Patient treatment with photon beam

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SUMMARY

The discussion focuses on calculating the number of monitor units (MU) required to deliver 150cGy to a patient using a 6MV photon beam with a 12.5 x 12.5cm field at a 110cm Source to Axis Distance (SAD). The machine is calibrated for 1.0cGy per MU at 100cm for a 10 x 10cm field. The correct answer for the required monitor units is 221, derived using the formula MU = DOSE / (OUTPUT FACTOR X SAD FACTOR), where the SAD factor must be accurately applied to account for the depth of 9cm.

PREREQUISITES
  • Understanding of radiation therapy concepts, specifically photon beam treatment.
  • Familiarity with monitor units (MU) and dose calculations in radiotherapy.
  • Knowledge of Tissue Maximum Ratio (TMR) for 6MV photons.
  • Proficiency in applying algebraic equations in medical physics contexts.
NEXT STEPS
  • Study the calculation of monitor units in radiation therapy using various field sizes.
  • Learn about the application of Tissue Maximum Ratio (TMR) in dose calculations.
  • Research the significance of Source to Axis Distance (SAD) in radiation treatment planning.
  • Explore the inverse square law and its implications in radiation dose distribution.
USEFUL FOR

This discussion is beneficial for medical physicists, radiation therapists, and students in medical physics who are involved in treatment planning and dose calculations for photon beam therapy.

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