Room: Exhibit Hall | Forum 4
Purpose: To evaluate the feasibility of a trajectory-based gantry-static couch motion optimization technique (GsCMOT) for treating single brain met or lesion proximal to brainstem and optical chiasm.
Methods: The proposed GsCMOT consists of multiple intensity modulated radiation therapy (IMRT) fields which were combined to simulate an arc at various couch angles and at static gantry position. This simulated arc represented a Ï€-geometry. Further advancement in VMAT planning has contributed to the development of this technique with the introduction of dynamic couch motion into treatment delivery. Arbitrarily planning target volumes (1-PTV and 2-PTV) were contoured posterior to the brainstem (1-PTV) and between the optical chiasm and brainstem (2-PTV). The dosimetric indices of two standard rapid arc techniques [(coplanar (CP) and non-coplanar (NCP)] were compared with our proposed new gantry static couch motion optimization technique (GsCMOT). Dose volume histograms (DVHs) were compared for all three approaches. Comparisons of the dose to the OARs (Timmerman guidelines) and PTV coverage (D95 = 100%) were used as meaningful endpoints.
Results: A percentage reduction of dose (D50 of -11%Â±5% and D30 of -20%Â±6%) to brainstem for 1PTVs, and to brainstem (D50 of -5%Â±3% and D30 of -10%Â±6%) and chiasm (D50 of -20%Â±8% and D30 of -9%Â±5%) for 2PTVs was noticed for GsCMOT with same volume max point dose to all techniques. Conformity Index (CI) for GsCMOT was 1.4 compared to 1.0 for CP and 1.0 for NCP because of dose spillage laterally. This is justified by the mean gradient measure (mm) of 4.0 as compared to 6.0 (CP) and 5.5 (NCP) in anterior-posterior direction.
Conclusion: Our novel GsCMOT results in a significant dose reduction to the organs-at-risk as compared to CP and NCP approaches.