Ion chambers Acceptance Testing and Evaluation
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Ion chambers Acceptance Testing and Evaluation

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Ion chambers Acceptance Testing and Evaluation Ion chambers Acceptance Testing and Evaluation Presentation Transcript

  • By     Vibha  Chaswal,  Ph.D.  
  • The  ion-­‐chamber  should  be  tested  along  with   the  electrometer  and  the  cable  it  is  going  to  get   used  with       For  reproducibility  of  results  the  ion-­‐chamber   should  also  be  tested  against  different   electrometers  and  the  results  should  have   minimum  variability       It  is  important  to  be  aware  of  various  sources  of   noise  in  the  system  that  contribute  to  charge-­‐ collection  at  the  ion-­‐chamber  electrodes  in   addition  to  charge  collected  due  to  ionizations   by  radiation      
  •   Acceptance  testing  is  recommended  when   the  chamber  is  back  from  ADCL  before  using   it  for  reference  or  absolute  dosimetry.      
  •   PTW  0.6cc  SN1315  farmer  type  ion-­‐chamber  by  PTW  FREIBERG.  Model  #   TN  30013-­‐1315  
  •   IC  set-­‐up:  100  cm  SSD;  ion-­‐chamber  active   volume  at  the  center  of  a  20cmx20cm  field;   solid-­‐water  phantom  with  1.5  cm  build-­‐up   and  5  cm  back  scatter       Electrometer:  -­‐300  V  (-­‐100%)  bias       Energy:  6  MV  photon  beam       Procedure:  Record  charge  collection   measurements  for  MU  delivery  ranging   between  2  MU  to  200  MU    
  • The  uncertainty  in  the  charge  collection  due  to   stem  effect  should  be  less  than  0.5%     This  can  be  checked  by  taking  exposures  using  a   field  size  that  irradiates  just  the  thimble  and   comparing  it  with  charge  collection  reading   taken  when  the  whole  stem  is  in  the  field.       Set  up:  100  cm  SSD;  FS  5cmx30cm;  tape  the  IC  in   two  orientations  –  IC  parallel  to  the  30  cm  dim  of   field,  IC  perpendicular  to  it  with  only  thimble   inside  direct  radiation  beam    
  •            stem  effect  slightly  greater  than  0.5%  but  5cmx30cm  has  greater   penumbra  uncertainties  repeat  test  using  7cmx30cm  or  10cmx30cm  FS            stem  effect  is  0.2%,  within  manufacture’s  specifications    
  •        Collect  charge  readings  using  both  bias  polarities   with  all  other  set-­‐up  parameters  constant  
  •   Which  means  check  the  cylindrical  symmetry   of  the  ion  chamber’s  active  volume’s   construction       Ion  chamber  suspended  in-­‐air  with  the  ion   chamber’s  build-­‐up  cap  on       This  provides  same  build-­‐up  from  all  different   directions  of  irradiation    
  • (left)  Ion  chamber  major  axis  perpendicular  to  the  CAX.  (right)  Ion  chamber  major   axis  parallel  to  the  CAX.  
  • ion chamber response for an orientation perpendicular to the central beam axis
  • Ion chamber response for an orientation parallel to the central beam axis (Set up: 10 x 10 field, 100 SAD, 06 MV X-rays, 100 MU, CAX parallel)
  •   Measurements  involving  ionizing  current  at  2   different  voltages  are  used  to  assess  the   collection  efficiency  of  an  ion  chamber  [Boag   2-­‐volatge  technique]       This  is  ‘Pion’  or  recombination  correction   factor  [TG  51]       Use  electrometer    100  %  and  50  %  voltage   settings  using  negative  and  positive  bias  (Vh   and  Vl  settings)    
  • Collection  Efficiency  (Pion):    
  •       A  feel  of  your  ion  chamber   Physicist’s  extended  hand  and  mind  in  clinical  reference   dosimetry   JEB’s  way  of  keeping  things  in  head  for  an  easy  reference      *Average  nC/cc  MU  for  a  0.6  cc  farmers  chamber  =  0.2092  nC/cc/  MU   *Average  nC/cc  MU  for  a  0.125  cc  farmers  chamber  =  0.1833  nC/cc/  MU     *Average  nC/cc  MU  for  a  0.015  cc  farmers  chamber  =  0.1714  nC/cc  /MU   OR,  simply  put…   ….the  average  charge  collected  per  cubic  centimeter  of  the  chamber’s   active  volume  for  a  monitor  unit  of  radiation  is  roughly  ~0.2  nC   So,  expected  charge  collection  for  100  MU  ~  20  nC…..right,  Happy  Physicist!   *  Averages  derived  from  detailed  linearity  measurements  using  3  chamber  types    
  • The  SNR  is  derived  from  the  same  irradiation  data  set   over  the  three  ion-­‐chambers       SNR  =  mean  signal  (your  data)/  Std  Deviation   SNR  test  Results  observations:     SNR  generally  increases  with  the  increase  in  Signal       Normalized  SNR  values  show  that  the  SNR  decreases   with  the  decrease  in  ion  chamber  collection  volume   (reason  I  wouldn’t  try  to  use  a  0.015  cc  ion-­‐chamber  to   assess  doses  at  very  low  signals)     At  10  MU  using  0.015  pin-­‐point  -­‐  noise  and  the   detected  signal  are  almost  equal      
  •   1.  2.  3.    To  quantify  the  leakage  from  various  components  the   following  were  quantified:   Ionizations  in  the  ion-­‐chamber  cable  in  the  field   Ionizations  in  the  triax  cable  in  the  field   Leakage  when  no  irradiation  but  power  supply  is  on.   This  was  quantified  with  and  without  the  ion-­‐ chamber  being  connected  to  the  electrometer.   Leakage  only  due  to  electrometer  electronics  can  be   characterized  by  letting  the  electrometers  run  on   battery  for  a  given  duration  of  time  without  any   connecting  wires  or  ion-­‐chamber.