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Doses to Organs-At-Risk in the Presence and Absence of a 1.5 T Magnetic Field for NSCLC Patients Undergoing SABR

S Al-Ward

S Al-Ward*, A Kim , C McCann , M Ruschin , P Cheung , A Sahgal , B Keller , Sunnybrook Health Sciences Centre, Toronto, Ontario


SU-F-T-451 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To determine whether the electron return effect (ERE) has deleterious effects on lung SABR plans optimized in the presence of an orthogonal 1.5 T magnetic field.

Methods: Data from five NSCLC-SABR patients were used. The Dose was modeled with a 2.5 mm dose grid in the presence and absence of a magnetic field using the Monaco (Elekta) TPS with the Monte Carlo GPUMCD (v5.1) algorithm. For each patient, two plans were generated, one using our conventional Elekta Agility linac beam model and another using the Elekta MRI Linac (MRL) model. Both plans were generated on the average CT using similar dose constraints and a 5 mm PTV. The optimization was performed using our clinic’s planning criteria, with normalization of the targets such that their V99% was equal to 99%. The OAR DVHs were compared for each patient.

Results: The DVH plots revealed that there were limited differences when optimizing plans in the presence or absence of the magnetic field. The mean of the absolute differences, between the two planning types, in the equivalent uniform doses (EUDs) for the OARs were: 0.3 Gy (range of 0.0 - 1.0 Gy) for the esophagus, 0.6 Gy (range of 0.1 – 1.9 Gy) for the heart, 0.5 Gy (range of 0.2 - 0.8 Gy) for the lungs, and 0.6 Gy (range of 0.2 – 1.5 Gy) for the spinal canal. Regarding the maximum doses to the serial organs, the mean of the differences were 3.0 Gy (esophagus) and 0.9 Gy (spinal canal). No trends in the differences were observed.

Conclusion: This study has demonstrated that there were no major differences between plans optimized using a conventional linac and those optimized using an MRI linac with an orthogonal 1.5 T magnetic field. This is attributed to the consideration of the ERE in the optimization.

Funding Support, Disclosures, and Conflict of Interest: This project was made possible with the financial support of Elekta

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