Question
On occasion, myQA iON shows spots delivered in areas/regions where there should be no dose delivered. Why is this?
Answer
myQA iON allows to conduct log file-based quality assurance. The software compares the spot position and monitor units that are recorded by the nozzle ionization chambers and saved as log files to the planned spots from the treatment plan. myQA iON also reconstructs and compares the dose distribution from the delivered dose information (again, from the log files) to the planned dose distribution from the TPS. As myQA iON uses these log files to display the analysis results between the TPS and the vendor-provided raw log files, proton machine vendors must ensure that the log files indicating spot positions and monitor unit data accurately reflect what their machine delivers.
Recently, a few customers have noticed the presence of spots that shouldn't exist while performing log-based dose reconstruction in myQA iON. The reason this occurs is due to a known issue/bug that exists in some vendor log files (currently only observed with IBA machines) that originate from ionization nozzle data. Therefore, it should be known that these spots are not actually delivered nor are they part of the planned data from the treatment planning system. Consequently, there is no defect with the myQA Ion software as it fulfills its function of accounting for all the raw log file data from the machine vendor and identifying spots that are unable to be delivered.
To summarize, this effect is not due to an issue with the myQA iON Software, which works as intended. Furthermore as the aforementioned spots, are negligible in terms of dose intensity, scale, and overall dosimetric impact, the upcoming release of myQA iON 2.2 includes a methodology designed to identify and automatically ignore such points that have potential to appear in log files similar to that as described above. This is similar to the method already in use by the machine vendor (IBA) to remove the data from their log file analyzer at the source.