Session Item

Intrafraction prostate motion affects the accuracy of radiotherapy treatment delivery. Motion correction is especially important in stereotactic body radiotherapy (SBRT) due to few fractions with high fraction doses. This study investigates the dosimetric benefit of continuous motion monitoring based correction strategy with the Calypso system (Varian Medical Systems). The Calypso correction strategy is compared to pre-treatment image-based strategies using only a setup CBCT scan (CBCT strategy) or a CBCT with additional kV imaging of fiducial markers (FM) to correct for motion occurring during the CBCT analysis (CBCT+kV strategy).

Treatment plans and motion data were gathered from 22 patients treated with 5 x 7-7.25Gy fractions using two 10 MV flattening filter free VMAT-beams. CTV-to-PTV margins were 3 mm posteriorly and 5 mm elsewhere. Patient setup included initial Calypso localization, CBCT check of rectum and bladder filling and additional kV or Calypso based position corrections prior to irradiation. Beam gate-off and couch corrections were applied during treatment if the Calypso-measured motion exceeded 2 mm posteriorly or 3 mm elsewhere. After treatment motion trajectories for the CBCT and CBCT+kV correction strategies were simulated by correcting the motion data for Calypso-guided couch adjustments. The kV-based couch adjustments were maintained in the simulated CBCT+kV strategy and removed in the CBCT strategy. The dosimetric effect of the motion was assessed by motion-including dose reconstruction. The dose reconstruction involved emulation of the 3D prostate motion as multiple isocenter shifts and recalculation of the motion-encoded plans by the treatment planning system. Motion inclusive DVH parameters for target and OAR structures (bladder and rectum) were compared to planned values.

The total number of analyzed fractions with dose reconstruction was 103 for the Calypso- and CBCT+kV correction strategies and 102 for the CBCT strategy. Motion simulation is illustrated in fig.1. Table 1 presents differences in target and OAR DVH parameters between motion inclusive and planned dose distributions. In general, the PTV margins covered most of the motion and CTV dose deficits were small in each correction strategy. For individual fractions, larger dose deviations were seen, especially in the CBCT strategy. The volume of the OARs that received high doses increased due to motion, whereas lower dose volumes were less affected.

Single CBCT-guided pre-treatment setup without further correction may lead to inaccurate treatment delivery with clinically relevant dose deficits in prostate SBRT. Additional pre-treatment position correction with kV imaging increases the accuracy and is adequate for most of the fractions. Position corrections based on continuous monitoring ensure high target dose coverage and minimizes the OAR doses best, although the dosimetric benefit of gating during the irradiation is small.