When delivering a radiotherapy tretament,parameters such as volume and dose have to bespecified for different purposes: prescription,recording, and reporting. It is important that clear,well defined and unambigous concepts andparameters are used for reporting purposes toensure a common language between differentcenters.
INTERNATIONAL COMISSION ONRADIATION UNITS ANDMEASUREMENTS ( ICRU )ICRU 50 (1993): 1993 to 1999ICRU 62 (1999): 1999 to till date
AIM OF THERAPY1. Radical treatment of Malignant disease2. Palliative treatment of Malignant disease3. Non-malignant diseases
ICRU 50Volumes 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 - Treated Volume - Irradiated Volume
GROSS TUMOR VOLUME ( GTV )• Gross palpable or visible/demonstrable extent andlocation of the malignant growth.• It consists of : - Primary tumor - Metastatic lymphadenopathy - Other metastasis• Corresponds to those parts of the malignant growthwhere the tumor density is largest.• If the tumor has been removed prior to radiotherapythen no GTV can be defined.
• GTV can be determined by using either clinicalexamination (inspection, palpation) and byvarious imaging techniques (X-rays, CT, MRI etc.)• Method used for determination of GTV shouldmeet the requirements for staging the tumoraccording to the clinical TNM (UICC and AJCCS).
Reasons for identification of GTV :1. An adequate dose should be delivered to the whole of GTV to obtain local tumor control in radical treatments.2. To allow for recording of tumor response in relation to dose and its variation, and to other relevant factors.
CLINICAL TARGET VOLUME ( CTV ) • 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).
The delineation of GTV and CTV arebased on purely anatomic-topographicand biological considerations withoutregard to technical factors oftreatment.
PLANNING TARGET VOLUME ( PTV )• It is a geometrical concept, and is defined to selectappropriate beam sizes and arrangements, taking intoconsideration the net effect of all possible geometricalvariations, in order to ensure that the prescribed dose isactually absorbed in the CTV.• It is used for dose planning and for specification of dose.• It has to be clearly indicated on sections used for doseplanning and the dose distribution to the PTV has to beconsidered to be representative of the dose to the CTV.
TREATED VOLUME• It is a volume enclosed by an isodose surface, selectedand specified by the radiation oncologist as beingappropriate to achieve the purpose of treatment ( tumoreradication or palliation ).• It may closely match to the PTV or may be larger thanthe PTV.• If, however, it is smaller than the PTV, or not whollyenclosing the PTV, then the probability of tumor controlis reduced and the treatment plan has to be reevaluatedor the aim of the therapy has to be reconsidered.
Reasons for identification of Treated Volume are :1. The shape and size of the Treated Volume relative to the PTV is an important optimization parameter.2. Also, a recurrence within a Treated Volume but outside the PTV may be considered to be a “true”, “in- field” recurrence due to inadequate dose and not a “marginal” recurrence due to inadequate volume.
IRRADIATED VOLUME ( IrV )• It is that tissue volume which receives a dose that isconsidered significant in relation to normal tissuetolerance.• It depends on the treatment technique used.
ORGANS AT RISK ( OR ) • These are normal tissues whose radiation sensitivity may significantly influence the treatment planning and/or prescribed dose. • They may be divided into 3 classes : 1. Class I : Radiation lesions are fatal or result in severe morbidity. 2. Class II : Radiation lesions result in mild to moderate morbidity. 3. Class III : Radiation lesions are mild, transient, and reversible, or result in no significant morbidity.
ICRU REPORT 62 ( Supplement to ICRUREPORT 50 )• Gives more detailed recommendations on the differentmargins that must be considered to account for anatomicaland geometrical variations and uncertainties.• Introduces a Conformity Index ( CI )• Gives information about how to classify Organs at Risk.• Introduces a Planning Organ at Risk Volume ( PRV )• Gives recommendations on graphics.
ICRU 62Volumes defined prior to treatment planning : - Gross Tumor Volume (GTV) Same as ICRU 50 - Clinical Target Volume (CTV)Volumes defined during the treatment planning : - Planning target Volume (PTV) - Treated Volume - Irradiated Volume - Planning Organ at Risk Volume (PRV)
INTERNAL MARGIN ( IM ) AND INTERNALTARGET VOLUME ( ITV )• It is the margin given around the CTV to compensate forall variations in the site, size and shapes of organs andtissues contained in or adjacent to CTV.• These may result from respiration, different fillings of thebladder and rectum, swallowing, heart beat, movements ofbowel etc.• These are physiological variations which are very difficultto control and result in changes in the site, size and shapeof CTV.
SET-UP MARGIN ( SM )• There can be many uncertainties ( inaccuracies and lack ofreproducibility ) in patient positioning and alignment of thetherapeutic beams during treatment planning and throughall treatment sessions.• These uncertainties depend on factors like : • variations in pt. positioning • mechanical uncertainties of the equipment (sagging of gantry, collimators, and couch) • dosimetric uncertainties • transfer set-up errors from CT & simulator to the treatment unit • human factors
SET-UP MARGIN ( SM ) is the margin that mustbe added to account specifically for uncertainties(inacuracies and lack of reproducibility) inpatient positioning and aligment of thetherapeutic beams during treatment planningand through all treatment sessions.
ITV = CTV + IMPTV = CTV + combined IM & SM
CONFORMITY INDEX ( CI )• It is defined as the quotient of the Treated Volume andthe volume of PTV.• It can be employed when the PTV is fully enclosed bythe Treated Volume.• It can be used as a part of the optimization procedure.
ORGANS AT RISK• According to the functional models based on the FSU(Functional Sub Unit) concept [ Withers et al., Kallman et al.,and Olsen et al. ] for the purpose of evaluation of thevolume-fractionation-response, the tissues of an Organ atRisk are considered to be functionally either “ serial”“parallel” or “serial-parallel” structures.• eg : Spinal cord has a “high relative seriality” meaning that adose above tolerance limit to even small volume of this ORmay be deleterious. On the other hand, Lung has a “lowrelative seriality” meaning that the most importantparameter is the relative size of volume that is irradiatedabove tolerance level.
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.
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
ADSORBED DOSE DISTRIBUTION• The dose given to the tumor should be as homogenous aspossible.• In cases of heterogeneity of doses, the outcome of thetreatment cannot be related to the dose. Also, thecomparison between different patient series becomesdifficult.• However, even if a perfectly homogenous dosedistribution is desirable, some heterogeneity is accepteddue to technical reasons.
The heterogeneity should be foreseen whileprescribing a treatment, and, in the besttechnical and clinical conditions should bekept within +7% and -5% of prescribed dose(Wittkamper et al., Brahme et al., Mijnheer et al.).
MAXIMUM DOSE ( Dmax )• It is the maximum dose to the PTV and the Organ atRisk.• The maximum dose to normal tissue is important forlimiting and for evaluating the side-effects of treatment.• Dose is reported as maximum only when a volume oftissue of diameter more than 15mm is involved (smallervolumes are considered for smaller organs like eye, opticnerve, larynx).• When the maximum dose outside PTV exceeds theprescribed dose, then a “Hot Spot” can be identified.
MINIMUM DOSE ( Dmin ) • It is the smallest dose in a defined volume. • In contrast to maximum adsorbed dose, no volume limit is recommended when reporting minimum dose.
HOT SPOTS• It represents a volume outside the PTV which receives adose larger than 100% of the specified dose.• A Hot Spot is considered significant only if theminimum diameter exceeds 15mm (in smaller organs likeeye, optical nerve, larynx etc. a diameter smaller than15mm is also considered significant).
ICRU REFERENCE POINT• It has to be selected according to the following generalcriteria : - the dose at the point should be clinically relevant. - the point should be easy to define in a clear and unambiguous way. - the point should be selected so that the dose should be accurately determined. - the point should be in a region where there is no steep dose gradient.
The recommendations will be fulfilled if theICRU reference point is located :• always at the centre ( or in the central part ) ofPTV, and• when possible, at the intersection of the beamaxes.
ICRU REFERENCE DOSEIt is the dose at the ICRU Reference Point andshould always be reported.
REPORTING • It should be done in order to make exchange of information between different centers. • It is important that the treatments performed in various centers be reported in the same way, using the same concepts and definitions.
• According to the recommendations of ICRU, as abasic requirement, the following doses shouldalways be reported : • the dose at ICRU reference point • the maximum dose to the PTV • the minimum dose to the PTV
Additional information which is considered asrelevant should also be added. This could berelated to :• a more accurate and detailed description ofdose distribution e.g., average dose and itsstandard deviation, dose - volume histograms(DVH) etc.• an accurate description of the dose at differentanatomical sites (including Organs at Risk).
Reporting these additionalinformation ultimately contributesto the developments andimprovements in Radiotherapy.
CONCLUSIONS • 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.