Feasibility of longitudinal prostate R2 ⁎ measurements in patients undergoing stereotactic ablative radiotherapy on an MRI-linac.

BACKGROUND AND PURPOSE: Tissue R2* (1/T2*), sensitive to deoxyhaemoglobin, is a proposed hypoxia biomarker in prostate cancer. This exploratory work investigates prostate R2* and volume throughout and after hypo-fractionated radiotherapy and the effects of androgen deprivation therapy (ADT).

MATERIALS AND

METHODS

Fifteen patients (seven with ADT) received five-fraction stereotactic-ablative-radiotherapy (SABR) on a 1.5 T Elekta Unity MRI-linac, with R2* mapping at MR-Simulation and treatment visits 1, 3, 5. Six patients had follow-up imaging post-treatment.Treatment-induced whole-prostate and tumour R2*/volume differences were investigated using repeated-measures analysis-of-variance.ADT effects on baseline whole-prostate R2*/volume were investigated using unpaired t-tests. Spearman correlations between whole-prostate R2* and volume were calculated before and during treatment.

RESULTS

Whole-prostate R2* (mean difference [95% CIs] = -1.77 [-2.17 to -0.498] s-1, p = 0.003) and tumour R2* (-3.11 [-5.78 to -1.44] s-1, p = 0.003) decreased, whole-prostate volume (8.05 [4.76 to 11.7] cm3) increased after two fractions.Follow-up whole-prostate R2* (p = 0.62) and volume (p = 0.16) were not significantly different from baseline.ADT patients had higher baseline whole-prostate R2* (p = 0.006) and lower volume (p = 0.003). ADT did not impact treatment-induced changes (p = 0.51).Baseline whole-prostate R2*/volume were negatively correlated (r = -0.75, p = 0.002); treatment-induced R2*/volume changes were not correlated r = 0.44, p = 0.11).

CONCLUSIONS

Treatment-induced whole-prostate and tumour R2* changes were measured after two SABR fractions. Post-treatment whole-prostate R2* and volume were not significantly different from baseline.Further work is required to confirm the mechanism(s) causing observed treatment effects.