Hsc 340 10 28

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Hsc 340 10 28

  1. 1. HSC 340 10-28 • Treatment Concepts and Calculations
  2. 2. Concepts to know • Treatment planning team, site specific • MU’s • One more review of prescription
  3. 3. Concepts in calcs • Treatment time vs. MU’s • Dose – cGy, Gy, rads • Depth – Point vs. volume • Seperation
  4. 4. Concepts in calcs cont. • SSD vs. SAD • Field size • Scatter • Dmax (24-1) • Output • Dose rate • Output factor – Sc & Sp
  5. 5. Inverse Square Law • Change in beam intensity due to divergence of beam • Dose rate of beam is inversly proportional to the square of the distance – I1/I2=(d2)2 /(d1)2 – I can be replaced with dose rate
  6. 6. Equivalent squares • Treatment calcs need this info • Most fields are rectangular • Sterlings formula – 4 (Area/Perimeter) – Does not account for loss of scatter back to CAX • Effective Field Size – Use in MLCS/Blocks • 24-2
  7. 7. Calculating changes in field size with distance Field size A = Field size B Distance A Distance B
  8. 8. Tissue Absorption Factors • PDD • TAR • SAR • BSF and PSF • TMR and TPR
  9. 9. Percentage Depth Dose • PDD=(Absorbed dose at depth/Absorbed dose at Dmax) *100% • Dependence on – Energy – Depth – Field Size – SSD
  10. 10. Tissue-Air Ratio • TAR= Dose in tissue/Dose in air • Dependent on – Energy – Field size – Depth
  11. 11. Scatter-Air Ratio • Ratio of the scattered dose at a given point in a medium to the dose in air at the same point • Dependent on – energy – field size – depth
  12. 12. BSF and PSF • Ratio of dose rate in phantom/water to the dose rate at the same point without phantom/water at Dmax • Dependent on – energy – field size
  13. 13. TPR and TMR • TPR= dose in tissue/dose in phantom (at any depth) But if the dose is at Dmax then it is referred to as: • TMR=dose in tissue/dose in phantom (at Dmax)
  14. 14. Dose rate modifiers • Tray Factor • Wedge Factor • Compensator Factor
  15. 15. SSD calcs • SSD- six step process – Determine equivalent square – Determine EFS, Sp, PDD – Determine appropriate table – Look up factors – Use appropriate equation
  16. 16. Equations for SSD calcs • Co60 Minute setting= Prescribeddose RDR*ISCF*Sc*Sp*(PDD/100)*otherfactors • Linac MU w/PDD= Prescribeddose RDR*ISCF*Sc*Sp*(PDD/100)*other factors
  17. 17. SAD calcs • TAR, TMR and TPR work best • PDD can be used with some difficulty
  18. 18. Equations for SAD calcs • TAR calculations Prescribed dose RDR*ISCF*Sc*TAR*Other Factors • TMR calculations Prescribed dose RDR*ISCF*Sc*Sp*TMR*Other Factors • TPR calculations Prescribed dose RDR*ISCF*Sc*Sp*TPR*Other Factors
  19. 19. Gap Calcs • Why the need? • Common methods – Dosimetric isodose matching – Junction shift – Half-beam blocking – Geometric matching
  20. 20. Equation for gap calc • Gap= {(L1/2)*(d/SSD1)}*{(L2/2)*(d/SSD2)} • L1=length of the first field • SSD1= treatment SSD of first field • D= depth of abutment • L2=length of second field • SSD2= treatment SSD of second field

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