(Sunday, 7/29/2018) 3:00 PM - 3:30 PM
Room: Exhibit Hall | Forum 7
Purpose: Conventional IMRT QA is based on phantom irradiation. The dose in the patient is not verified. We have developed a new IMRT QA method using the Monte Carlo simulation with measured actual IMRT fluence distributions to verify dose calculation and the dose delivered to the patient.
Methods: Five head and neck IMRT plans delivered on the Varian TrueBeam linear accelerator were chosen for evaluation. Portal dosimetry was performed to record the portal dose pattern for each field. Deconvolution techniques were used to extract the actual fluence from the measured portal dose. The actual fluence and the expected fluences calculated by Eclipse were compared with one another in MapCheck (tolerance: 3%/3mm). Monte Carlo simulations were executed with the actual fluences and patient CT data. Dose to critical structures (brainstem, cord, parotids) and targets (PTVs) were compared between Monte Carlo simulations and Eclipse looking at the following criteria: max, min, mean, D95, and D50.
Results: The actual fluences evaluated with MapCheck had passing rates of 94.1% or above. Max, min, mean, and D95 dose differences of Monte Carlo compared to Eclipse for the targets ranged from 0.5%-16.3%, 3.5%-17.5%, 1.0%-4.5%, and 3.6%-9.3% respectively. Max, min, mean, and D50 dose differences for critical structures ranged from 0.3%-10.5%, 0.03%-4.9%, 0.0%-13.0%, and 0.1%-27.3% respectively.
Conclusion: The Monte Carlo incorporating portal dosimetry and patient CT has included the dose delivery information and patient geometry, and thus is able to verify the dose delivered to the patient. The results indicated that actual fluence patterns were not exactly the same as the expected, and doses to targets and critical structures could be significantly different from the calculated values. This study has shown this new IMRT QA method is feasible for patient dose verification without phantom measurement.
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