tissue markers were 1.90mm in LR, 5.25mm in SI and 7.17mm AP directions. Prostate bed motion is independent of pelvic bone anatomy and soft tissue matching has shown to be superior compared with pelvic bone matching. With a required PTV margin of 7.17mm in the AP direction, the use of hydrogel tissue marker can significantly improve treatment set up accuracy even in patients with optimal bowel and bladder preparations who undergo image guidance with pelvic bone matching. This will provide the best CTV coverage whilst minimising the current PTV margin and mitigating bladder and rectal toxicities.