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Evaluation On the Out-Of-Field Irradiation by Magnetic Field During MR Image-Guided Radiation Therapy: A Phantom Study

H An1*, J Kim1 , C Choi1 , S Kang2 , J Cho1 , S Lee1 , M Kim1 , J Park1 , (1) Seoul National University Hospital, Seoul, SEOUL, (2) Seoul National University Bundang Hospital, Seongnam-si, Gyeonggi-do,


(Tuesday, 7/31/2018) 3:45 PM - 4:15 PM

Room: Exhibit Hall | Forum 7

Purpose: The scattered electrons in the air generate electron-streams by the magnetic field during magnetic resonance (MR) image-guided radiation therapy (IGRT). If this electron-stream reaches the patient body outside the treatment field, unwanted exposure can occur. Therefore, we measured out-of-field doses of the ViewRay system and compared to the calculated doses with the treatment planning system (TPS).

Methods: A custom-made support-phantom was fabricated to generated slopes with angles of 10°, 20°, and 30° between the couch top and the surface of the solid water phantom (SWP, 30 cm x 30 cm x 10 cm). The isocenter was located at the centroid of the tilted SWP by the support-phantom. With a field size of 6.3 cm², three beams with gantry angles of 0°, 30°, and 330° delivered to the tilted SWP (150 cGy to the isocenter). The EBT3 films positioned at the distance of 17 cm from the centroid of the SWP to the film surface. The doses in the circles with radii of 1 cm (Dᵣ₌�), 2 cm (Dᵣ₌₂), and 3 cm (Dᵣ₌₃) from the maximum dose point were acquired. Areas of the isodose lines of 90% (V₉₀), 70% (V₇₀), and 50% (V₅₀) of the maximum dose were acquired.

Results: The calculated values were generally larger than the measured values (over-estimation of the TPS calculation). As increasing the slope of the SWP, the out-of-field doses increased. At the 30° slope with gantry angle of 0°, the measured values of Dᵣ₌�, Dᵣ₌₂, and Dᵣ₌₃ were 80.9 cGy, 91.4 cGy, and 96.2 cGy, respectively. The values of V90, V70, and V50 were 6.3 cm², 15.7 cm², and 21.0 cm², respectively.

Conclusion: As the slopes of the SWP increased, out-of-field doses and irradiated areas increased. When treating superficial tumors with MR-IGRT technique, the out-of-field irradiation should be considered to avoid unwanted exposure.

Funding Support, Disclosures, and Conflict of Interest: Current work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2017M2A2A7A02020643).


Treatment Planning, Validation, Film


IM/TH- MRI in Radiation Therapy: MRI for treatment planning

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