Session Item

Online Adaptive Radiotherapy (ART) with daily MR imaging has the potential to improve dosimetric accuracy by accounting for anatomical changes during radiotherapy. Existing daily ART prostate studies have diverse conclusions, but lack information regarding integrated boosts to the dominant intraprostatic lesion, which has shown an increase in biochemical disease-free survival. The purpose of this study is to determine the potential dosimetric gain of daily ART to the dominant lesion and other targets as well as potential benefits to the OARs.

Five patients with prostate cancer treated with MR-Guided SBRT are included. Prescription to the planning target volume (PTV) was 36.25Gy in 5 fractions and the prescription to the intraprostatic boost (GTV) ranged between 40-45Gy. The normalization for the PTV ranged from 90-95% coverage at the Rx dose and >90% coverage for the GTV.  All patients underwent a planning MRI and planning CT as well as daily setup MRIs prior to each fraction. The organs-at-risk (OARs) included bladder, anorectum, bowel, femoral heads, and penile bulb. The CTV encompassed the prostate and proximal seminal vesicles, the PTV is an expansion of the CTV by 4-5mm in all directions except 3mm posteriorly. The GTV was defined using mpMRI and confirmed by MRI guided TRUS biopsy (no margin). The initial treatment plan was generated using step-and-shoot IMRT. The electron density map and contours from the planning MR were propagated to the daily setup MRI using deformable image registration and the contours were then edited. Each adapted fraction was obtained by re-optimizing based on the contours on daily setup MRI. Non-adaptive fractions were simulated by re-calculating the dose on each daily setup MRI. Non-adaptive and adaptive fractions were compared using target coverage and OAR dose-volume metrics.

For non-adapted fractions the GTV coverage ranged from 55-97% at the boost dose level, where 14/20 fractions had 90% or more coverage. All adapted fractions had greater than 90% coverage. For non-adapted fractions the CTV coverage ranged from 90-100%, where 18/25 fractions had greater than 95% coverage. The CTV coverage was 98% or greater for all adapted fractions.  For the non-adapted fractions the PTV coverage ranged from 71-99%, while for the PTV coverage constraint was met for all adapted fractions. Very few OAR constraints were exceeded in the non-adaptive setting, and none were exceeded in the adaptive setting. The anorectum constraint was exceeded 1/25 fractions and the bladder constraint was exceeded 3/25 fractions.

Online adaptive MR-Guided SBRT of prostate cancer using daily re-optimization resulted in better target coverage as compared with non-adaptive SBRT, this was especially true for the GTV where the non-adaptive coverage ranged from 55%-97%. Generally, the OARs fared well in the non-adaptive setting with moderately increased sparing for adaptive fractions.