Question

Table 2.5.1 Depth/PDD 5 cm 6x6 cm 91.8 83.2 71.8 15 x 15 cm 92.3 84.5 20x20 cm 92.5 84.9 7 cm 74.7 10 cm Output Dmax (cGy/MU) 100 cm SSD 100 cm SAD 0.972 1.025 1.034 1.072 1.053 1.094 Answer the following two questions with the table above: 6. Calculate the Mus needed to deliver 250 cGy at a depth of 7cm, at 100 cm SSD. The field size is 15 x 15 cm, the energy is 10 MV, and there is a single PA beam. 7. Calculate the dose received at 7 cm if the prescription dose from a single PA 10 MV photon beam was 180 Gy to a depth of 5 cm, 100 cm SSD, with a 20 x 20 cm field size.

Answer 1

Equation MU + [having quantities prescribed dose, dose rate ] ro MU = dose rate at prescrip prescribed dose dose rate at prescription point + e[used equation | dose rate at prescription point] MU - prescribed dose (dose rate) (PDD/100) 250 CGY 1.034 CGY/MU +a (with specific amount Prescribed dose Dose rate at SSD 100 cm for field size 15 x 15 cm PDD at 7 cm for field size 15 x 15 cm 84.5 M 250 MU = 1.034 x 84.5/100 MU = 286

Dose + [having quantities received dose, prescribed dose, PDD ] received dose at da (PDD/100) at de prescribed dose at dy (PDD/100) at dy c + a (at specific depth | d1 = 5 cm, d2 = 7 cm) received dose at 7 cm PDD/100 at 7 cm = PDD/100 at 5 cm x prescribed dose at 5 cm 180cGy 84.9 + a with specific amount Prescribed dose PDD at 7 cm for field size 20 x 20 cm PDD at 5 cm for field size 20 x 20 cm 92.5 received dose at 7 cm = 84.9/100 92.5/100 received dose at 7 cm = 165 CGY