Comments ICRP MDCT Intended Audience and Report Organization It is not clear for whom this document is intended. Lines 387-389 state that “the primary audience for this document is imaging professionals- radiologists, radiological technologists, medical physicists and researchers involved in patient dose management”. However, in lines 424-427, it says that “the intended audience of this document is medical professionals and not necessarily medical physicists”. The discrepancy must be eliminated. It is not a linguistic issue, but a question of emphasis. If the real audience is medical professionals, most of the technical material presented is inappropriate; as many physicians neither will understand it nor do they care. Even if the audience includes technical people, many explanations dealing with MDCT technology should be summarized. It is helpful to understand how MDCT work and the different dose reduction approaches used by the various manufacturers, but the document, as it is written, is more a technical document on CT than a radiation safety guide. There are so many details on how the CT scanners work, that what the user can do to reduce dose gets lost. In order to make the radiation safety recommendations clear to the CT user, technical issues should be condensed and repetitions eliminated –at times it appears that different sections have been written by different authors who say the same thing albeit slightly differently. This is particularly true in Section 4.3. Many of the examples given are preceded by technical explanations which have been given before. There should be cross-references, not repetition of concepts. Some rearrangement of sections would also help in understanding some of the technical issues. For example, Section 3.1 deals with a very important topic, tradeoffs between dose and image quality; perhaps it would be better placed in Chapter 2. Sections 3.2.1 Overbeaming and 3.2.2 Overranging are misplaced in Chapter 3, they really belong to either Chapters 1 or 2, as they describe CT scanner characteristics. The whole document needs a good cross-reference method for the various sections to avoid duplications. Accuracy In addition of condensing technical aspects, the document needs a thorough screening regarding oversimplifications that may lead to inaccuracies. For example, all throughout the document, image noise is equated to quantum mottle. Quantum mottle is only one component of CT scanner image noise, albeit the one that impacts on dose. The distinction should be made clear in line 717. Dosimetry Lines 425-427 state that: “it was deemed most appropriate to use the term dose in this document in a more generic manner similar to as in ICRP publication number 87. A detailed description of the dose quantities used in CT is provided in Appendix A.” While using the generic use of the term dose, seems appropriate, when it comes to CT dose descriptors, the dosimetric quantities proposed by ICRU 74 (ICRU 2005) must be followed. The document should be revised using the new dosimetry terms, and Appendix A needs to be rewritten to incorporate the new terms. The change will not impact on any of the conclusions, but it will align the document with international dosimetry standards. The intended audience in this context is irrelevant. For a physician it is immaterial whether the dose descriptor is called CT air kerma-length product or CT dose-length product. Doses resulting from various CT scanner techniques are compared calculating the effective dose quantity. The use of effective dose is not recommended by ICRU (ICRU 2005) for medical exposures arising from medical imaging, since the wT and wR values are not applicable to patient populations. Effective dose in medical exposures is also discouraged by the current draft of the new ICRP Recommendations. If the Commission insists on using this quantity in the MDCT publication, it should be emphasized somewhere in the document that this is only “valid for prospective radiological protection purposes” and that “it should not be used for retrospective dose assessments” or for “the detailed estimation of specific individual human exposures and risks” (ICRP Committee 2. Basis for dosimetric quantities used in radiological protection). Justification Principle Chapter 4 starts with the justification of the practice, which is appropriate in a radiation safety report. However, Section 4.1 needs considerable revisions. Lines 1358-1359 state that “only a qualified” medical practitioner generates requests for CT examinations”. Who is to decide whether a medical practitioner is qualified or not? Are regulatory authorities going to demand that practitioners who wish to request CT scans proof their competency? The issue is serious as it interferes with the role of medical licensing boards. The word “qualified” should be removed. Lines 1379-1389 suggest that, before undergoing a CT examination, patients should sign an informed consent for radiation risks. To support this recommendation, the example given is one in radiation therapy (lines 1398-1401). The comparison is not valid; doses in radiation therapy are at least one order of magnitude higher than in CT. Requiring patients to sign an informed consent form for radiation risks would deter them from seeking and/or undergoing needed radiodiagnostic tests. Lines 1379-1401 should either be deleted or rephrased. Lines 1402-1407 can be incorporated somewhere in Chapter 2. Editing The document also needs linguistic editing and the replacement of bombastic terms such as the ones underlined here: “magnificent computing power” (line 183}, “phenomenal increase in use of MDCT” (line 210), “Since the revelation in 2001 that exposure factors in CT of children are sometimes kept the same as for adults” (lines 620-621); “educate physicians and medical physicists to realms of radiation dose associated with MDCT (line 1724-1725) by more scientific ones. There are two aspects of the Report that are very good, one is the Summary Points; the second one is Chapter 4. One recommendation regarding the Summary Points is to eliminate the one in line 129, expanded in 402-404, as it does not contribute to any radiation safety issue. As far as Chapter 4, since all the data come from 4 manufacturers, it would be helpful if the manufacturer to which each particular example applies be identified. Other Specific Comments Lines 629-630 state that “The ICRP and the International Basic Safety Standards (BSS) “require generic and individual justification”. That is not true. The ICRP does, the BSS does not. Lines 658-659 state that “manufacturers did not consider radiation dose to patient an important issue until the media highlighted the issue”. The sentences should be eliminated as it untrue, inflammatory and unnecessary. Line 1254: What are “reduced settings”? References In press publications (lines 2027-2028) should not be cited. Missing references should be completed (line 1478). Some additional references should be incorporated (in line 1374, the reference for the European Commission is: EUROPEAN COMMISSION. Referral guidelines for imaging, Radiation Protection 118, Luxembourg, Directorate-General for the Environment, Office for Official Publications of the European Communities ). Others, like the one in line 1041 need to be verified (Does ICRP-60 discuss noise vs improved inherent contrast?).