Ano-rectal wall dose-surface maps localise the dosimetric benefit of hydrogel rectum spacers in prostate cancer radiotherapy

Background and Purpose To evaluate spatial differences in dose distributions of the ano-rectal wall (ARW) using dose-surface maps (DSM) between prostate cancer patients receiving intensity-modulated radiation therapy with and without implantable rectum spacer (IMRT+IRS; IMRT-IRS, respectively), and to correlate this with late gastro-intestinal (GI) toxicities using validated spatial and non-spatial normal-tissue complication probability (NTCP) models. Materials and Methods For 26 patients DSMs of the ARW were generated. From the DSMs various shape-based dose measures were calculated at different dose levels: lateral extent, longitudinal extent, and eccentricity. The contiguity of the ARW dose distribution was assessed by the contiguous-DSH (cDSH). Predicted complication rates between IMRT+IRS and IMRT-IRS plans were assessed using a spatial NTCP model and compared against a non-spatial NTCP model. Results Lateral extent, longitudinal extent and cDSH were significantly lower in IMRT+IRS than for IMRT-IRS at high-dose levels. Largest significant differences were observed for cDSH at dose levels >50Gy, followed by lateral extent at doses >57Gy, and longitudinal extent. Significant decreases (p = 0.01) in median rectal and anal NTCPs were predicted when using an IRS. Conclusions: Local-dose effects are predicted to significantly reduce by an IRS. Differences in predicted GI toxicity rates exist between spatial and non-spatial NTCP models.