For a single spine field, treated with 6MV (dmax = 1.5cm) to a depth of 5cm at 100cm SSD. What is the Mayneord F Factor for the following extended SSD setups? a. 110cm b. 120cm c. 130cm d. 140cm e. 150cm
For a single spine field, treated with 6MV (dmax = 1.5cm) to a depth of 5cm at 100cm SSD. What
is the Mayneord F Factor for the following extended SSD setups?
a. 110cm
b. 120cm
c. 130cm
d. 140cm
e. 150cm
Mayneord F factor is used to correct for the change in beam size when extending the SSD for a certain depth. The formula for calculating Mayneord F factor is given as:
F = (SAD + d_max) / (SSD + d_max)
Where SAD is the source-to-axis distance (distance from the source to the treatment machine isocenter), SSD is the source-to-skin distance (distance from the source to the patient's skin), and d_max is the depth of maximum dose.
For the given setup, the SAD is 100 cm and the depth of maximum dose is 1.5 cm. Therefore, we can calculate the Mayneord F factor for each of the extended SSD setups as follows:
a. SSD = 110 cm
F = (SAD + d_max) / (SSD + d_max) = (100 cm + 1.5 cm) / (110 cm + 1.5 cm) ≈ 0.879
b. SSD = 120 cm
F = (SAD + d_max) / (SSD + d_max) = (100 cm + 1.5 cm) / (120 cm + 1.5 cm) ≈ 0.824
c. SSD = 130 cm
F = (SAD + d_max) / (SSD + d_max) = (100 cm + 1.5 cm) / (130 cm + 1.5 cm) ≈ 0.774
d. SSD = 140 cm
F = (SAD + d_max) / (SSD + d_max) = (100 cm + 1.5 cm) / (140 cm + 1.5 cm) ≈ 0.728
e. SSD = 150 cm
F = (SAD + d_max) / (SSD + d_max) = (100 cm + 1.5 cm) / (150 cm + 1.5 cm) ≈ 0.685
Therefore, the Mayneord F factor for the extended SSD setups are:
a. 0.879
b. 0.824
c. 0.774
d. 0.728
e. 0.685
These values can be used to correct the beam size at the depth of maximum dose when using extended SSD setups for treatment planning.
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