Correspondence Address:
Pratik Kumar
Additional Professor, Medical Physics Unit Institute Rotary Cancer Hospital, All India Institute of Medical Sciences New Delhi
India

Full Text

By D.G. Sutton, C.J. Martin, J.R. Williams and D.J. Peat

(2 nd Edition, Report of a BIR working party Oct 2010 - April 2012, published in 2012 by The British Institute of Radiology)

Medical imaging has always played a pivotal role in the diagnosis, evaluation of treatment, and management of various diseases. In recent years, interventional fluoroscopy has assumed the role of treatment modality for many ailments. All these have brought the focus on the radiological safety due to use of X-ray radiation in these diagnostic and therapeutic processes. One of the intrinsic requirements for safety has been a properly designed and adequately shielded radiological imaging installation which makes the staff and public safe. Keeping the need for an all-inclusive compendium on the shielding requirements for diagnostic radiology into consideration, the British Institute of Radiology (BIR) took initiative to form a working party on the subject which published its first report in 2000 that soon gained the status of a treatise. Since then, there have been much path-breaking technological advancements in imaging devices like digital imaging [computed radiography (CR) and digital radiography (DR)]. These inventions, on one hand, decreased the patient's radiation dose but increased the patient throughput (and hence workload). Helical CT scanner and multi-slice CT scanner are the other technology developments which, coupled with faster computing ability, made the workload increase tremendously. Interventional fluoroscopy has also incorporated flat panel which may decrease patient dose but may increase the energy due to increased filtration. Positron emission tomography (PET)-CT has presented a new challenge for shielding owing to the use of isotope-emitting gamma rays with very high energy in MeV range. All these changes and advancements in the scenario of medical imaging necessitated the present expanded and thoroughly revised report of a new BIR working party in 2012 and have been explained thoroughly in preface. The book also deals with the issue of shielding in mammography, dental radiology, bone densitometers, and mobile imaging equipments like mobile CT, mobile mammography, and mobile catheterization laboratory.

This sleek comprehensive book has altogether 11 chapters in addition to the bibliography (references). Chapter 1 is an elaborate introduction to the book which outlines the scope of the book, deliberates on the ideas behind the methodologies for design criteria and the treatment of primary, scattered, and leakage radiation. It also explains the basic concept of dose, occupancy factor, workload, etc., which lays down a firm foundation for further development of the edifice of ideas. This chapter remarkably cautions about the use of personal dose equivalent H p (d) and ambient dose equivalent H*(d) in place of Air Kerma in shielding calculation, for both H p (d) and H*(d) overestimate the Effective Dose which is the ultimate cornerstone for all calculations. It again argues with references for selecting 30% of public dose limit, i.e., 0.3 mSv as dose constraint. Similarly it has suggested some practical and new occupancy factors which will lead to realistic shielding design.

Chapter 2 entitled "Methods for Primary and Secondary Radiation" presents various ways to assess the quantity of primary and secondary radiation supposed to reach at the point to be shielded. It deals with traditional and digital detectors. The chapter would have been more complete had DR-related attenuation been also provided. It gives a few practical advices like ignoring leakage radiation from X-ray tube in certain conditions. Another review of this book refers that for most of the installations scatter from patients is likely to be quite dominant with respect to leakage for the energies employed (L.T. Dauer. Book Review. Radiation Shielding for Diagnostic Radiology, Rad Protec Dosim 152, 472-474, 2012). The chapter states that leakage radiation is frequently specified at 150 kVp while in reality radiography X-ray tube is operated below 100 kVp most of the time. This results in very conservative design (over-shielding) up to 8300 times (Simpkin DJ and Dixon RL, 1998, Secondary shielding barriers for diagnostic X-ray facilities: scatter and leakage revisited. Health Phys, 74, 350-65). The chapter also suggests easy workable way to include tertiary scatter (sky shine) from flooring and walls in case of multi-slice CT and interventional suits.

Chapter 3 has the details regarding various building materials and shielding materials, their use and properties. It has very lucid information regarding the commercially available lead sheet. It enumerates designs for fabricated lead plywood panels, lead plaster-board, barium-boards, in addition to the usual lead-lined doors. The chapter ends with a list of various building-and construction-related terms and their meanings in the form of glossary.

Chapter 4 deals with the transmission characteristics of various building materials and presents the method to calculate primary and secondary radiation transmission through these materials. In presenting these data, it takes care of broad-beam geometry which is practically encountered rather than experimental narrow-beam geometry.

Chapter 5 gives the explanation of practical methods of assessment of the integrity of shielding using radioactive source or X-ray unit. It describes the transmission curves using radioactive sources 241 Am and 99m Tc. The chapter, however, cautions that 99m Tc may be used for determining thickness of lead shielding but not that of barium plaster. It indicates that 18 F should be used for PET scanner.

Chapters 6 and 7 provide explanation of radiation shielding in radiographic room and fluoroscopy room, respectively. For radiography, Chapter 6 explains the method of calculation based upon both KAP workload and ESD workload. This may be quite useful as many radiography machines may not be equipped with KAP meter. Proper and realistic (and not overestimated) shielding design presents a complex task in case of interventional suits and Chapter 7 deals with it by presenting examples complete with typical layout. An interesting suggestion has been referred in this chapter which is regarding the room size of 38 m 2 for specialized radiology room (NHS Estates 2001, Health building note 6: Facilities for diagnostic imaging and interventional radiology, Norwich: The stationery office). This chapter contains a loose sheet of errata. It should be fixed in the book itself so that its probable loss is averted.

Chapter 8 deals with the CT installations and advocates the method based upon DLP rather than the method based upon dose plots of scatter (from scanning of a cylindrical phantom) provided by the manufacturer for the latter significantly differs when anatomical phantom is scanned. This chapter summarizes the typical workload of 38 CT scanners in terms of DLP as well, which may be used by the reader. For example, the mean DLP per exam has been quoted by other reviewer of the book as 850 mGy.cm for body and 870 mGy.cm for the head scan (L.T. Dauer. Book Review. Radiation Shielding for Diagnostic Radiology, Rad Protec Dosim 152, 472-474, 2012). However, the book cautions the reader about the use of this DLP workload to newer technologies (likely to be available in future) since patient throughput may increase in newer CT scanners further. The chapter also includes tertiary scatter (Sky shine) from roof in CT scanner. The chapter has many worked out practical examples for proper understanding.

Chapter 9 is related with the shielding facilities of PET/CT which has caught the attention of the physicians (in Nuclear Medicine or Oncology) due to its potential application in diagnosis and treatment evaluation of cancer and metastasis. PET uses 18 F ( 18 FDG to be precise) which emits gamma rays of 511 keV. At this energy, Compton scattering dominates. The chapter underlines the fact that at this energy, the first TVL of lead is 16 mm in contrast with the first TVL for 99m Tc being 1 mm. The chapter emphasizes that whole path of administration, uptake, rest, and scanning should be taken into consideration. The chapter has dealt with the calculation based upon the administration of 400 MBq 16 FDG to a patient, which may cause 45 μGy/h at 1 m from the patient. Over the next hour, it may fall to 37 μGy/h at 1 m. Just before scanning, the patient is asked to use the toilet (taking 15% excretion), and therefore, during scan period, the dose rate may be 24 μGy/h at 1 m from the patient. The chapter contains various scenarios (single scanner and single/double uptake rooms) and presents practical solved examples of shielding puzzle.

Chapter 10, is a miscellaneous chapter with deliberation of shielding requirements on specialized application. In mammography section, it does not mention any specific room size but cites scatter factor 7.6 μGy per image at 1 m from image receptor (Simpkin DJ 1996, Scatter radiation intensities about mammography units, Health Phys, 70, 238-45), but with a caveat that it may be a worst case scenario, i.e., overestimation. It suggests that window with normal glass may be inadequate and may need 0.25 mm lead equivalent. For doors in mammography room, it points out that strategic placement of door may avoid the need of lead lining but cautions that no lead lining may be a bit risky if not placed properly. For dental radiology, it emphasizes on the proper evaluation of shielding due to high volume of work and probable multiple units in smaller places. The chapter has devoted some portion on cone-beam dental CT (a relatively new technology), dual-energy X-ray absorptiometry (DEXA for bone density measurement), and mobile CT, Cath Lab, and mammography as well. The Chapter 11 which is the last chapter contains all references. In a nutshell, the book under review has effectively covered all areas of medical imaging in a very comprehensive manner. It starts with the basics and provides unambiguous recommendations. The strength of the book lies in providing practical problems and solutions with properly drawn layouts. The book gives many useful data for the future utilization by the shielding experts. This compendium would certainly serve as a reference guide for shielding work in diagnostic radiology.