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VMAT Vs 7-Field-IMRT with MR-Linac: Comparison of Dosimetric Parameters for Prostate Cancers

D Lee1*, S Lee1, S Oh1, R Fuhrer1, J Sohn1,2, (1) Allegheny Health Network, Pittsburgh, PA, (2) Drexel University College of Medicine, Philadelphia, PA

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: The Elekta Unity MR-Linac (Elekta AB, Stockholm, Sweden) has been clinically introduced and (over) eighteen MR-linac(s) will be available by the end of 2020. However, treatment planning for MR-linac(s) accounts for machine specific characteristics (i.e., IMRT only and 7FFF). This study aims to investigate (1) the feasibility of MR-linac treatment planning with 7-field-IMRT and (2) the comparability of dosimetric parameters between VMAT and 7-field-IMRT plans using 7FFF for prostate cancer patients.
Methods: VMAT plans of five prostate cancer patients received external beam radiotherapy were exported from Monaco v5.11.02 (Elekta AB, Stockholm, Sweden) as a DICOM format and imported to Monaco v5.40.01 for the development of 7-field-IMRT plans with the same structures and prescriptions (4 Gy with 23 fractions for 4 plans and 70Gy with 28 fractions for a plan). During plan development, a 7FFF beam was used with plan parameters (i.e., Gantry angles at every 51o, collimator 0o, MR-coil, MR-couch components, beam isocentre at 143.5 mm and dose rates up to 450 MU/min). To evaluate plan quality, 7-field-IMRT plans were compared with corresponding VMAT plans using the dosimetric parameters (volume, prescription dose constraints and mean dose) for PTV, Rectum, Bladder and left/right Femoral Heads.
Results: Across five plans, an absolute difference of mean dose was negligible (= 2%) to PTV V100RX > 98%, but it varied in Rectum V38Gy (= 66%), Bladder 35Gy (= 49%) and left/right Formal Heads 28Gy (= 22%). The maximum difference of reference volume also varied in PTV (0%), Rectum (= 22%), Bladder (= 26%) and left/right Femoral Heads (= 0.5%).
Conclusions: This study developed 7-field-IMRT plans with a minimal difference of mean dose on PTV, compared with clinical VMAT plans. Larger differences of mean dose were found in rectum, bladder and left/right femoral heads but they are still lower than dose tolerance (dose objectives).

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