(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: Both virtual phantom and MVCT phantom (i.e., virtual phantom with uniform density of 1.25 g/cm³ and MVCT phantom scanned by megavolt x ray computed tomography (MVCT) of ArcCHECK are used in clinic. The purpose of this study is to evaluate their performances in commissioning and patient specific QA with or without the presence of magnetic field.
Methods: The measured dose for 15 simple plans with different field size and gantry angles and 14 patient’s treatment plans were obtained on MR-Linac, Unity, using the ArcCHECK-MR. They were compared with the planned doses calculated by Monaco treatment planning system using two phantoms of ArcCHECK-MR, respectively. The similar experiments were performed on the conventional Linac, Versa HD. The performances of each phantom were evaluated by comparing the gamma passing rate using paired t test. Dose profiles in different phantom were also compared, especially near the diode plane.
Results: For MR-Linac, the gamma passing rates between measured dose and planned dose calculated with MVCT phantom were significantly higher than those between measured dose and planed dose calculated with virtual phantom (92.79% vs. 85.63% for simple plans with criteria of 2mm/2% and 99.61% vs. 98.49% for patient plans with criteria of 3mm/3%, both Ps<0.01). However, the differences in gamma passing rate were not significant (both Ps>0.05) for conventional Linac (94.14% vs. 95.83% for simple plans and 98.33% vs. 97.44% for patient plans, respectively). The main difference in profiles appeared at the diode plane due to the electron return effect (ERE) in presence of magnetic field.
Conclusion: Both MVCT phantom and virtual phantom of ArcCHECK-MR can be used for commissioning and patient specific QA for conventional Linac. However, MVCT phantom of ArcCHECK-MR is prefer for MR-Linac because it more accurately reflect the nonuniform density distribution inside the phantom, especially near the diode plane.
Keywords
Magnetic Fields, Quality Assurance, Phantoms
Taxonomy
IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined Quality Assurance
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