(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: This study investigates the impact of magnetic field on plan quality and dose at tissue-air interfaces in MR-guided radiotherapy of head and neck cancer.
Methods: patients with hypopharyngeal carcinoma were reviewed. The skin and interfaces of tissues containing air cavities were contoured. Three plans using 9 fields intensity modulated radiation therapy (IMRT) were created for each patient according to a protocol for conventionally fractionated radiotherapy in Monaco treatment planning system (TPS). The first plan was optimized without magnetic field (plan0T), the second and the third plan were re-calculated (plan1.5T_reCal) and re-optimized (plan1.5T_reOpt) in presence of 1.5 T magnetic field, respectively. A plan quality metric (PQM) was defined according to the clinical requirements for plan assessment. The dosimetric differences between the three types of plans were conducted.
Results: The quality score of plan1.5T_reCal (67.9 ± 10.0) was significantly lower than that of plan0T (81.9 ± 7.1), and the plan1.5T_reOpt (79.8 ± 7.8) achieved similar score compared to plan0T. The skin dose was increased in the presence of 1.5T magnetic field, whereas the increase amplitude of plan1.5T_reOpt (?Dmean 1.30 ± 0.40 Gy, ?Dmax 1.53 ± 1.35 Gy) was smaller than that of plan1.5T_reCal (?Dmean 1.80 ± 0.80 Gy, ?Dmax 5.37 ± 2.51 Gy). There were no significant differences for the mean dose of interfaces of tissues containing air cavities.
Conclusion: The magnetic field has an apparent impact on the dose distribution, especially for electron return effect (ERE) induced increase of skin dose. After the plan is re-optimized in presence of 1.5 T, we can achieve similar quality as the plan optimized without magnetic field.
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