Samir Laoui, Ph.D. 2/08/2016

Basic Principles: Equivalent Square

Field o Calculation data is tabulated according to square field size

o Sterling’s Formula

S = 4xAREA/PERIMETER

o The equivalent square is used to look up dosimetric parameters related to the primary collimator settings

L

W

S

S

Basic Principles: Effective Square

Field

o Field defining blocks (or multi-leaf collimators) further modify the field

o The effective square concept allows one to determine a square field size that is “effectively” equal to the blocked field as relates to dosimetry

12% Blocking

Basic Principles: SAD Setup – SSD Setup

oThe PDD is the primary parameter used to

calculate dose for SSD setups oThe TPR is the primary parameter used to

calculate dose for SAD setups

Basic Principles: Sc

o Collimator scatter factor

o Sc: In-air output ratio, The ratio

of the output (i.e., energy fluence)

in air for a given field size to that

for the reference field size

o 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

TG-71: Introduction

Introduction

o The photon beam normalization depth, d0, is

different from reference depth

oProtocol recommends using normalization and reference

depth to be equal to 10 cm

oThis decreases the difference in programmed MUs

when moving patients from one machine to another

(assuming machines’ characteristics are the same?)

o For electron beams, normalization depth is taken to

be the depth of maximum dose along the central

axis

Monitor Unit equations: Photons

oMay be performed using either TPR (isocentric) or

PDD (nonisocentric)

oTPR (isocentric)

oPDD (nonisocentric)

Field size determination

o Equations (1), (2) and (3) are field size dependent

o Field equivalent 4.A/P estimation

o for Sc: 3 methods to determine equivalent square

field size Equivalent square of jaw settings

1. Equivalent square of jaw settings

2. Point’s eye view (PEV) model of collimating jaws

3. PEV model of all collimators

o For Sp, the field size is proportional to that

incident on the patient

Field size determination

oWedge factor

oPhysical Wedges: Investigations have determined that

the WF for rectangular fields is closely approximated by

the WF of the equivalent square for both external and

internal wedges, regardless of orientation

oNonphysical wedges: WF represent the fractional

change in dose per MU at the calculation depth after the

treatment field is completed

Radiological depth determination

o To correct for heterogeneities within the patient, a

correction factor

oMethods to determine CF

1. Method 1: uses water-equivalent or radiological depth,

deff

2. Method 2: The power law TAR

Electrons

o Se: Electron output factor is defined in TG-70

Electron calculation at extended SSDs o The effect of treatment distance not equal to the

standard SSD can be accounted for in two ways, as

described in the AAPM Task Group 70 report

At dmax

Field size determination

o However, the above techniques do not predict accurately the

output factors or percentage depth-dose values for irregularly

shaped electron fields lateral buildup ratios LBR

Arc therapy

Example: Brain Case

78.7 MU

TG 71 Summary

oNormalization depth of 10 cm be chosen for MU

calculations for photons

o For electrons, normalization depth is the depth of

maximum dose along the central axis for the same

field in water phantom

oMU calculations for patients prior to first

treatment, if not, then before third fraction, 10% of

the dose has been delivered, whichever occurs first

o Patient Safety is the ultimate goal for doing MU

verification

o In the modern era, the purpose and methodology

for the MU verification have come into question

oGuidelines are needed to help the physicist set

clinically reasonable action levels for agreement

Scope

o Recommendations on how to perform verification

of MU in modern clinic

oActions levels

oDoes not pertain to IMRT

Objective of MUV

o IAEA: “Results provided by the TPS need to be checked,

and this should include verification by manual calculation

of the treatment time and dose to the selected point. This

verification should be part of the QA programme.”

o ICRP 2009: “A simple secondary MU calculation,

independent from the TPS, has proven for many years to be

an efficient tool for prevention of major errors in dose

delivery.”

Errors o Errors can be classified into random and

systematic errors

o Random: Incorrect energy, wrong dose

o Systematic: defect in part of the calculation

procedure

Limitations of MUV

o The MUV is not a check of the accuracy of the

entire calculated dose distribution, but rather to a

single point

o In addition to the MUV, the physicist’s plan review

should confirm that the dose, beam energy,

fractionation, and dose point location are

consistent with the physician’s prescription

Aspect of a MUV program

o Each center performing after-hours emergency

treatments should have a policy detailing how

planning, including the MUV, is to proceed when a

physicist is not immediately available for plan and

calculation review

Recommended Methods of MUV

o It is recommended that MUV point need not

coincide with the plan normalization point

o Point placement in regions containing a high dose

gradient should be avoided; as well, a calculation

point should not be placed near a field edge, >2 cm

o The factors that are likely to introduce the greatest

uncertainty in the verification calculation are

blocked field scatter, patient contour, and patient

heterogeneity effects

Recommended Methods of MUV

oManual calculational methods using look-up tables

provide the basis for most MUV

oAll radiation therapy clinics should maintain an

accurate set of dosimetry tables

i. Reference dose rate

ii. Output ratios

iii. Attenuation factors

iv. OAR

v. Depth dependence

Action levels and remedial actions

Remediation of MU discrepancies

o Review parameters in both calculations

oAll factors are taken into account

oDosimetric point placement

UCI EXAMPLES

Shoulder Case

The end

“The price of Safety in Radiotherapy is

an Eternal Vigilance”