SAD Calculations

• Calculation Point at SAD (100)– Usually multiple beams

– Ideal methodology for isocentric calculations

– Most treatments are isocentric100 cm

Depth = d cm

Source

calculation point

Depth = d cm

SAD Equation

• MU Setting = Prescribed Dose .

RDR x ISCF x Sc x Sp x TMR x Other Factors

RDR = reference dose rate usually cGy/MU (cGy/min for Co-60)

ISCF = inverse square correction factor

Sc = collimator scatter factor

Sp = phantom scatter factor

TMR = tissue maximum ratio

Other factors = trays, wedges, etc…

Case 1: 180 cGy to Dmax, 10x10 Open Field

• MU Setting = Prescribed Dose .

RDR x ISCF x Sc x Sp x TMR x Other Factors

RDR = reference dose rate usually cGy/MU (cGy/min for Co-60)

ISCF = inverse square correction factor

Sc = collimator scatter factor

Sp = phantom scatter factor

TMR = percent depth dose

Other factors = trays, wedges, etc…

RDR = 1 cGy/MU

ISCF = 1.000

Sc = 1.000

Sp = 1.000

TMR = 1.000

Other factors = N/A = 1.000

• MU Setting = 180 .

1 x 1.000 x 1.000 x 1.000 x 1.000 x 1.000

= 180 MU

100 cm

Depth = dmax = 1.5 cm

10x10 Field Size

Case 2: 180 cGy to 5 cm, 10x10 Open Field

• MU Setting = Prescribed Dose .

RDR x ISCF x Sc x Sp x TMR x Other Factors

RDR = reference dose rate usually cGy/MU (cGy/min for Co-60)

ISCF = inverse square correction factor

Sc = collimator scatter factor

Sp = phantom scatter factor

TMR = percent depth dose

Other factors = trays, wedges, etc…

RDR = 1 cGy/MU

ISCF = 1.000

Sc = 1.000

Sp = 1.000

TMR = 0.929

Other factors = N/A = 1.000

• MU Setting = 180 .

1 x 1.000 x 1.000 x 1.000 x 0.929 x 1.000

= 194 MU

100 cm

Depth = 5 cm

10x10 Field Size

Case 3: 180 cGy to 5 cm from two equally weighted 10x10 Open Fields

• MU Setting = Prescribed Dose .

RDR x ISCF x Sc x Sp x TMR x Other FactorsRDR = reference dose rate usually cGy/MU (cGy/min for Co-60)ISCF = inverse square correction factorSc = collimator scatter factorSp = phantom scatter factorTMR = percent depth doseOther factors = trays, wedges, etc…

RDR = 1 cGy/MUISCF = 1.000Sc = 1.000Sp = 1.000TMR = 0.929 (AP Field), 0.788 (PA Field)Other factors = N/A = 1.000

• AP MU Setting = 90 . 1 x 1.000 x 1.000 x 1.000 x 0.929 x 1.000

= 97 MU

• PA MU Setting = 90 . 1 x 1.000 x 1.000 x 1.000 x 0.788 x 1.000

= 114 MU

100 cm

Depth = 5 cm

10x10 Field Size

Depth = 10 cm

Case 4: 180 cGy to 5 cm from two 10x10 open fields weighted 2:1 AP to PA

• MU Setting = Prescribed Dose .

RDR x ISCF x Sc x Sp x TMR x Other FactorsRDR = reference dose rate usually cGy/MU (cGy/min for Co-60)ISCF = inverse square correction factorSc = collimator scatter factorSp = phantom scatter factorTMR = percent depth doseOther factors = trays, wedges, etc…

RDR = 1 cGy/MUISCF = 1.000Sc = 1.000Sp = 1.000TMR = 0.929 (AP Field), 0.788 (PA Field)Other factors = N/A = 1.000

• AP MU Setting = 120 . 1 x 1.000 x 1.000 x 1.000 x 0.929 x 1.000

= 129 MU

• PA MU Setting = 60 . 1 x 1.000 x 1.000 x 1.000 x 0.788 x 1.000

= 76 MU

100 cm

Depth = 5 cm

10x10 Field Size

Depth = 10 cm

Case 6: 180 cGy to 5 cm, 10x10 Open Field, extended distance of 125 cm

• MU Setting = Prescribed Dose .

RDR x ISCF x Sc x Sp x TMR x Other Factors

RDR = reference dose rate usually 1cGy/MU (cGy/min for Co-60)

ISCF = inverse square correction factor

Sc = collimator scatter factor

Sp = phantom scatter factor

TMR = percent depth dose

Other factors = trays, wedges, etc…

RDR = 1 cGy/MU

ISCF = (1002/1252) = 0.640

Sc = 0.987 (based upon 8x8 field size defined by the collimators)

Sp = 1.000 (based upon a 10x10 field size at the calculation point)

TMR = 0.929 (based upon a 10x10 field size at d=5cm)

Other factors = N/A = 1.000

• MU Setting = 180 .

1 x 0.640 x 1.000 x 1.000 x 0.929 x 1.000

= 302 MU

125 cm

Depth = 5 cm

10x10 Field Size

Case 7: Rx 180 cGy to d = 5cm, what is the dose to Dmax

In an SAD setup, the doses inside the patient are related by the ratio of TMRs and the inverse square factor.

Dose Dmax = 180 cGy x (TMR (dmax, 9.65x9.65 field size) / TMR (d=5, 10x10 field size)) x (SCPD (d=5))2/(SCPD(d=dmax))2

= 180 x 1.000/0.929 x 1002/96.52 = cGy

100 cm

Depth = 5 cm

10x10 Field Size

9.65x9.65 Field Size96.5 cm