Session Item

To investigate the effect of intrafraction translation and rotation motion extracted from online 3D cine-MRI on the delivered dose distributions of the first prostate patients treated on the Elekta Unity MR-linac at our institute.

Five low/intermediate prostate patients were treated on a 1.5 T MR-linac (20 × 3.1 Gy) using daily contour/plan adaptation based on online MRI. During the delivery of each fraction, intrafraction 3D cine-MRI using a balanced 3D gradient echo sequence was acquired at 8 or 16 sec intervals. For each fraction a soft tissue tracking algorithm (ESTRO 38) was used to obtain the rigid prostate intrafraction motion of subsequent dynamics relative to the first cine-MRI of that session. The prostate position from the first cine-MRI was then registered to the daily MRI, transferring the local cine-MRI motion to the reference coordinate system. The linac treatment log files of each fraction were used to reconstruct the delivered dose on the respective cine-MRI dynamics. The necessary parameters —including MLC/gantry positions and Monitor Units— corresponding to the timepoint of each dynamic were extracted yielding several partial plan/3D volume combinations. For each partial plan, a pseudo-CT volume was created by bulk density assignment of the corresponding cine-MRI. Then the partial dose was calculated using our research treatment planning system and was warped back to the reference volume by using the inverse rigid transformation for the purpose of dose accumulation. For each fraction the partial doses were summed leading to the accumulated fraction dose (INTRA) which was compared to the daily reference (REF) dose.

Dose delivery took 5.5 min per fraction, corresponding to 22 cine-MRI dynamics on average. The mean±SD translations (mm) were 0.1±0.7 (LR), 1.0±1.9 (AP), -1.0±2.0 (CC) and rotations (degrees) were -0.9±2.5 (LR), -0.2±0.8 (AP), 0.01±1.2 (CC) among all timepoints of these 100 fractions. Figure 1 shows the D99% point of the target structures between REF and INTRA dose. The average drop in D99% coverage for the PTV, EBV and CTV was 10.9%±9.3%, 7.3%±7.3% and 2.1%±2.8% respectively. Figure 2 shows boxplots of the V62Gy DVH point for bladder and rectum, undergoing an average increase of 0.6cc ± 1.0cc and 0.1cc ± 1.0cc respectively.

We present the first dosimetric impact analysis due to rigid prostate intrafraction motion for prostate MR-linac patients. This dose reconstruction based on soft-tissue tracking from online 3D cine-MRI and linac log files, enables accurate dosimetric evaluation with high spatial and temporal resolution. We are now evaluating the adequacy of the clinical margins in these treatments. These results allow us to investigate the optimal inter- and intrafraction adaptation methods for MRI-guided prostate radiotherapy, towards delivering individualized dynamic patient treatments in every fraction.