Dosimetric Impact of the Positional Imaging Frequency for Hypofractionated Prostate Radiotherapy – A Voxel-by-Voxel Analysis

Abstract

Background To investigate deviations between planned and applied treatment doses for hypofractionated prostate radiotherapy and to quantify dosimetric accuracy in dependence of the image guidance frequency. Methods Daily diagnostic in-room CTs were carried out in 10 patients in treatment position as image guidance for hypofractionated prostate radiotherapy. Fraction doses were mapped to the planning CTs and recalculated, and applied doses were accumulated voxel-wise using deformable registration. Non-daily imaging schedules were simulated by deriving position correction vectors from individual scans and used to rigidly register the following scans until the next repositioning before dose recalculation and accumulation. Planned and applied doses were compared regarding dose-volume indices and TCP and NTCP values in dependence of the imaging and repositioning frequency. Results Daily image-guided repositioning was associated with only negligible deviations of analyzed dose-volume parameters and conformity/homogeneity indices for the prostate, bladder and rectum. Average CTV T did not significantly deviate from the plan values, and rectum NTCPs were highly comparable, while bladder NTCPs were reduced. For non-daily image-guided repositioning, there were significant deviations in the high-dose range from the planned values. Similarly, CTV dose conformity and homogeneity were reduced. While TCPs and rectal NTCPs did not significantly deteriorate for non-daily repositioning, bladder NTCPs appeared falsely diminished in dependence of the imaging frequency. Conclusion Using voxel-by-voxel dose accumulation, we showed for the first time that daily image-guided repositioning resulted in only negligible dosimetric deviations for hypofractionated prostate radiotherapy. Regarding dosimetric aberrations for non-daily imaging, daily imaging is required to adequately deliver treatment.

Cone beam CT-based dose accumulation and analysis of delivered dose to the dominant intraprostatic lesion in primary radiotherapy of prostate cancer

“…As strict adherence to organ preparation protocols is still mandatory, selecting the closest temporal CBCT in case of missing CBCTs is expected to have no excessive impact on the dose accumulation accuracy. Nonetheless, daily instead of non-daily CBCT may further improve the dose accumulation workflow and will be explored in sequential studies [ 20 , 21 ]. Our data is also limited by the absence of intra-fractional motion analysis and correction, which was not available for the analyzed patient cohort.…”

Advantages and Robustness of Partial VMAT with Prone Position for Neoadjuvant Rectal Cancer Evaluated by CBCT-based Offline Adaptive Radiotherapy

“…Images were transferred from the Eclipse 15.6 treatment planning system to the Velocity software V4.0 (Varian Medical Systems, Palo Alto, CA) for image registration and dose reconstruction. The accuracy of deformable image registration and dose reconstruction using computer simulations and patient data has been demonstrated in previous papers, and has been tested and validated separately [18,19] . The clinical shift was used for rigid alignment between partial KVCBCT images and the corresponding planned CT.…”

Advantages and robustness of partial VMAT with prone position for neoadjuvant rectal cancer evaluated by CBCT-based offline adaptive radiotherapy

“…Images were transferred from the Eclipse 15.6 treatment planning system to the Velocity software V4.0 (Varian Medical Systems, Palo Alto, CA) for image registration and dose reconstruction. The accuracy of deformable image registration and dose reconstruction using computer simulations and patient data has been demonstrated in previous papers, and has been tested and validated separately [ 20 , 21 ]. The clinical shift was used for rigid alignment between partial KVCBCT images and the corresponding planned CT.…”