Exploring QA methods for realtime MRI guided radiotherapy

Purpose:The MR-linac is a novel hybrid system that is used in radiotherapy (RT) and combines irradiation with MR imaging. Novel adaptive image-guided radiotherapy techniques are developed for this machine. However, new qualityassurance (QA) techniques are required that check if the (adapted) planned radiation dose is the same as the dose given to the patient. Dose reconstruction, based on the MR-LINACs logfile, can be a new powerful QA tool. Here, we verify logfile based dose reconstruction with the treatment planning system (TPS) dose and Delta4 phantom MR+ on various timescale and discuss its potential as a new QA tool for the MR-LINAC.Method:Software was developed and validated for comparison of the different dose distributions. A patient RT-plan was selected with the intensity modulated RT step-and-shoot technique. The first experiment TPS dose – Logfile was performed at timescales: 1. Total RT-plan 2. Per beam. The second experiment Delta4 – Logfile was performed at timescales: 1. Total RT-plan 2. Per beam 3. Per segment 4. Per 200ms.For analysis, visual comparison, dose profiles, dose difference (DD) and gamma-index (90% of the data) were used.Results:The patient RT-plan had 7 fields and 57 segments. Outcomes of the first experiment were: 1. Total RT-plan: DD = -1.5 to 1.5% and gamma-index (DD=1% & DTA=1mm) of 0.89. 2. Per beam: DD = -3.3 to 2.4% and gamma-index (DD=2% & DTA=2mm) of 0.87.Outcomes of the second experiment were: 1. Total RT-plan: DD of -2.7 to 2.4% and a gamma-index (DD=2% and DTA=2mm) of 0.69. 2. Per beam: maximum DD of -4.7 to 2.5% and gamma-index (DD=2% and DTA=2mm) of 0.99considering all beams. 3. Per segment: maximum DD of -5.9 to 6.0% and gamma-index (DD=3% and DTA=3mm) of 1.0 considering all segments. 4. Per 200ms: the cumulative DD in a high irradiated area was -4.0 to 7.1%, after the2nd segment and -5.2 to 5.3%,. after the 7th segment.Conclusion:Logfile based dose reconstruction might be newest QA tool for the MR-LINAC. Accuracies meet current accepted criteria for IMRT treatment plans considering the total RT-plan and per beam timescale. Smaller timescales have a slightly higher error and require further optimization