This document discusses SSD calculations for radiation therapy treatment planning. It provides examples of calculating monitor units (MUs) needed to deliver a prescribed dose to different depths using various field sizes and source-to-surface distances (SSDs), accounting for factors like percent depth dose (PDD), inverse square, scatter, and blocking. It also gives an example of calculating the dose to the depth of maximum dose (dmax) given a prescribed dose to another depth using the ratio of PDD values.

1. SSD Calculations
• Patient surface at 100cm
– (80cm for Co-60)
– Usually a single beam
– Most simple of all calculations
– Most likely to be called upon to do
100 cm
Dose to a point at depth
Depth = d cm
Source
Patient Surface

2. The SSD Equation
• MU Setting = Prescribed Dose .
RDR x ISCF x Sc x Sp x (PDD/100) x Other Factors
RDR = reference dose rate usually 1 cGy/MU (cGy/min for Co-60)
ISCF = inverse square correction factor
Sc = collimator scatter factor
Sp = phantom scatter factor
PDD = percent depth dose
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 (PDD/100) 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
PDD = percent depth dose
Other factors = trays, wedges, etc…
RDR = 1 cGy/MU
ISCF = N/A = 1.000
Sc = 1.000
Sp = 1.000
PDD = 100%
Other factors = N/A = 1.000
• MU Setting = 180 .
1 x 1.000 x 1.000 x 1.000 x (100/100) x 1.000
= 180 MU
100 cm
Depth = dmax
= 1.5 cm
10x10 Field Size

4. Case 2: 180 cGy to d = 5 cm, 10x10 Open Field
• MU Setting = Prescribed Dose .
RDR x ISCF x Sc x Sp x (PDD/100) 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
PDD = percent depth dose
Other factors = trays, wedges, etc…
RDR = 1 cGy/MU
ISCF = N/A = 1.000
Sc = 1.000
Sp = 1.000
PDD = 87.1%
Other factors = N/A = 1.000
• MU Setting = 180 .
1 x 1.000 x 1.000 x 1.000 x (87.1/100) x 1.000
= 207 MU
100 cm
Depth = 5 cm
10x10 Field Size

5. Case 3: 180 cGy to d = 5 cm, 10x20 Open Field
• MU Setting = Prescribed Dose .
RDR x ISCF x Sc x Sp x (PDD/100) 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
PDD = percent depth dose
Other factors = trays, wedges, etc…
RDR = 1 cGy/MU
ISCF = N/A = 1.000
Sc = 1.013 (Based Upon 13x13 equivalent square)
Sp = 1.008 (Based Upon 13x13 equivalent square)
PDD = 87.7% (Based Upon 13x13 equivalent square)
Other factors = N/A = 1.000
• MU Setting = 180 .
1 x 1.000 x 1.013 x 1.008 x (87.7/100) x 1.000
= 201 MU
100 cm
10x20 Field Size
Depth = 5 cm

6. Case 4: 180 cGy to d = 5 cm, 10x20, 20% blocked field
• MU Setting = Prescribed Dose .
RDR x ISCF x Sc x Sp x (PDD/100) 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
PDD = percent depth dose
Other factors = trays, wedges, etc…
RDR = 1 cGy/MU
ISCF = N/A = 1.000
Sc = 1.013 (Based Upon 13x13 equivalent square)
Sp = 1.005 (Based Upon 11.7x11.7 effective square)
PDD = 87.4% (Based Upon 11.7x11.7 effective square)
Other factors = 0.970 (a blocking tray transmission factor)
• MU Setting = 180 .
1 x 1.000 x 1.013 x 1.005 x (87.4/100) x 0.970
= 208 MU
100 cm
10x20 Field Size
Depth = 5 cm

7. Case 5: 180 cGy to d = 5 cm, 10x20, 20% blocked field, SSD = 110 cm*
*Ignoring Mayneord Factor
• MU Setting = Prescribed Dose .
RDR x ISCF x Sc x Sp x (PDD/100) 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
PDD = percent depth dose
Other factors = trays, wedges, etc…
RDR = 1 cGy/MU
ISCF = (101.52
/111.52
) = 0.828
Sc = 1.007 (Based Upon 11.8x11.8 equivalent square)
Sp = 1.005 (Based Upon 11.7x11.7 effective square)
PDD = 87.4% (Based Upon 11.7x11.7 effective square)
Other factors = 0.970 (a blocking tray transmission factor)
• MU Setting = 180 .
1 x 0.828 x 1.007 x 1.005 x (87.4/100) x 0.970
= 253 MU
110 cm
10x20 Field Size on the patient
Depth = 5 cm
9.1x18.2 Field Size on the machine

8. Case 6: 180 cGy to d = 5 cm, 10x20, 20% blocked field, SSD = 110 cm*
*Including Mayneord Factor
• MU Setting = Prescribed Dose .
RDR x ISCF x Sc x Sp x (PDD/100) 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
PDD = percent depth dose
Other factors = trays, wedges, etc…
RDR = 1 cGy/MU
ISCF = (101.52
/111.52
) = 0.828
Sc = 1.007 (Based Upon 11.8x11.8 equivalent square)
Sp = 1.005 (Based Upon 11.7x11.7 effective square)
PDD = 87.4% x (1052
)/(101.52
) x (111.52
)/(1152
) = 87.9%
Other factors = 0.970 (a blocking tray transmission factor)
• MU Setting = 180 .
1 x 0.828 x 1.007 x 1.005 x (87.9/100) x 0.970
= 252 MU
110 cm
10x20 Field Size on the patient
Depth = 5 cm
9.1x18.2 Field Size on the machine

9. Case 7: Rx 180 cGy to d = 5cm, what is the dose to dmax?
In an SDD setup, the doses inside the patient are related by the
ratio of PDDs.
Dose Dmax = 180 cGy x PDD(dmax) / PDD(d=5)
= 180 x 100 / 87.1 = 206.7 cGy
100 cm
Depth = 5 cm
10x10 Field Size
dmax

10. Mayneord Factor