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Homework Help: Patient treatment with photon beam

  1. Jan 14, 2016 #1
    1. The problem statement, all variables and given/known data
    Doing some past papers but I got stuck to this question! Any medical physicists can give me a hand please?

    A patient is treated with a 6MVphoton beam, 12.5 x 12.5cm at 110cm (Source to Axis Distance: SAD) to a 9cm depth. The machine is calibrated to deliver 1.0cGy per MU at 100cm for a 10 x 10cm field. The number of monitor units required to deliver 150cGy at the isocentre is?

    Given the information below:
    Tissue Maximum Ratio (TMR) for 6Mv Photons
    Depth (cm) ---- 10 x 10cm ---- 12 x12cm ----14 x 14cm
    --------------- 1.0000 ------- 1.0000 ------ 1.000
    9.0 --------------- 0.810 ---------- 0.818 --------- 0.824

    A. 183
    B. 210
    C. 247
    D. 221

    2. Relevant equations
    SAD FACTOR = (SAD/ SAD + D)2

    3. The attempt at a solution
    Tried using the equations to solve but still not getting the correct answer which is supposed to be 221.
    I got 183 but without using the SAD factor.
  2. jcsd
  3. Jan 19, 2016 #2
    Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?
  4. Jan 19, 2016 #3


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    Gold Member

    You have not provided enough explanation for a nonspecialist to follow. E.g. what is D in your second relevant equation?
    Please either include enough explanation that it becomes just an applied algebra problem, or find a more specialised forum.
  5. Jan 20, 2016 #4
    What is your reason for not using the inverse sq. law correction?
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