Room: AAPM ePoster Library
Purpose: Improved anatomical visualization of glioblastoma multiforme (GBM) cancers with the MRI-Linac (MRL) makes it feasible to adapt plans to tumor changes. This study looks at the effect of the tumor changes as treatment progresses and the value of replanning at specific time intervals of 2 weeks (2W), 4 weeks (4w), and 6 weeks (6w) for two different adaptive strategies: segment weight optimization (SWO) and segment shape optimization (SSO).
Methods: Four GBM patients were selected for this IRB approved retrospective study. All patients have four MRI scans; scheduled at before treatment (0W), 2W, 4W and 6W. These patients received 60 Gy in 30 fractions. First, GTV/CTV treatment volumes were contoured on the 0W T1W MRI fused to the treatment planning CT. Then these volumes were propagated sequentially using deformable registration to subsequent MR scans (0W to 2W, 2W to 4W, 4W to 6W). Three different plans were created for the subsequent MRIs (2, 4 and 6W) using recalculation (no optimization), SWO and SSO. All plans were performed in Philips Pinnacle v16.2 utilizing a coplanar seven 6xFFF field step-and-shoot beams with 90 degree collimator angle to mimic the Elekta Unity platform. Target coverage, OAR doses, conformality and homogeneity indices were evaluated.
Results: All four patient pre-treatment plans were deemed clinically acceptable. Two patients have PTVs directly overlapping OARs. Given the proximity to the OARs, SWO tended to have higher brainstem and optic nerve doses compared to SSO. The other two patients (PTV not abutting OARs) demonstrated clinically acceptable plans for SWO and SSO for all time points.
Conclusion: Given a PTV that is not near any OAR, a simple SWO is deemed clinically acceptable. When the target volume is near an OAR such as brain stem and optic nerves, it is best to proceed with caution and perform SSO.
Not Applicable / None Entered.