Journal of the ICRU Vol 4 No 1 (2004) Oxford University Press

DOI: 10.1093/jicru/ndh004

PREFACE the variations in size, shape, and position of the CTV, but also for all uncertainties, and possible errors, in positioning. The Organs at Risk (OARs) were identified and, as in the case of the PTV, a safety margin was applied to compensate for the movements of the OARs and the uncertainties in positioning. This led to the concept of Planning Organ at Risk Volume (PRV). For reporting the treatments, Reports 50 and 62 recommended the selection of an ICRU Reference Point which should fulfill the following requirements:


The dose at that point should be clinically relevant and representative of the dose distribution throughout the PTV. The point should be easy to define in a clear and unambiguous way. The point should be selected where the dose can be accurately determined. The point should be in a region where there is no large dose gradient.

A point located at the center (or central part) of the PTV generally fulfills these requirements and is recommended as the ICRU Reference Point. In addition, the maximum and the minimum dose to the PTV should also be reported. Reporting only the minimum dose to the PTV (i.e., the isodose surface encompassing the PTV) represents only part of the ICRU recommendations. Furthermore, the dose gradient at the border of the PTV is usually large and difficult to determine accurately. Reporting only the minimum dose to the PTV, instead of the dose at the center, introduces a systematic difference of 10, 15, and sometimes 20 percent in the dose to the patient, with erroneous interpretations and damaging consequences when comparing the treatment outcomes. The present Report was originally planned to be part of ICRU Report 62 on Prescribing, Recording, and Reporting Photon Beam Therapy, since the general recommendations for prescribing, recording, and reporting electron beam therapy should be consistent with, and similar to, the recommendations for photon beam therapy. However, the dose

ª International Commission on Radiation Units and Measurements 2004

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For several decades, the ICRU has been involved in an effort to improve harmonization in reporting radiation treatments. The use of the same language and definitions of terms and concepts is indeed needed for an accurate, effective, and safe exchange of information, and is a prerequisite for progress in the development of radiation therapy. The first Report in this series, ICRU Report 29, Dose Specification for Reporting External Beam Therapy with Photons and Electrons, was published in 1978. In Report 29, the ICRU was the first to clearly define and distinguish the Tumor Volume and the Target Volume. For reporting treatments, Report 29 recommended specification of the dose at the point of intersection of the beam axes (when possible) but always in the center (or central part) of the Target Volume, in an area where the dose is in general homogeneous and representative of the dose distribution in the Target Volume. Fifteen years later, in Report 50 (1993) and its Supplement Report 62 (1999), Prescribing, Recording and Reporting Photon Beam Therapy, the ICRU again approached the problem of harmonization in describing and reporting. In the meantime, radiation therapy techniques had significantly changed and improved. At that time, two-dimensional treatment planning was the standard and threedimensional dose computation and planning was being developed and used in leading centers. Simultaneously, the dramatic improvement of imaging techniques focused interest on the different volume concepts. Several volumes were defined (or their definition refined) in Reports 50 and 62. For the Gross Tumor Volume (GTV), the need to specify the method used for its delineation was stressed. Around the GTV, two types of safety margin were identified. A first safety margin was added around the GTV to take into account subclinical malignant involvement, leading to the concept of Clinical Target Volume (CTV). A second type of safety margin was introduced to take into account all types of geometrical uncertainties in patient----beam positioning, leading to the concept of Planning Target Volume (PTV). This second type of safety margin compensates for



treatment planning such as inverse dose planning. Dose computation must be fast, safe, and reliable. As a consequence of this evolution, the clinical indications of the different radiation therapy modalities have to be reconsidered continuously, as well as the methods for reporting the treatments. In particular, the recommendations contained in ICRU Reports 50 and 62 need to be adapted. With the novel and complex irradiation techniques, the dose distribution in the central part of the PTV does not always exhibit a homogeneous plateau where an ICRU Reference Point could be selected according to the requirements mentioned above. In addition, owing to the dramatic progress in imaging, the volume concepts tend to overwhelm the traditional approach of dose specification at defined point(s). Presently, there are trends to specify the isodose encompassing a certain percentage of the PTV (e.g., 90 percent) instead of the whole PTV and to consider the volume that receives a certain percentage (e.g., 90 percent) of the prescribed dose. These approaches and their rationales need to be discussed and evaluated. To deal with these new issues, the ICRU has established a Reporting Committee on ‘‘Prescribing, recording, and reporting conformal photon beam therapy’’. Two other reporting committees, newly established, have similar goals for proton beam therapy and intracavitary brachytherapy, respectively. These three committees have in common that they have to deal with techniques achieving dose distributions which are sometimes inhomogeneous throughout the PTV. This inhomogeneity may result from the technique or may even be intentional and based on very accurate diagnostic or functional imaging. The ultimate goal is to develop recommendations and agreement on methods of reporting that would be, on the one hand, specific and accurate but, on the other hand, broad enough to be applied to the different techniques. For safe exchange of information and correct interpretation of the clinical results, it is indeed essential to keep, whenever possible, a common language and a common definition of terms and concepts between teams involved with different radiation therapy techniques.

The electron energy is usually selected in such a way that the maximum of the depth-dose curve is located at (or close to) the center of the PTV. The maximum of the depth-dose curve is, in general, situated on a rather homogeneous plateau, with most energies and beam delivery systems. This approach agrees with the general approach recommended for photons.

In contrast, at the level of the 90, 85, or 80 percent isodose, the dose gradient is so great as to make selection of the ICRU Reference Point at that level impossible. However, the minimum dose to the PTV (i.e., the isodose encompassing the PTV) should be reported in addition to the dose at the Reference Point at the maximum (peak dose). At present, the situation of radiation oncology is changing dramatically. Novel and complex irradiation techniques are now rapidly developing and being implemented, e.g., intensity-modulated radiation therapy (IMRT) and other techniques such as gamma knife, tomotherapy, Cyberknife1, and proton beam therapy. Their aim is to achieve more tightly any prescribed dose distribution, in particular to match as closely as possible the Treated Volume to the PTV even for complex shapes. This also applies to electron beam therapy. One section of the present Report is devoted to developments in electron beam therapy. These novel and complex irradiation techniques require, in turn, novel and complex approaches in

ACKNOWLEDGMENTS The ICRU expresses thanks and appreciation to Mr. David Carpenter, Division of Radiation Therapy, A. James Cancer Hospital and R. Solove Research Institute at the Ohio State University Medial Center, for reviewing and editing the manuscript and illustrations. Our appreciation also to 12

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distributions of electron beams are so different from those of photon beams that they imply different selection of clinical indications, and require different beam arrangements and often a combination with photon beams. Therefore, the ICRU believed that it was worthwhile to devote a special report to Electron Beam Therapy. This Report extends the concepts and recommendations of ICRU Reports 50 and 62 from photons to electrons. Today most of modern linear accelerators offer the possibility to apply electron beam therapy. The finite range of the electron beams in tissues provides a significant advantage in some clinical situations and for some series of patients. It is, however, recognized that electron beam therapy may raise complex technical and dosimetric problems and is often difficult to use efficiently and safely. The recommendations for electrons should, in principle, follow the same logic used for photons whenever possible. Selecting the ICRU Reference Point for prescribing and reporting on the beam axis at the maximum of the depth-dose curve (‘‘peak dose’’) appears to be a reasonable choice:

PREFACE and Waste Safety), the Ohio State University Comprehensive Cancer Center (Grant P30 CA 16058). The contents of this Report are the sole responsibility of the ICRU and do not necessarily represent the official views of the above Commissions or Agencies.

Mr. Ron McLean and Ms. Marga Garcia-Hunter, Graphic Design Services, Ohio State University for their work on the graphics. The ICRU also expresses gratitude and appreciation to Mr. Macy Block, who most generously supported work on this Report. This Report was made possible, in part, by the support of the National Cancer Institute (NCI, Grant Number R24 CA74296-05), the European Commission (EC, Grant Number FIRI-CT-200020037), the International Atomic Energy Agency (IAEA, Division of Human Health and Radiation

Andre´ Wambersie Paul DeLuca Gordon Whitmore June 1, 2004

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