icru 50_62_83

International Commission on Radiation Units and

Measurements

(ICRU) Report 50, 62 and 83

SAMIR LAOUI, Ph.D.

Principal objective of ICRU

Is to develop concepts, definitions and

recommendations for the use of quantities

and their units for ionizing radiation and its

interaction with matter, in particular with

respect to the biological effects induced by radiation

History of ICRU 50, 62 and 83

ICRU Report No: 29 (1978)

“Dose specification for reporting external beam therapy in photons and

electrons

ICRU Report – 50 (1993)

Supersedes and updates Report 29

Prescribing, Recording, and Reporting photon beam therapy


Supplement to ICRU Report No: 50


Prescribing, Recording, and Reporting Photon-beam IMRT

ICRU 50

When delivering a radiotherapy tretament,

parameters such as volume and dose have

to be specified for different purposes:

Prescription

Recording

Reporting

DESCRIBED VOLUMES

Gross target volume

Clinical target volume

Planning target volume

Organs at risk

Treated volume

Irradiated volume

Defined prior

to T/t planning

During T/t

planning

Recommendations for

reporting volumes

GROSS TUMOR VOLUME ( GTV )

Gross demonstrable extent and location of the

malignant growth

It consists of : Primary tumor(GTV primary)

Metastatic lymphadenopathy(GTV nodal)

Other metastasis(GTV M)

If the tumor has been removed prior to radiotherapy

then no GTV can be defined

The clinical Target Volume

CTV

The clinical Target Volume (CTV) is a tissue volume that

contains a demonstrable GTV and/or subclinical

microscopic malignant disease, which has to be

eliminated

2 types of Subclinical extension:-

Around the GTV-CTV I

At a distance (Regional lymph nodes)-CTV II

Planning Target Volume (PTV)

The PTV is a geometrical concept, and it is defined to

select appropriate beam sizes and beam arrangements,

taking into consideration the net effect of all the possible

geometrical variations and inaccuracies in order to ensure

that the prescribed dose is actually delivered to the CTV

Affected by:

Size and shape of the GTV & CTV

Effects of internal motions of organs and the tumor

Treatment technique (beam orientation and patient fixation, daily

setup errors)

Intrafractional errors (During a single session)

Interfractional errors (From one session to another)

TREATED VOLUME

It is the volume enclosed by an isodose surface that is

selected and specified by the radiation oncologist as

being appropriate to achieve the purpose of treatment

(palliation or cure).

Usually taken as the volume enclosed by the 95%

isodose curve.

Ideally dose should be delivered only to the PTV but due

to limitations in the radiation treatment technique.

The shape and size of the Treated Volume relative to the

PTV is an important optimization parameter.

2 field(AP-PA) 3 field

4field(box) Arc

IRRADIATED VOLUME(IRV)

It is the volume that receives a dose considered

significant in relation to normal tissue tolerance

Usually taken as the volume enclosed by the 50%

isodose curve.

It depends on the treatment technique used

ICRU 50: Organs at risk (OR)

OR - Normal tissues whose radiation sensitivity may

significantly influence treatment planning and/or

prescribed dose

Class I – radiation lesions are fatal or result in severe

morbidity

Class II – radiation lesions result in moderate to mild

morbidity

Class III – radiation lesions are mild, transient, and

reversible, or result in no significant morbidity

ICRU 50: Dmax

One can identify the maximum dose within the PTV, and

the maximum dose at tissue outside the PTV

In most cases, high dose to a volume with smallest

diameter <15mm is not clinically meaningful in terms of

normal tissue tolerance

However, maximum dose assessment is important for

organs at risk with small dimension (<15mm) such as

optic nerve

ICRU 50: Dmin, Davg, Dmedian,

and Dmodal Minimum Dose: In contrast to the Dmax, no minimum

volume is recommended when reporting Dmin

Average Dose: is the average of the dose values in the

lattice points

Davg =

The median dose is the central value of the doses at all

lattice points

Modal Dose: is the dose that occurs most frequently at

the lattice points

ICRU 62: Supplement to ICRU

50

ICRU 62 scope

Gives more detailed recommendations on the different

margins that must be considered to account for

anatomical & geometrical variations & uncertainties.

PTV has been separated into two components: an

internal margin and set-up margin.

Classified organs at risk depending on response to

radiation.

Defined planning organ at risk volume (PRV)

Report dose to the OAR/PRV

Introduced conformity index

Gives recommendations on graphics

Volumes

A “Normal” Treatment plan

Effect of Motion

Internal Margin (IM)

Compensates for expected physiologic movements and

variations in size, shape , and position of the CTV in relation to

an Internal Reference Point

Commonly asymmetric

May result from respiration, different filling of rectum/bladder,

swallowing, heart beat

These variations cannot be easily controlled

Internal Target Volume (ITV) = CTV + IM

ICRU 62: IM and ITV

PTV = ITV + SM

SET-UP MARGIN ( SM )

Accounts for uncertainties, inaccuracies, and lack of

reproducibility in patient positioning and alignment of

the therapeutic beams during treatment planning and all

treatment sessions

Referenced to the external coordinate system

May result from patient positioning variation, mechanical

uncertainties of the equipment, dosimetric uncertainties,

transfer set-up errors, and human factors

Size of this margin might be reduced with record and

verify systems, patient immobilization devices, and

increased skill

ICRU 62: Components of the PTV

ICRU 62: Treated Volume (TV)

Due to limitations of RT techniques, the volume receiving the prescribed dose does not generally match the PTV

TV - The tissue volume that (according to the approved treatment plan) is planned to receive at least a dose selected and specified by the radiation oncology team as being appropriate to achieve the purpose of treatment

AKA: The volume that receives at least 100% of the dose

ICRU 62: Conformity Index

(CI) Can be employed when the PTV is fully

enclosed by the TV, then it is the quotient

of the TV and the volume of the PTV

CI = TV/PTV

This is concept is used during the

optimization procedure

CLASSIFICATION OF

ORGANS AT RISK Classified as :

Serial – whole organ is a continuous unit and damage at one

point will cause complete damage of the organ (spinal cord,

digestive system). So even point dose is significant

Parallel – organ consists of several functional units and if one part

is damaged, the rest of the organ makes up for the loss (lung,

bladder). Dose delivered to a given volume or average/mean dose

is considered

Serial-parallel – kidney (glomerulus- parallel, tubules- serial),

heart (myocardium- parallel, coronary arteries- serial).

PLANNING ORGAN AT RISK

VOLUME(PRV) PRV to OAR is analogous to the PTV for the CTV.

Aim is to account for movements of the OAR due to

movements, changes in size and shape and setup

uncertainties.

PTV and PRV may overlap, then it is the responsibility of

the radiation oncologist to decide depending on the

importance of the treatment versus risk of critical organ

damage.

GRAPHICS

These are used to delineate the different volumes and the

other landmarks

These are in different colors for an easy and uniform

interpretation

The convention recommended and used in ICRU 62 are: GTV - Dark Red

CTV – Light Red

ITV – Dark Blue

PTV – Light Blue

OR – Dark Green

PRV – Light Green

Landmarks - Black

ICRU 50

Prescribing, Recording, and Reporting

Photon Beam Therapy

ICRU 62

Supplement to ICRU 50

ICRU 83

Prescribing, Recording, and Reporting

Photon-beam IMRT

ICRU Reports 50 and 62 deal with

conventional external photon-beam

irradiation, including 3D-CRT.

ICRU 83 deals with more complex IMRT

IMRT METHODS

Objective functions in IMRT

Based on soft tissue constraints using least-square

minimization

Tends to enforce absorbed-dose homogeneity within PTV

and reduce it in normal tissue

Where IPTV is the relative importance of the PTV.

TPTV is the number of voxels

ICRU 83: Volumes

GTV

CTV

Dose Prescribing and

reporting for IMRT Absorbed-dose and volume information obtained from

DVHs

Dose Homogeneity and

Conformity

Conformity index

CI= TV/PTV

Sub structures for optimization

Priority rules when overlapping

ICRU 83 Update on Organs at

Risk Remaining Volume at Risk (RVR) = Imaged volume within

the patient excluding OAR and the CTVs

RVR aids in optimization

RVR Can be used to estimate late effect

Treatment planning

Volumes +Objectives

Optimization

Dose calculations

Correction-based

Model based

Correction-based dose

computation Based on measured dose distributions in a

water phantom. Corrections for:

Beam modifiers Surface contours

Tissue heterogeneities

Model based calculation

Compute patient dose considering the actual

physics interactions in the radiation transport

process.

Consider patient contour/homogeneity from

start, as opposed to correcting water dose

distribution

Model-based dose computations use monitor

unit calculations based on beam intensity (e.g.,

energy fluence) rather than dose in a phantom.

The beam has to be modeled

Summary of Changes Between ICRU

50 & 62 and IMRT ICRU (83)

More emphasis on statistics.

No longer use ICRU-Reference Point.

Want median dose D50 reported.

Use model-based dose calculations

Include the effect of tissue heterogeneities

icru 50_62_83

Health & Medicine