ESTRO 2023 - Abstract Book

S1448

Digital Posters

ESTRO 2023

The sensitivity and specificity of the SDC is comparable to that of phantom QA. A combination of both methods leads to higher sensitivity keeping the same specificity. Decisions based on DVH criteria should be given preference over gamma analysis. Further improvements are expected after fine-tuning the beam model in RadCalc.

PO-1730 Logfile based automatic patient QA for daily MR guided adaptive radiotherapy

D. Tilly 1,2 , S. Fransson 2,3 , M. Lundmark 4 , U. Isacsson 2,5 , A. Johansson 2,6,3

1 Uppsala University, Immunology, Genetics and pathology, Uppsala, Sweden; 2 Uppsala University Hospital, Medical Physics, Uppsala, Sweden; 3 Uppsala University, Radiology and Surgery, Uppsala, Sweden; 4 Uppsala University Hospital, Medical Physics, Uppsala University, Sweden; 5 Uppsala University, Immunology, Genetics and Pathology, Uppsala, Sweden; 6 Uppsala University, Immunology, Genetics and Pathology, Uppsala, Sweden Purpose or Objective Patient QA for daily adaptive radiotherapy with the MR-Linac requires new methods compared to conventional radiotherapy as the plan cannot be measured using a phantom prior to delivery. Currently the daily plan is measured using a phantom after delivery to assess if any corrective measures are necessary. However, measuring every adapted plan is very time consuming. Therefore, we have implemented an alternative automatic QA workflow based on logfiles from the MR-Linac. Materials and Methods The analysis compares a logfile from the daily specific fraction with the Dicom treatment plan created online for that same fraction. The online plan is exported to a local Dicom server and in-house software automatically identifies the corresponding logfile and proceeds with the comparison. The result is easily inspected in a result file produced by the software. The logfiles contains the complete information of the collimating elements, i.e. the position of each individual MLC leaf as well as jaw, together with the delivered MU at a frequency of 25 Hz. All plans were also delivered on a Delta4 phantom and evaluated with the gamma criteria 3%/3mm vs the dose calculated in the TPS. Results The logfile analysis produced a result in 224 of the 226 fractions. In 90.2% of the 224 logfiles all test criteria passed. The frequency of reasons for logfile not passing the patient QA can be seen in figure 1. In 13 of the failed 22 plans (6% of all plans) the cause was an interrupted treatment which produces multiple logfiles which broke the automatic workflow. In most of the 9 plans with Jaw mean position failure both Jaws failed and are counted twice in Figure 1. The errors in the Jaw mean position did not result in an error in the MU map comparison. The combined distribution, from all fractions without delivery interruptions, of errors in the positioning of the Jaw and MLC according to the machine logfiles can be seen in figure 2. The mean is very close to 0 mm which means that no systematic errors could be found. Not a single plan failed the Delta4 measurement criteria, so the errors found in the logfile analysis did not translate into errors in the phantom measurement. The current work reports on the result of the logfile analysis based on the 226 latest fractions delivered on the MR-Linac. The comparison criteria used in our institution when comparing the logfile vs the plan are, • Position of each Jaw and every individual MLC leaf (max error < 1 mm, mean error < 0.5 mm) • Total delivered MU (error < 2% of treatment plan MU) • Gantry angle (error < 0.5 degrees) • Monitor unit maps (MU maps) per beam (max error < 2% compared to total plan MU)