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Ssd calculations

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Ssd calculations

1. 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. 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. 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. 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. 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. 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. 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. 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. 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. 10. Mayneord Factor