Effect on therapeutic ratio of planning a boosted radiotherapy dose to the dominant intraprostatic tumour lesion within the prostate based on multifunctional MR parameters

Objective: To demonstrate the feasibility of an 8-Gy focal radiation boost to a dominant intraprostatic lesion (DIL), identified using multiparametric MRI (mpMRI), and to assess the potential outcome compared with a uniform 74-Gy prostate dose. Methods: The DIL location was predicted in 23 patients using a histopathologically verified model combining diffusion-weighted imaging, dynamic contrast-enhanced imaging, T-2 maps and three-dimensional MR spectroscopic imaging. The DIL defined prior to neoadjuvant hormone downregulation was firstly registered to MRI-acquired post-hormone therapy and subsequently to CT radiotherapy scans. Intensity-modulated radiotherapy (IMRT) treatment was planned for an 8-Gy focal boost with 74-Gy dose to the remaining prostate. Areas under the dose-volume histograms (DVHs) for prostate, bladder and rectum, the tumour control probability (TCP) and normal tissue complication probabilities (NTCPs) were compared with those of the uniform 74-Gy IMRT plan. Results: Deliverable IMRT plans were feasible for all patients with identifiable DILs (20/23). Areas under the DVHs were increased for the prostate (75.1 +/- 0.6 vs 72.7 +/- 0.3Gy; p < 0.001) and decreased for the rectum (38.2 +/- 2.5 vs 43.5 +/- 2.5Gy; p < 0.001) and the bladder (29.1 +/- 9.0 vs 36.9 +/- 9.3Gy; p < 0.001) for the boosted plan. The prostate TCP was increased (80.1 +/- 1.3 vs 75.3 +/- 0.9Gy; p < 0.001) and rectal NTCP lowered (3.84 +/- 3.65 vs 9.70 +/- 5.68Gy; p = 0.04) in the boosted plan. The bladder NTCP was negligible for both plans. Conclusion: Delivery of a focal boost to an mpMRI-defined DIL is feasible, and significant increases in TCP and therapeutic ratio were found. Advances in knowledge: The delivery of a focal boost to an mpMRI-defined DIL demonstrates statistically significant increases in TCP and therapeutic ratio.