GEMPIX in Radiotherapy

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Last Updated: Tuesday, 28 July 2015 17:59

We built a prototype of new 2D gas dosimeter for IMRT dose checking. It is based on GEMPIX detector which allows to obtain a sub-millimiter resolution with an on line control of data acquisition. We show here also the first results obtained on small radiotherapy gamma beams coming from a clinical linac.

 

  Mesaurements were performed on a IMRT clinical linac located at the radiotherapy department of Tor Vergata University General Hospital.
     
  The linac used is the Elekta Synergy S with three working energies (6, 10, 18 MV photons). Gamma beam coming from a clinical linac has temporal pulsed structure with a variable frequency (a parameter known as PRF). Elekta Synergy S linac can work at 7 different values (6, 12, 25, 50, 100, 200, 400 Hz). Pulse width is 3 µs and the number of particles per pulse estimated is about 6.7 x 1010ph/(mm2 sec).
     
 gempix radiotherapy   The GEMPIX detector shown in the picture was placed under a MV gamma beam coming from the linear. The ionization is produced mainly on the firts GEM foil. A very little fraction of the Copmton electron are backscattered in the active volume. The ones that comes near the GEM holes are multiplied and produce current pulse signals. So detector is able to work at very low efficiency but at the same time it provides a number of digital counts proportional to the photon beam fluence. Counts are linearly correlated to PRF which sets the number of MU/min.
     
 gempix gafcromic bw  

On the left can be seen the difference of the image obtained with the GEMPIX in 1 min of acquisition respect to an image taken in 10 min with gafchromic film.

The GEMPIX is sensitive to a single Linac pulse.

     
     
gempix gafcromic sqare   These plots show a comparison between  the 2D images obtained with GEMPIX (left) and radiochromic film (right) taken at the treatment isocenter with a beam spot of 1x1 cm2.  
      
gempix gafcromic 1  

In this picture the setup of the lead leafs for the definition of the gamma spot is shown on the left.
The two images obtained respectivelly with GEMPIX and gafchromic filmare compared at the level of 160 micron pixels and the results is shown in the right picture. The gamma index is less than 1% in the 90% of the beam spot.  

     
gempix gafcromic 2  

Here the results obtained with another lead leafs configuration. An optimal accordance between the two detectors is obtained working in a bi-GEM configuration and applying the suitable thresholds to cut the halos around the mainbeams.