Dosimetric calculations

Dosimetric Calculations

Radiation Therapy Department

Physician

Physicists

Dosimetrists

Therapists

Nurses

Physicist

• Calibrations

• Radiation Safety

• Machine QA

• Clinical QA

• Treatment Planning

• Research

• Technology


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


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

Education

Dosimetric calculations