ICRU Volumes for Prescribing, Recording, and ReportingPhoton-Beam Intensity-ModulatedRadiation Therapy (IMRT)

ALTHAF JOUHAR KM.Sc. RADIATION PHYSICSCALICUT UNIVERSITYICRU 50,62,83

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ICRU Guidelines Internationally accepted recommendations on Radiation-related quantities and units, measurementprocedures, and reference data for the safe andefficient application of ionizing radiation to medical diagnosis and therapy, radiation science andtechnology, and radiation protection of individuals and populations

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INTERNATIONAL COMMISSION ON RADIATION UNITS AND MEASUREMENTS ( ICRU )ICRU 50 (1993): 1993 to 1999

ICRU 62 (1999): 1999 to till date

ICRU 83 (2010): 2010 on wards

For more than 20 years, Duarte has developed presentations3

Volumes defined prior to treatment planning :

- Gross Tumor Volume (GTV)- Clinical Target Volume (CTV)

Volumes defined during the treatment planning :

- Planning target Volume (PTV)- Organs at Risk (OAR)- Treated Volume (TV)- Irradiated Volume (IrV)ICRU 50

GROSS TUMOR VOLUME ( GTV) Gross palpable or visible/demonstrable extent and location of the malignant growth.

It consists of :- Primary tumor- Metastatic lymphadenopathy- Other metastasis

If the tumor has been removed prior to radiotherapy then no GTV can be defined.

GTV can be determined by using either clinical examination (inspection, palpation) and by various imaging techniques (X-rays, CT, MRI etc).

Method used for determination of GTV should meet the requirements for staging the tumor according to the clinical TNM.

It is a tissue volume that contains a GTV and/or subclinical microscopic disease, which has to be eliminated.

This volume has to be treated adequately in order to achieve the aim of therapy : cure or palliation.

The delineation of this volume requires consideration of factors like local invasive capacity of the tumor and its potential to spread to different regions ( eg: regional lymph nodes).

CLINICAL TARGET VOLUME ( CTV )

The delineation of GTV and CTV are based on purely anatomic-topographic and biological considerations without regard to technical factors of treatment.

GTV

CTV

PLANNING TARGET VOLUME ( PTV )

The planning target volume is a geometrical concept, and it is defined to select appropriate beam arrangements, taking into consideration the net effect of all possible geometrical variations, in order to ensure that the prescribed dose is actually absorbed in the CTV .

It is used for dose planning and for specification of dose.

The PTV is linked to the reference frame of the treatment machine. It is often described as the CTV plus a fixed or variable margin (e.g., PTV = CTV + 1 cm).

Usually a single PTV is used to encompass one or several CTVs to be targeted by a group of fields.

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Planning Target Volume (PTV)

It is a volume enclosed by isodose surface, selected and specified by the radiation oncologist as being appropriate to achieve the purpose of treatment .

It may closely match to the PTV or may be larger than the PTV.

If, however, it is smaller than the PTV, or not wholly enclosing the PTV, then the probability of tumor control is reduced and the treatment plan has to be re-evaluated or the aim of the therapy has to be reconsidered. TREATED VOLUME

It is that tissue volume which receives a dose that is considered significant in relation to normal tissue tolerance.

It depends on the treatment technique used.IRRADIATED VOLUME ( IrV )

ORGANS AT RISK (OAR)These are normal tissues whose radiation sensitivity may significantly influence the treatment planning and/or prescribed dose.

They may be divided into 3 classes :

Class I : Radiation lesions are fatal or result in severe morbidity.

Class II : Radiation lesions result in mild to moderate morbidity.

Class III : Radiation lesions are mild, transient, and reversible, or result in no significant morbidity.

ICRU 50

Irradiated VolumeTreated VolumePlanning Target Volume (PTV)Clinical Target Volume (CTV)Gross Tumor Volume (GTV)

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ICRU REPORT 62 ( Supplement to ICRU REPORT 50 )

Gives more detailed recommendations on the different margins that must be considered to account for anatomical and geometrical variations and uncertainties.

Introduces the concept of Conformity Index ( CI )

Gives information about how to classify Organs at Risk.

Introduces a Planning Organ at Risk Volume ( PRV )

Gives recommendations on graphics.

Consists of the CTV plus an internal margin.

It is the margin given around the CTV to compensate for all variations in the site, size and shapes of organs and tissues contained in or adjacent to CTV.

The internal margin is designed to take into account the variations in the size and position of the CTV relative to the patients reference frame (usually defined by the bony anatomy), i.e., variations due to organ motions such as breathing, bladder or rectal contents, etc.

Internal Target Volume (ITV)

Internal target volume (ITV) = CTV+IM

PLANNING TARGET VOLUME ( PTV ) by ICRU 62

PTV=CTV+IM+SMIntroduce the concept of setup margin.

Setup margin is the Uncertainty in patient positioning and mechanical uncertainty of the equipment which arise due to sagging of gantry or collimator or couch, Dosimetric uncertainty, transfer setup error , human error, etc.

Planning Organ at Risk Volume (PRV) This is a volume which gives into consideration the movement of the Organs at Risk during the treatment.

An integrated margin must be added to the Organ at Risk to compensate for the variations and uncertainties, using the same principle as PTV and is known as the Planning Organ at Risk volume ( PRV ).

A PTV and PRV may occasionally overlap.

ICRU REPORT 83 ( Supplement to ICRU REPORT 62 )

Gives more detailed recommendations on the different margins that must be considered to account for anatomical and geometrical variations and uncertainties

Introduces a Remaining Volume at Risk ( RVR )

Ideally when delineating the OAR, especially forIMRT, all normal tissues that could potentially beirradiated should be outlined. The imaged volumewithin the patient, excluding any delineated OARand the CTV(s), should be identified as the RVRRemaining Volume at Risk (RVR)

It is defined as the quotient of the Treated Volume (TV) and the Planning Target Volume (PTV).

It can be employed when the PTV is fully enclosed by the Treated Volume.

It can be used as a part of the optimization procedure.

Dose conformity characterizes the degree to which the high-dose region conforms to the target volume, usually the PTV.CONFORMITY INDEX ( CI )

Dose HomogeneityDose homogeneity characterizes the uniformity of the absorbed-dose distribution within the target volume.

Homogeneity index is defined as, Dose-volume reporting - D50% (Dmedian), Dose received by 50% of PTV- D98% : Dose received by 98% volume of PTV - D2% : Dose received by 2% volume of PTVHI = D2%-D98% D50%

Delineation of these volumes is an obligatory step in the planning process, as absorbed dose cannot be prescribed, recorded, and reported without specification of target volumes and volumes of normal tissue at risk.

The ICRU previously recommended that the absorbed dose in the PTV be confined within 95 % to 107 % of the prescribed absorbed dose(ICRU, 1999). With IMRT, these constraints can be unnecessarily confining if the avoidance of normal tissue is more important than target-dose homogeneity. In the present Report, it is recommended that the extent of high- and low-dose regions is specified using dosevolume quantities such as D2 % and D98 % for regions of high and low absorbed dose, respectively.Dose Volume Specifications

DVH summarizes the entire dose distribution into a single curve for each anatomic structure of interest.

The main use of DVH is as a plan evaluation tool.

IMRT is an approach to conformal therapy that not only conforms (high) dose to the target volume but also conforms (low) dose to sensitive structures.

Volume out side PTV that receives dose larger than 100% of PTV dose is called Hot Spot.

Volume inside the PTV that receives dose lower than 95% that of PTV dose is called Cold Spot.

Additional information which is ultimately contributes to the developments and improvements in Radiotherapy.

Proper identification and delineation of GTV is the most important factor in treatment.

Other volumes like CTV, PTV, ITV should also be properly delineated.

The errors like set-up error and human errors should be kept to a minimum.

Dose prescription, fractionation and calculation should be done in the same way by all the different centers throughout the world for the proper exchange of information and reporting.CONCLUSIONS

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