(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: study simulated and evaluated the feasibility of adaptive strategies (adapt to position(ATP), adapt to shape(ATS)) for Elekta Unity for local advanced cervical cancer(LACC) SBRT treatment.
Methods: patients (FIGO stage IB2-IIIC1) with pre-RT CT and during-RT MRI were included. The boost volume (CTVboost) consisted of T2/T1-visible gross tumor plus areas of expected microscopic disease with a 0.5cm margin added to create the PTVboost. A prescription dose of 30Gy/5 fractions was used for SBRT planning. All plans were created in Pinnacle 16.2 using 7 step&shoot IMRT beams with 90° collimator angle, 6XFFF and DMPO optimization to simulate the Elekta Unity platform. The ATP strategy (without re-contouring or re-optimization) was performed using rigid image registration between the CT and MRI with MRI contours mapped to the CT dataset, and assessed by recalculating the original CT plan on the MRI. CTVboost coverage (V100,V90), D2cc to bladder, rectum, sigmoid and bowel were reviewed. A re-optimized plan based on MRI structures was created the same way as the CT plan to evaluate the feasibility and improvement using ATS.
Results: the original CT plans met the OAR constraints (ABS LACC HDR guideline), and CTVboost_CT V90 for three patients was >96% with V100 >91%. One patient had low CTV coverage (V100=75%, V90=85%) due to proximity of OARs to CTVboost_CT. In the re-calculated plan on the MRI, CTVboost_mri coverage for 2 patients was reduced by at least 10%, and all patients had OARs exceeding dose limits. With re-planning on the MRI, all CTV boost_mri V100 was greater than 96% and all OARs met dose constraints.
Conclusion: is not feasible for treating LACC SBRT on MRI-Linac due to low CTV coverage and/or risk of high OAR toxicity. Use of ATS which requires re-contouring on the daily MR and re-planning, should be adopted for LACC SBRT.
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