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Sad calculations

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  • 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