A Dosimetric Assessment of Magnetic Resonance-Guided Stereotactic Body Radiotherapy: Examples from Liver Metastases and Prostate Cancer

The planning target volume (PTV) concept in radiotherapy ensures that the clinical target volume (CTV) receives the intended dose despite geometrical uncertainties. However, the price of a PTV is that the dose intended for the CTV is delivered to healthy tissue. With the introduction of magnetic resonance (MR) linear accelerators and MR-guided radiotherapy, came a new potential to reduce the PTV, and thus also to reduce the radiation of healthy tissue, without compromising the treatment efficacy. In this PhD project, we conducted dosimetric assessments of key aspects of MR-guided stereotactic body radiotherapy (SBRT) of prostate cancer and liver metastases.
In Study I we assessed the impact of MR-guided beam-gating in SBRT of prostate cancer on (i) the dose delivered and (ii) necessary PTV margins. We found that beam-gating had a negligible effect on both the dose delivered and on the necessary PTV margins compared to treatment monitoring based on magnetic resonance imaging.
In Study II we examined the inter-algorithm dose warping reproducibility in MR-guided SBRT of liver metastases. A high agreement among the seven included algortihms would have increased our confidence in the algortihms and opened up possibilities to utilize information about delivered dose in previous treatment fractions in daily plan optimizations. However, we found low inter-algorithm agreements for some patients and fractions.
In Study III we used an analytical margin recipe and a dose accumulation approach for estimating necessary PTV margins accounting for intrafractional motion for MR-guided and beam-gated SBRT of liver metastases in three fractions. Both methods were applied on (i) British and (ii) Danish guidelines. We found that the dose accumulation approach generally gave smaller PTV margins than the analytical approach and that high doses in the center of the target (Danish guidelines) could reduce the necessary PTV margins compared to more homogeneous dose distributions (British guidelines).