Question20. A patient is delivered 4000 cGy over 20 fractions through a single field via SSD technique. The patient is treated on a 10 MV unit to a depth of 12 cm . The patient is treated at 115 SSD, the PDD is $75.5 \%$ , the collimator scatter factor is 1.025 , the phantom scatter is 1.012 , and the reference dose rate is 1.000 . What is the MU of the field?

Studdy Solution

STEP 1

1. The patient receives a total dose of 4000 cGy over 20 fractions.
2. The treatment is delivered through a single field using SSD (Source to Skin Distance) technique.
3. The treatment is on a 10 MV unit to a depth of 12 cm.
4. The SSD is 115 cm.
5. The Percent Depth Dose (PDD) is $75.5\%$ .
6. The collimator scatter factor is 1.025.
7. The phantom scatter factor is 1.012.
8. The reference dose rate is 1.000.
9. We need to calculate the Monitor Units (MU) for the field.

STEP 2

1. Calculate the dose per fraction.
2. Use the formula for MU calculation in SSD technique.
3. Substitute the known values into the formula.
4. Solve for the MU.

STEP 3

Calculate the dose per fraction.
The total dose is 4000 cGy over 20 fractions, so the dose per fraction is:
$\frac{4000 \, \text{cGy}}{20} = 200 \, \text{cGy}$

STEP 4

Use the formula for MU calculation in SSD technique.
The formula for MU in SSD technique is:
$\text{MU} = \frac{\text{Dose per fraction}}{\text{PDD} \times \text{Collimator Scatter Factor} \times \text{Phantom Scatter Factor} \times \text{Reference Dose Rate}}$

STEP 5

Substitute the known values into the formula.
$\text{MU} = \frac{200}{0.755 \times 1.025 \times 1.012 \times 1.000}$

STEP 6

Solve for the MU.
First, calculate the denominator:
$0.755 \times 1.025 \times 1.012 \times 1.000 = 0.780765$
Now, calculate the MU:
$\text{MU} = \frac{200}{0.780765} \approx 256.18$
Therefore, the MU of the field is approximately:
$\boxed{256.18}$

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