MRI-guided focal boost to dominant intraprostatic lesion using volumetric modulated arc therapy in prostate cancer. Results of a phase II trial.

Objective: To determine morphological and biological control as well as toxicity and quality of life (QoL) of men with localized prostate cancer (PCa) treated with MRI-guided focal boost radiotherapy.

Material And Methods: 30 patients with PCa and a visible dominant intraprostatic lesion (DIL) identified on mpMRI were included in a prospective Phase II trial. Matching point registration of planning CT and T2W, diffusion-weighted and a gradient-recalled echo (GRE) MRI images made in treatment position was used for prostate and tumour delineation. Treatment consisted on 35 daily fractions of 2.17 Gy with a concomitant focal boost to the DIL of 2.43 Gy using volumetric modulated arc therapy (VMAT) and image-guided radiation therapy (IGRT) with intraprostatic fiducial markers. Biochemical failure was analysed using PSA nadir +2 ng/mL criteria and local control using mpMRI evaluation at 6-9 months following RT. Acute and late toxicity were defined according to CTCAE v.4.0 and RTOG/EORTC scales and QoL was assessed using IPSS, EPIC short-form and UCLA-PCI questionnaires.

Results: The median radiation dose to the prostate was 77.6 Gy (IQR 77.3-78.1), and to the DIL was 85.5 Gy (IQR 85.0-86.0). With a median follow up of 30.0 months (IQR 25.5-40.27), all patients remain free of biochemical relapse. An mpMRI complete response was observed in 25 patients during the first post-treatment evaluation at 6 months. The remaining five patients achieved a complete disappearance of the DIL both on T2 and DWI on the second mpMRI performed at 9 months following treatment. Six out of 30 (20%) patients presented acute Grade 2 urinary toxicity with no Grade 3 acute complications. Acute rectal toxicity was only found in 2 (6.6%) patients (both Grade 1). Only late Grade 1 urinary and rectal complications were observed in 3/30 patients, respectively, with no Grade 2 or more late toxicity. The urinary, bowel and sexual bother EPIC scores were slightly and insignificantly increased in the first 3 months post-treatment, returning to normal afterwards.

Conclusions: mpMRI-guided focal boost using VMAT hypofractionated technique is associated with an excellent morphological and functional response control and a safe toxicity profile.

Advances In Knowledge: In the present trial, we examined the potential role of mpMRI for radiological assessment (functional and morphological) of treatment response in high-risk prostate cancer patients treated with MRI-guided focal radiotherapy dose intensification to dominant Intraprostatic lesion.