Patient treatment with photon beam

[Shinobi]

Homework Statement

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
1.5 --------------- 1.0000 ------- 1.0000 ------ 1.000
9.0 --------------- 0.810 ---------- 0.818 --------- 0.824

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

Homework Equations

MU = DOSE / ( OUTPUT FACTOR X SAD FACTOR)
SAD FACTOR = (SAD/ SAD + D)2

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.

 

[haruspex]

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.

 

[gleem]

What is your reason for not using the inverse sq. law correction?