Dose Intensification to the Dominant Intraprostatic Lesion During Prostate Stereotactic Body Radiation Therapy Delivered on a Magnetic Resonance Imaging Linear Accelerator: Feasibility and Early Clinical Outcomes.

Purpose: This study aims to evaluate the feasibility of delivering escalated doses to the dominant intraprostatic lesion (DIL) as noted on T2-weighted magnetic resonance imaging and diffusion-weighted imaging while maintaining dose to the surrounding normal tissue structures within dose-volume constraints. Methods and Materials: A total of 50 consecutive patients were treated with prostate stereotactic body radiation therapy (SBRT), via a simultaneous integrated boost to the DIL, on a 1.5-Tesla magnetic resonance imaging linear accelerator platform. These patients were treated with SBRT to 40 Gy in 5 fractions every other day, and the DIL was simultaneously boosted to 45 Gy in 5 fractions. No patient was treated with androgen deprivation therapy. The normal tissue structures and the prostate and DIL were recontoured, and a postfraction plan was generated to retrospectively generate the doses delivered to the prostate and the DIL boost target for each of these 250 therapy sessions. Results: On postfraction dosimetric analysis, the median dose to 95% (D95) of the DIL was 45.3 Gy (initial plan: 45.9 Gy; Conclusions: With real-time adaptive planning on a magnetic resonance imaging linear accelerator, dose escalation was achieved in most cases with the intended doses without significantly compromising the dose-volume constraints of the surrounding normal tissue structures. These dosimetric findings were associated with an excellent tolerance profile at 12 months and a low incidence of urinary or rectal toxicity.