ICRU 50

HISTORY

The ICRU (originally known as the International X-Ray Unit Committee and later as the International Committee for Radiological Units) was conceived at the First International Congress of Radiology (ICR) in London in 1925 and officially came into being at ICR-2 in Stockholm in 1928. The primary objective was to propose a unit for measurement of radiation as applied in medicine. From 1950 the ICRU expanded its role significantly to embrace a wider field. Initially meetings were held every 3 years at ICR congresses (excluding the 13-year period encompassing World War II) with one physicist and one radiologist from each participating country having the right of attendance with the Chairman being nominated by the ICR host country. A permanent Commission was elected in 1953. L S Taylor (USA) served ICRU as a member [1928 – 1934] and then Secretary [1934 – 1953], first permanent Chairman [1953 – 1969] and then Honorary Chairman [1969 until his death in 2004]. Subsequent ICRU Chairmen have been: H O Wyckoff 

(USA) [1969 – 1987]; A Allisy (France) [1987 – 1994]; A Wambersie (Belgium) [1994 – 2006]; and P M DeLuca, Jr (USA) [2006 – 2009]. H-G Menzel (Switzerland) is the current Chairman

ICRU 50 REPORT

· ICRU 50 (1993) - "Prescribing, Recording, and Reporting Photon Beam Therapy"

o   Supercedes and updates Report 29

o   Aim of Therapy: Should be defined as radical, palliative, or non-malignant

     o   Volumes are defined:

§ Gross Tumour Volume (GTV):
Gross palpable or visible/demonstrable extent and location of malignant growth

§ Clinical Target Volume (CTV):
Anatomical concept. Tissue volume that contains a GTV and/or subclinical microscopic malignant 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 CTV is an anatomical-clinical concept, that has to be defined before a choice of treatment modality and technique is made

§ Planning Target Volume (PTV):Geometrical concept. 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 absorbed in the CTV. Its size and shape depend on the CTV but also on the treatment technique used, to compensate for the effects of organ and patient movement, and inaccuracies in beam and patient setup

§ Treated Volume:
Volume enclosed by an isodose surface (e.g. 95% isodose), selected and specified by radiation oncologist as being appropriate to achieve the purpose of treatment. Ideally, Treated Volume would be identical to PTV, but may also be considerably larger than PTV

§ Irradiated Volume:
Tissue volume which receives a dose that is considered significant in relation to normal tissue tolerance. Dose should be expressed either in absolute values or relative to the specified dose to the PTV

§ Organs at Risk:
Normal tissues whose radiation sensitivity may significantly influence treatment planning and/or prescribed dose

o   Recommendations for Reporting Dose

§  ICRU Reference Point:

· Dose at the point should be clinically relevant and representative of the dose throughout PTV

· The point should be easy to define in a clear and unambigous way

· The point should be selected where the dose can be accurately determined (physical accuracy)

· The point should be selected in a region where there is no steep dose gradient

·  ICRU Reference point should be located at the center of the PTV and when possible at the intersection of the beam axes

· The dose at the ICRU Reference Point is the ICRU Reference Dose

§ Dose at/near center of PTV, maximum dose to PTV, and minimum dose to PTV should always be reported

§ Maximum Dose:
Highest dose in PTV. A volume is considered clinically meaningful if its minimum diameter exceeds 15 mm; however, if it occurs in a small organ (e.g. the eye, optic nerve, larynx), a dimension smaller than 15 mm has to be considered

§ Minimum Dose:
Lowest dose in PTV. In contrast to Maximum Dose, no volume limit is recommended.

§ Hot Spots:
Volume outside the PTV which receives dose larger than 100% of the specified PTV dose. In general considered significant only if minimum diameter exceeds 15 mm; however, if it occurs in a small organ (e.g. the eye, optic nerve, larynx), a dimension smaller than 15 mm has to be considered

§  Levels of Dose Evaluation for Reporting:

· Basic:
Only dose at ICRU Reference Point and its variation along a central beam axis is available

· Advanced:
Dose distribution can be computed for plane(s)

· Developmental:
Dose distribution can be computed for volume(s)