Session Item

Online MR-guided adaptive radiation therapy (ART) offers the ability to generate an adapted treatment plan tailored to daily anatomy. With the extended time required for online planning, internal motion may require an additional adaptation, which together with daily ART creates a complex paradigm for PTV margin design. Here we employ a dose accumulation approach to estimate the cumulative delivered dose to optimize PTV margin reduction for online MR-guided adaptive prostate SBRT. 

We performed dose accumulation for 10 patients treated using 30 Gy in five fractions on a 1.5 T MR-Linac. Patients also received a single fraction 15 Gy intra-prostatic brachytherapy boost. We included our clinical treatment plans employing a 5 mm uniform PTV margin and additionally simulated pre-treatment reference plans on reference MRs and daily adapted plans on localization MRs for two additional PTV margins: 4 mm margins (3 mm LR), and 3 mm margins (2 mm LR). We performed adapt-to-shape (ATS) adaptation with deformable image registration to map contours to the localization MR followed by manual contour editing and full inverse optimization. We performed a soft tissue prostate rigid registration between the localization and a verification MR scan collected during adaptation steps, and for fractions with > 3 mm translation in any direction, we generated an adapt-to-position (ATP) plan based on a rigid body isocentre shift to mitigate intra-fraction motion. To estimate delivered dose per fraction we computed the ATS or ATP plan on a 3D MR collected during beam delivery. We then accumulated dose by deforming each fraction dose to the reference MR using deformable registration and summed across the treatment course. We compared target and organ-at-risk (OAR) DVH metrics on the estimated accumulated delivered dose for the various margin designs using Wilcoxon signed-rank tests with Bonferroni Correction for multiple comparisons. 

Visually, we observed modest improvements in normal tissue sparing when moving to the smaller PTV margins, (Figure 1). CTV coverage met our clinical goals of D98 > 2850 cGy and D95 > 3000 cGy for all PTV margins for the estimated delivered dose with no significant differences (Figure 2). Although we observed a trend for lower OAR and integral dose (evaluated as the external within 2 cm SI of the CTV), the differences did not reach statistical significance (Figure 2). 

Here we demonstrate the utility of dose accumulation for PTV margin reduction in the complex paradigm of online MR-guided ART. Our next step is to prospectively implement the 4 mm (3 mm LR) PTV margin with offline dose accumulation. Further, we plan to investigate the impact of our geometric threshold for performing additional ATP adaptations on the accumulated dose to co-optimize dosimetric benefit and daily adaptation time.