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