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