Prostate cancer tumour control probability modelling for external beam radiotherapy based on multi-parametric MRI-GTV definition

Abstract

Purpose: To evaluate the applicability and estimate the radiobiological parameters of linear-quadratic Poisson tumour control probability (TCP) model for primary prostate cancer patients for two relevant target structures (prostate gland and GTV). The TCP describes the dose–response of prostate after definitive radiotherapy (RT). Also, to analyse and identify possible significant correlations between clinical and treatment factors such as planned dose to prostate gland, dose to GTV, volume of prostate and mpMRI-GTV based on multivariate logistic regression model.Methods: The study included 129 intermediate and high-risk prostate cancer patients (cN0 and cM0), who were treated with image-guided intensity modulated radiotherapy (IMRT) +/- androgen deprivation therapy with a median follow-up period of 81.4 months (range: 42.0 - 149.0) months. Tumour control was defined as biochemical relapse free survival according to the Phoenix definition (BRFS). MpMRI-GTV was delineated retrospectively based on a pre-treatment multi-parametric MR imaging (mpMRI), which was co-registered to the planning CT. The clinical treatment planning procedure was based on prostate gland, delineated on CT imaging modality.Results: Our results indicated an appropriate α/β = 1.3 Gy for prostate gland and α/β = 2.9 Gy for mpMRI-GTV. Only for prostate gland, EQD2 and gEUD 2Gy were significantly lower in the biochemical relapse (BR) group compared to the biochemical control (BC) group. Fitting results to the linear-quadratic Poisson TCP model for prostate gland and α/β = 1.3 Gy were D 50 = 66.8 Gy with 95%CI [64.6 Gy, 69.0 Gy], and γ = 3.81 with 95%CI [2.58, 5.20]. For mpMRI-GTV and α/β = 2.9 Gy, D 50 was 68.1 Gy with 95%CI [66.1 Gy, 70.0 Gy], and γ = 4.45 with 95%CI [3.00, 6.12]. The fitness of the model was better for prostate gland. For the multivariate logistic model, the gEUD 2Gy for prostate gland showed a very high significant predictive value ( p = 0.001), whereas regarding mpMRI-GTV only its volume showed a significance ( p = 0.01).Conclusion: The linear-quadratic Poisson TCP model was better fit when the prostate gland was considered as responsible target than with mpMRI-GTV. This is compatible with the results of the comparison of the dose distributions among BR and BC groups and with the results achieved with the multivariate logistic model regarding gEUD 2Gy . Probably limitations of mpMRI in defining the GTV explain these results. Another explanation could be the relatively homogeneous dose prescription and the relatively low number of recurrences.

Impact Analysis of the Interobserver Variability in CBCT-Based Soft-Tissue Matching using TCP/NTCP Models for Prostate Cancer Radiotherapy

“…The TCPs and NTCPs were calculated using the linear-quadratic Poisson model and the Lyman-Kutcher-Burman model, respectively. These calculations were performed using the open-sourced pyradiobiology package 9 and the physical dose exported from the Eclipse planning system. The radiobiology parameters and endpoints used for TCP and NTCP calculations were speci ed in previous studies [10][11][12] .…”