An interview with Roberta Nigro and Silvana Riccardi, medical physicists at S.Camillo De Lellis, Radiotherapy Department, Rieti, Italy.
How much time do you need to spend performing a Delta4 Machine QA session?
We usually perform machine QA once per week, it takes from 30 min- 1 hour. Before we started to use the Delta4 Machine QA option we did Machine QA with a matrix chamber and due to the software, the warm up time and its weight, we spent much more time, at least twice as much time when not three times compared to the time it takes today.
What current machine QA routines can you replace with the Delta4 Machine QA option?
We start our machine QA process with some basic tests, these basic tests we perform with a simpler daily QA device (Output, Flatness, Symmetry, Radiation field vs. Light field), whereas with the Delta4 Machine QA option, as we still have an IMRT only linac, we perform a picket fence test and control of dlg (dosimetric leaf gap). We test both energies, 6 and 18 MV. These tests took 2-3 times longer when we used our previous equipment (EPID and Gafchromic Film). Today, we are finished in 30 min - 1 hour.
Which is the greatest advantage with the Delta4 Machine QA?
The accuracy , the speed of the test, the automatic results and the report.
The results from a picket fence test, showing the deviation for each leaf pair.
Can you perform tests which you could not before with the Delta4 Machine QA option?
The control of dlg ( dosimetric leaf gap ) on Varian has a specific procedure, based on Ion Chamber measurments. To test the constancy of this, film can also be used when necessary but in our experience, is too time consuming and not so accurate in dose. The Delta4 Machine QA option is a good compromise between accuracy in geometry and dose and of course, tremendous gain in operator time, saved compared to Gafchromic film analysis, process and evaluation. More over the results from the Delta4 Machine QA can be related to patient specific tests, in order to have a better comprehension of failure of tests related to MLC leaves failures.
A limit is the size of the field 20x20 that oblige to repeat at least the test in two shifted positions, to enclose all leaves, but can be done also with a couch option shift from consolle ( we do not have that option).
In what way could the usage of Delta4 Machine QA increase the quality and safety of patient treatments?
It is a complete tool to check MLC and main parameters of the linac in a fast way; then with the same program it is possible to carry out the patient's specific QA and correlate the eventual fault of some leaves and the effects on the quality of the plan.
The use of a 3D dosimeter to assess dosimetric leaf seperations constancy over time