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; P < .05), and 44 Gy or more was delivered to the DIL in 84% of the treated fractions. The median D95 to the prostate was 40.4 Gy (initial plan: 40.7 Gy; P < .05). Despite excellent target coverage, the rectum, urethra, and bladder dose constraints were generally maintained. At 6 months from completion of therapy, the median prostate-specific antigen result was 1.1 ng/mL (range, 0-5.6 ng/mL) compared to the median pre-SBRT prostate-specific antigen of 6.8 ng/mL (range, 3.45-31 ng/mL). No patient developed late grade 3 or higher urinary or rectal toxicities at median follow-up of 10.8 months (range, 6.4-15.7 months).

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.