Radiation Therapy Calculations

A patient is delivered a dose of 150 cGy at a depth of 13 cm with 10 MV photons at 100 cm SSD. That PDD is 60%. Calculate the dose delivered to the depth of maximum dose.

(Dose @ Point A / PDD @ Point A) = (Dose @ Point B / PDD @ Point B) PDD at Dmax is 100%, so (x / PDD or x / 1) Dose at Dmax is 250 cGy

A deep seated tumor is treated posteriorly with a high energy photon beam. The tumor is 13 cm deep and the %DD is 65. The spinal cord is in the field and is at a depth of 5 cm and the %DD is 85. If 200 cGy is delivered to the tumor, the calculated dose to the spinal cord would be:

(Dose @ Point A / PDD @ Point A) = (Dose @ Point B / PDD @ Point B) The spinal cord will receive 261.54 cGy

What would be the MU per degree (aka speed of gantry) when the MU is 255 for an anterior arc of 180 degrees?

(Treatment MU / Degrees of treatment arc) = Gantry Speed 255/180 = 1.4166 1.42 MU/degree

If the MU are 255 and the MU/degree is 1.42 and the gantry starts at gantry angle of 270, travels clockwise. Where is the finishing gantry angle for this treatment?

1. Determine the number of degrees in the arc (Treatment MU / Degrees of treatment arc = Gantry Speed) (255 / x ) = 1.42 255 / 1.42 = 180 degrees in the arc 2. Look at gantry angle orientation AND direction of the gantry movement Starts at 270 ==> 180 clockwise Finish at 90 degrees

Calculate the MU necessary to deliver 180cGy to a 5cm depth 10x10 field size, 6Mv Linac 100cmSAD TMR = 97.7% Machine output at 100cm from source is 1cGy/MU

184.24 MU

Calculate the Monitor Unit necessary to deliver 180cGy to a 5cm depth TAR at D5 = 95.2% 10x10 field size, 100cmSAD Machine output at 100cm from source is 1cGy/MU

189.08 MU

Calculate the MU necessary to deliver 200cGy to a depth of 3cm (PDD value = 95.1%) 10x10 field size 6Mv Linac 100cmSSD Reference Dose Rate at 101.5cm from source is 1.0cGy/monitor unit

210.3 MU

For a 6Mv beam, what is the dose to the depth of 5cm when the dose at 3cm is 200cGy? • PDD value at D3 = .951 • PDD value at D5 = .876

• To calculate the dose at some point along the central axis - use direct proportion. • (Dose at Point A / PDD at Point A) = (Dose at Point B / %DD at Point B) x = 184.23 cGy

Setup for Entire Femur: Largest field length at 100cm is 40cm • Need 56cm length • What would be the SSD required?

140cm SSD

What is the equivalent square of a 10 x 14 cm field size?

FS eq. = 11.667 cm

A 10 x 14 cm field size has a 2 cm square blocked on one side, and another 3 cm square contralaterally. What is the field size equivalent of the 10 x 14 cm rectangle?

FS eq. = 4x(A/P) Area = (1014)−(22)−(33) Perimeter =8+2+2+12+7+3+3+11 FS 10.58 cm

For a 6Mv beam, what is the dose to the D/Max when the dose at 3cm is 200cGy? • PDD value at D3 = .951

dose at D/Max x = 210.30 cGy