Dose Linearity from

[crcrewso]

Is your feature request related to a problem? Please describe. Real world problem - We noticed that the inital dose rate for one of our Linacs changed. This can be seen easily in the SNC Profiler software but not easily quantitatively recorded.

Describe the solution you'd like An additional metric added to the SNC Profiler package and the corresponding outputs to produce the relevent aapm metrics

[jrkerns]

I didn't measure this in the clinic so I'm unfamiliar with the AAPM metrics. Can you point me to the report please? I'm guessing this can only be done w/ .prm files, not .prs files.

[crcrewso]

AAPM Practice Guideline for Linacs TG 142 Table III - Annual Dosimetry - X-ray monitor unit linearity: limits for MU linearity as ±5% (2–4 MU), ±2% (> 5 MU).

I'm not sure if the information would be in PRS files. I can't find any in our records to inspect.

[jrkerns]

Hmm. I never thought about doing it that way. We did it with an ion chamber and delivered X MU and compared the normalized result. Is this with a known MU? Or do you integrate the area?

[crcrewso]

I think that beam was 100 MU. I think that the generally accepted way to consider linearity and initial beam quality has changed within the last 10 years, the referenced extension of TG142 dates to 2017. CPQR guidance from 2016 also includes considerations for what the document describes as end effect. (AL9 & 10)

[jrkerns]

I'm not seeing implementation details there? In the US at least, in TG-198 (published 2021) an IC reading is still the default way. 2.D.1.8.3

[mchamberland]

I'm also curious about using the Profiler to calculate MU linearity, but I don't understand how you would do it or what metric you're calculating from that graph?

@crcrewso Do you have a procedure you can share that you are currently using to calculate it?

[crcrewso]

We are currently exploring how best impliment this clinically, I haven't seen consensus on how to report this detail; it appears to me to be a recommended tolerance with no suggested implimentation. We are keeping track and manually comparing the first MUs of a 100 MU field to the same field from the commissioning dataset to ensure that the starting peak isn't too extreme but would like to turn this into a quantitiative measure going forward.

[crcrewso]

For context here is the dose rate for the same field on a different linac, the spike is over the first 10 MU of the field.

And an electron field