1.1. BACKGROUND
The incidence of cancer throughout the world is increasing with the prolonged life expectancy that has resulted from improvements in standards of living. About half of all cancer patients receive radiotherapy, either as part of their primary treatment or in connection with recurrences or palliation. The IAEA has estimated that approximately 2500 teletherapy machines were in use in 1998 in developing countries and that 10 000 such machines may be needed by 2015. The preparation of this Safety Report was initiated as a result of an expected increase in the construction of radiotherapy facilities, and in response to Member States that have requested practical guidance regarding the design and shielding of such facilities.
1.2. OBJECTIVE
The objective of this report is to elaborate on the requirements for the design and shielding of radiotherapy facilities prescribed in Appendix IV of the International Basic Safety Standards for Protection against Ionizing Radiation and for the Safety of Radiation, Safety Series No. 115 (the BSS) [1].
1.3. SCOPE
This report is intended to be used primarily by health physicists, medical physicists and other radiation protection professionals in the planning and design of new radiotherapy facilities and in the remodelling of existing facilities. It draws together information from several reports [2, 3] with regard to the requirements of the BSS [1]. It provides guidance on the design of radio-therapy facilities and describes how the required structural shielding should be determined. Methods for determining the necessary structural shielding for external beam units (60Co units, linear accelerators, superficial and ortho-voltage units and simulators) are given as well as shielding for brachytherapy units. Data used for determining the structural shielding necessary for all types of radiotherapy facilities are reproduced in this report, and example calcula-tions are provided for each type of facility. The design of facilities so that security objectives for radioactive sources can be met is also addressed in this publication.
Since corrections or additions after facilities are completed can be expensive, it is important that security concerns be addressed and structural shielding be properly designed and installed during the original construction process. It is also advisable that the planning includes consideration of possible future needs for new equipment, higher radiation energies, and increased workloads.1 Special consideration needs to be given to the fact that workload can increase significantly with many new techniques which require more beam-on time per treatment or which enable many more patients to be treated per unit time.
The BSS [1] define a dose constraint as “…[a] prospective and source related restriction on the individual dose delivered by the source which serves as a bound in the optimization of protection and safety of the source.” When planning for the construction of a radiotherapy facility, the dose constraints for occupational and public exposures will be the doses in, respectively, controlled and supervised areas (see Section 2.2) for which the facility is designed. Two principles of radiation protection and safety on which the BSS [1] are based and that must be considered when choosing appropriate dose constraints are optimization of protection and dose limitation.
In ICRP Publication 60 [4], an acceptable level of optimization of protection is described in this way:
“If the next step of reducing the detriment2 can be achieved only with a deployment of resources that is seriously out of line with the consequent reduction, it is not in society’sinterest to take that step, provided that individuals have been adequately protected.”
Therefore, when considering anticipated dose rates in controlled and supervised areas in radiotherapy facilities, additional shielding can be added to the facility if the costs of the shielding are not significantly more than the resultant dose reduction. Further information on the complex task of cost–
benefit analysis is given in ICRP Publication 37 [5].
The BSS [1] provide a simplified rendering of the principle of limitation:
“individual doses due to the combination of exposures from all relevant
1 The degree of use of an X ray or gamma ray source. For X ray equipment operating below 4 MV, the weekly workload is usually expressed in milliampere minutes. For gamma ray beam therapy sources, and for X ray equipment operating at 4 MV or above, the workload is usually stated in terms of the weekly exposure of the useful beam and is expressed in gray (Gy).
2 Detriment can be taken to be effective dose as it is considered here.
practices should not exceed specified dose limits.” Therefore, dose constraints need to be selected so that, in addition to meeting the requirement for optimi-zation, limits on individual doses from occupational and public exposures are not exceeded. It should be noted that the possibility that public and occupa-tional exposures may result from more than one source must be taken into account.
There is no quantitative international standard with regard to dose constraints for radiotherapy facilities. However, two examples of dose constraints that are presented in this Safety Report are those used in the United Kingdom [6, 7] and the USA [8–10]. These dose constraints are based on the principles of optimization and limitation of doses and have been supported by many years of operational experience. Therefore, the use of the methods and data presented in this report should result in a cost effective room design consistent with radiation protection principles and requirements.
Shielding should be designed by a qualified expert, as defined in the Glossary of the BSS [1], to ensure that the required degree of radiation protection is achieved. The expert should be consulted during the early planning stages. Often, the shielding requirements affect the choice of location of radiation facilities and type of building construction. It is strongly recommended that a qualified expert approves the final shielding drawings and specifications before beginning construction. Other aspects of X and gamma ray facilities, such as interlocks, warning signs and lights, and room lighting, are mentioned in this Safety Report.
While specific calculational methods are used in the examples in this publication, alternative methods may prove equally satisfactory in providing radiation protection. The final assessment of the adequacy of the design and construction of structural shielding should be based on the radiation survey of the completed installation. If the radiation survey shows deficiencies, additional shielding or modifications of equipment and procedures will be required.
1.4. STRUCTURE
After a brief review of terminology in Section 2, the design features of radiotherapy facilities are described in Section 3, while Section 4 describes the materials used for shielding such facilities. Section 5 reviews the methods for calculating radiation barriers at radiotherapy facilities. Sections 6–9 provide examples of external beam facilities, simulators and orthovoltage units, brachy-therapy facilities, and special radiobrachy-therapy procedures. Finally, Section 10 .provides guidance on how to conduct a radiation survey of facilities.