Ssd calculations
10
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
Transcript of Ssd calculations
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
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…
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
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
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 cm10x20 Field Size
Depth = 5 cm
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 cm10x20 Field Size
Depth = 5 cm
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
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
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
Mayneord Factor
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