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Evaluation of Delivered Dose Using Synthetic CT for MR-Guided and CT-Guided Radiotherapy

H Wang1*, (1) UT MD Anderson Cancer Center, Houston, TX

Presentations

PO-GePV-M-227 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: To evaluate actual delivered dose on an Elekta Unity MR-linac versus a TrueBeam linac based on daily MR images and corresponding synthetic CTs for patients treated to their soft tissue targets that are difficult to distinguish from CBCT and subject to daily target volume variations.

Methods: One oropharynx patient (69.96Gy/33 fractions) and one pelvis intramuscular metastases patient (60Gy/15 fractions) treated with MR-Linac were evaluated in this study. Both patients have a reference plan created for MR-Linac treatment (MRL_Ref) and a backup plan created for Truebeam treatment (TB_Ref). Two daily MR images for each patient was randomly picked to generate synthetic CTs using an in-house cycle-consistent generative adversarial network model. For MR-Linac treatment with daily MR guidance, clinically delivered adapt-to-position (ATP) plans were recalculated (MRL_Calc) in Monaco on daily MR images with verified deformed contours from the reference plan. Meanwhile, the TB_Ref plans were also recalculated (TB_Calc) in Raystation on synthetic CTs simulating daily CT guidance. The recalculated plans were compared to their reference plans in terms of target V95 (percent volume receiving 95% of prescription dose), target mean dose, and maximum or mean dose to surrounding critical structures.

Results: MRL_Ref plans achieved similar CTV V95 as the TB_Ref but with up to 5% higher CTV mean dose for both patients and slightly higher brainstem maximum and parotid mean dose for oropharynx patient. Clinical ATP plans achieved all clinical goals. Both MRL_Calc and TB_Calc plans showed up to 2Gy higher CTV mean dose compared to their reference plans. Two of the TB_Calc plans showed 2-2.7% drop on CTV V95 due to target shape change (oropharynx patient) or CT based alignment error (pelvis patient). All MRL_calc plans had CTV V95 within 0.5% of MRL_Ref plans.

Conclusion: Dose delivery using MR-guidance is more consistent with the planning dose than using CT- guidance.

Keywords

MRI, Quality Assurance, Radiation Therapy

Taxonomy

TH- External Beam- Photons: Quality Assurance - IMRT/VMAT

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