Dosimetric calculations

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Dosimetric calculations

  1. 1. Dosimetric Calculations
  2. 2. Radiation Therapy Department Physician Physicists Dosimetrists Therapists Nurses
  3. 3. Physicist • Calibrations • Radiation Safety • Machine QA • Clinical QA • Treatment Planning • Research • Technology
  4. 4. Dosimetric Calculations Calibration Conditions Patient Conditions Water Phantom Patient
  5. 5. Why do you need to know this? • Basic fundamental knowledge in your field • May be called upon to perform calculations • Need to know how the parameters effect dose calculations • You need to be able to detect errors • Career opportunities
  6. 6. Why do you need to know this? • Recent news reports highlight errors in radiation oncology • Over reliance on technology • Therapists job has become increasingly segmented • An overall grasp of the basics is essential
  7. 7. Outline • Basic Principles • Non-Isocentric (or SSD) Calculations • SSD example • Isocentric (or SAD) Calculations • SAD example
  8. 8. Basic Principles: Dose and Prescription • Radiation Dose (cGy) • The Radiation Therapy Rx • 4500cGy @ 180cGy x 25
  9. 9. Basic Principles: Linear Accelerator / Cobalt Unit • Radiation Source • Rotating Gantry • Source to Axis Distance (SAD) • Field Defining Collimators • MLCs or Blocks • Treatment accessories (e.g. wedges)
  10. 10. Basic Principles: Machine Calibration Cobalt Unit (cGy/min) / Linear Accelerator (cGy/MU) Source Point of Dmax Reference Field Size 100 SSD Surface
  11. 11. Dosimetric Calculations Calibration Conditions Patient Conditions Water Phantom Patient
  12. 12. Basic Principles: Inverse Square Law 1/r2
  13. 13. Basic Principles: Equivalent Square • Collimators always define a square or rectangular field size • Calculation data is tabulated according to square field size • The equivalent square concept allows one to determine a square field size that is “equivalent” to the rectangular field as relates to dosimetry • Sterling’s Formula – S = 4xAREA/PERIMETER • Tables based upon measurement • The equivalent square is use to look up dosimetric parameters related to the primary collimator settings L W S S
  14. 14. Basic Principles: Effective Square • Most times a rectangular field from the primary collimators is not appropriate • Field defining blocks (or muli-leaf collimators) further modify the field • The effective square concept allows one to determine a square field size that is “effectively” equal to the blocked field as relates to dosimetry • Remember to incorporate a tray factor when using a block • The equivalent square is use to look up dosimetric parameters related to the field size on the patient’s surface 12% Blocking
  15. 15. Non-Isocentric or SSD Setup Isocentric or SAD Setup Basic Principles: Non-Isocentric and Isocentric Calculations Patient surface is at the axis of rotation Calculation point is at the axis of rotation SSD = SAD = 100 cm d = 5 cm d = 5 cm SSD = 95 cm
  16. 16. Basic Principles: SSD Setup - Percent Depth Dose (PDD) • Radiation dose decreases with depth • For high energy x-rays (Megavoltage), dose initially builds up to a maximum and then decreases with depth • The PDD is the primary parameter used to calculate dose for SSD setups •
  17. 17. Source Dmax Field Size Source Depth = d Field Size SAD = 100 cm SAD = 100 cm Tissue Maximum Ratio (TMR) = Ratio of dose at depth d to the dose at dmax for a given field size TMR is the parameter used to calculate dose for SAD setups Basic Principles: SAD Setup – Tissue Maximum Ratio (TMR)
  18. 18. Basic Principles: Sc • Collimator scatter factor • Quantifies the relationship between the field size setting and the dose resulting from scattering from the machine collimators • NOTE: Sc is a function of the field size defined in the treatment head, not the final field size that reaches the patient Scatter off the collimators
  19. 19. Basic Principles: Sp • Phantom scatter factor • Quantifies the relationship between the field size on the patients surface and the dose resulting from scatter within the patient • NOTE: Sp is a function of the field size as defined on the patient, not the field size as defined in the treatment head Field size on the patient surface
  20. 20. Basic Principles Review: Radiation Prescription • Prescribed total dose (cGy) • Prescribed dose per fraction (cGy) • Energy • Prescription point • Field Weighting (for multiple fields)
  21. 21. Basic Principles Review: Machine Parameters • Energy • Calibrated Dose Rate or RDR (usually 1cGy/MU) at dmax • Sc = collimator scatter factor • Sp = phantom scatter factor • Beam modifiers (blocks, wedges)
  22. 22. Basic Principles Review: Patient Factors • Parameters that quantify how the radiation acts within the patient • Sp = phantom scatter factor • PDD • TMR
  23. 23. The MU or Time Calculation The MU or time required to deliver the prescribed dose MU or time = Rx Dose . Dose rate at that point

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