1. SAD Calculations
• Calculation Point at SAD (100)
– Usually multiple beams
– Ideal methodology for isocentric calculations
– Most treatments are isocentric
100 cm
Depth = d cm
Source
calculation point
Depth = d cm

2. 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…

3. 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

4. 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

5. 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 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 (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

6. 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 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 (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

7. 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

8. 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 Size
96.5 cm