u/ClinicFraggle

After reading several papers and internal documents from other centers, I noticed that people normally use metallic needles for template calibration QC, even though the treatment uses plastic needles. The alleged reason is to avoid bending. I have done it with both metallic needles and plastic ones with metallic obturators inside, and I did not find any noticeable curvature. The only difference I saw was that metallic needles cause more reverberation artifacts. Is it a bad idea to use plastic needles for any other reasons? What kind of needles do you use for this?       

For Elekta users who perform the Hancock MLC test on Agility: have you noticed a lack of reproducibility with variations close to 1 mm in some positions between sucesive executions of the test? Like, you do it the first time and one of the segments appears with -0.4 mm absolute deviation, you repeat it after a while and you get about +0.4 mm in the same region, but in other segments the position does not change...

https://preview.redd.it/c6k4cw0mceah1.png?width=1194&format=png&auto=webp&s=58f719e923ec6860fd9b7ef7a910b98a74d7df90

Since each one of the three bands is actually the combination of two images, small differences between the upper and the lower half are not surprising, but differences between consecutive tests seem higher than desirable (even although the absolute deviation is within 0.5 mm for most leaves). I have seen this in three linacs.