TG51

Question 1

why do we need 30x30x30 tank?

Answer 1

because water should extend 10 cm beyond the field you are measuring on all 4 sides

Question 2

why do we have to correct for stopping power with depth for electrons but not for photons?

Answer 2

Electron stopping power ratios change a lot as a function of depth because they deposit energy along their path. Photons on the other hand have infrequent but powerful interactions- the new electrons set in motion have roughly the same energy

Question 3

why does TG51 use 10 cm depth?

Answer 3

beyond the depth of highest energy contaminant electrons, and TCPE exists for the highest energy beam

Question 4

issues with different detectors for electron PDD measurements

Answer 4

• in buiulkd-up region of electrons, cylindrical chambers are inaccurate compared to PP/diamond- issue is more pronounced for 6 and 9 MeV because you have a more prominent build up region – important when commissioning!
  
  • Cylindrical- correct for Prep and stopping power ratios
  • PP-correct for stopping power ratios
  • diamond - don’t correct at all cause it should be like water

Question 5

explain how TG21 measures the dose to the gas in the chamber (Dgas)

Answer 5

• kerma at point P in free space is Kc=X * (W/e)
• the collision kerma at an infinitesmal piece of wall placed at P is Kc,w = Kc * (uen/p) wall to air
• The little wall P is centered within the spherical. wall attenuation reduces Kc,w to AKc(uen/p)wall to air
• the absorbed dose in the wall material at P is Dw=Kc,w
• dose to gas in the sphere is Dg = Dw * (L/p) from gas to water