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The Impact On the Dosimetric of Online Replanning for Rectal Cancer Radiation Therapy On the 1.5T MR-Linac

Z Li1*, Y Yin1 , (1) Shandong Cancer Hospital Affiliated to Shandong University, Jinan,


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: The MR-linac (MRL) provides image guidance with high soft-tissue contrast. Online 1.5T MR imaging on the MRL gives better target visualization in realtime. However, due to the presence of a transverse magnetic field (1.5T), electron deflection may affect the target dose distribution, particularly at air cavities. The purpose of the study is to investigate the dosimetric impact of online replanning of rectal cancer.

Methods: 10 cases with rectal cancer were prospectively enrolled into a cohort study which underwent pretreatment plan and online planning using the research version of Monaco (V 5.40.00, Elekta AB, Stockholm, Sweden). For the pretreatment plan, a 1.5T, 7MV, 7 fields intensity modulated radiotherapy (IMRT) plan was created. For each online planning, the PTV V95% and V110% and bowel area V5Gy, V10Gy, V15Gy, V20Gy, and V25Gy were calculated to evaluate the changes in dose distribution from fraction to fraction. To evaluate the effects of gas cavities in Magnetic field, we generated artificial holes in the CTV and OARs for the initial plan CT.

Results: For the 10 patients, the online MRI-based replanning can better improve the target coverage and reduce dose of OARs. A comparison of DVH parameters between pre-treatment plans calculated on the simulated repeated MRI anatomy and online replanned plans shows that online replanning yields significant dosimetric benefits for OAR dose. The online planning show that the air cavities have much lower dose in cold spots and much higher dose in hot compared with the prescribed dose.

Conclusion: Online replanning reduces the number of unplanned violations of dose constraints for surrounding OARs. The presence of a 1.5T magnetic field can affect the dose distribution on the air holes. The study showed that the dose distribution difference in these parts were clinical acceptable.


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