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- 1. Dosimetric Calculations
- 2. Radiation Therapy Department Physician Physicists Dosimetrists Therapists Nurses
- 3. Physicist • Calibrations • Radiation Safety • Machine QA • Clinical QA • Treatment Planning • Research • Technology
- 4. Dosimetric Calculations Calibration Conditions Patient Conditions Water Phantom Patient
- 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. 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. Outline • Basic Principles • Non-Isocentric (or SSD) Calculations • SSD example • Isocentric (or SAD) Calculations • SAD example
- 8. Basic Principles: Dose and Prescription • Radiation Dose (cGy) • The Radiation Therapy Rx • 4500cGy @ 180cGy x 25
- 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. Basic Principles: Machine Calibration Cobalt Unit (cGy/min) / Linear Accelerator (cGy/MU) Source Point of Dmax Reference Field Size 100 SSD Surface
- 11. Dosimetric Calculations Calibration Conditions Patient Conditions Water Phantom Patient
- 12. Basic Principles: Inverse Square Law 1/r2
- 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. 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. 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. 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. 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. 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. 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. Basic Principles Review: Radiation Prescription • Prescribed total dose (cGy) • Prescribed dose per fraction (cGy) • Energy • Prescription point • Field Weighting (for multiple fields)
- 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. Basic Principles Review: Patient Factors • Parameters that quantify how the radiation acts within the patient • Sp = phantom scatter factor • PDD • TMR
- 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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