Effective dose from radiopharmaceuticals.

The effective dose, as defined by the International Commission on Radiological Protection (ICRP 1991), provides a possibility of expressing the radiation risk to patients undergoing different radiodiagnostic procedures by means of a single figure. This has been obtained by introducing organ or tissue weighting factors reflecting the radiation sensitivity of the organs. Such weighting factors were first published by the ICRP in publication 26 (1977), and have now been revised in publication 60 (1991). The effective dose for almost all radiopharmaceuticals in clinical use has been recalculated using the new weighting factors from ICRP 60 (1991) and compared with results from former calculations. A slight decrease in the numerical value for the effective dose has been observed, on average 11%. However, this does not correspond to a decrease in the estimated risk from the irradiation, since this has been re-evaluated and found to be higher than earlier believed (NAS 1990; ICRP 1991).     

不同版本国际放射防护委员会建议组织器官权重因子对心脏介入诊疗所致有效剂量的影响

目的 对比采用国际放射防护委员会(ICRP)60和ICRP 103组织器官权重因子计算冠状动脉血管造影术(CAG)及经皮穿刺腔内冠状动脉成形术(PCI)所致有效幅射剂量的变化.方法 采用在ART仿真人体辐照体模(fluke biomedical)躯干部分布放热释光剂量计的方法获得器官剂量,再将器官剂量按照不同版本ICRP组织器官权重因子加权求和获得有效剂量.分析有效剂量变化趋势及原因.同时计算有效剂量与剂量面积乘积(DAP)转换系数.结果 ICRP 103对组织器官权重因子进行调整后带来了有效剂量的增加:CAG(6.88%)和PCI(8.46%).对于CAG、PCI诊疗过程,权重因子的变化带来女性有效剂量的变化为7.25%(8.76%),男性有效剂量的变化为6.51%(8.17%);有效剂量对DAP的转换系数也从0.10(0.13)变为0.11(0.14).结论 ICRP 103对组织器官权重因子的调整导致了CAG和PCI诊疗过程所致患者器官剂量的增加,对于有效剂量增加幅度PCI略高于CAG,女性患者略高于男性患者.有效剂量的增加有两方面原因:器官权重因子变化小而器官当量剂量大和器官当量计晕小但器官权重因子变化大.有效剂量和DAP之间转换系数的变化表明在介入放射工作中用转换系数估算患者有效剂量时要考虑新版本ICRP对组织器官权重因子的调整.     

Effective dose evaluation of multidetector CT examinations: influence of the ICRP recommendation in 2007

We compared effective doses for recent computed tomography (CT) examinations calculated based on International Commission on Radiological Protection publication number 103 (ICRP 103) with those calculated based on ICRP publication number 60 (ICRP 60), and considered the usefulness of the effective dose in CT dose evaluation. After placing radiophotoluminescence glass dosimeters (RPLDs) inside or outside an anthropomorphic phantom, we examined from the chest to the pelvis, cardiac, and cranial regions of the phantom. The absorbed dose was calculated by multiplying calibrated dose values of RPLD by the mass energy coefficient ratio. The effective dose was calculated as the sum total of the value for each tissue, which was multiplied by the equivalent dose according to the tissue weighting factor recommended in ICRP 103 and ICRP 60. Calculated effective doses based on ICRP 103 were different by –11% to +82% compared with those based on ICRP 60. The values of absorbed doses for selective tissues were relatively higher than the values for the effective dose. The effective dose represents only a mean dose value for an average human. Therefore, assessing the absolute dose of particular individuals in CT examinations based exclusively on the effective dose is not recommended.     

Radiation dose and risk of patients through nuclear medical procedures in the GDR

For the years 1978 and 1981 we compared the radiation dose for the patients examined by in vivo methods after administration of radiopharmaceuticals (27 procedures). The somatic effective dose equivalent, the effective collective dose, the somatic radiation risk, and the number of induced malignancies were calculated according to ICRP publication No. 26.All the procedures give rise to a radiation-induced somatic risk from the 4th up the 7th order. In recent years we have seen an increase of the application of ^99mTc compounds and a decrease in the use of ¹³¹I-sodium iodide. A comparison of the results for the two years shows the expected reduction of radiation dose and risk.Dedicated to Ernst W. Doerffel, Honorary President of the GDR-Society of Nuclear Medicine, on occasion of his 75th anniversary     

Risk assessment from bitewing radiography.

The effective dose equivalent and the effective dose from bitewing radiography have been estimated for three different X-ray sets under 10 different exposure conditions using the ICRP 26 (1977) and the ICRP 60 (1990) recommendations. The results of dose measurements in the head and neck with an Alderson Rando phantom and thermoluminescence dosimeters (TLD-100 ribbons) were used (Velders XL et al. Dentomaxillofac Radiol 1991; 20: 161-5). The effective dose equivalent (ICRP 26) was calculated using the salivary glands and brain as remainder organs. The highest effective dose equivalent was 11 microSv for the Philips Oralix 50 unit with a round, pointed cone; the lowest was 2 microSv for the X-ray sets with a rectangular open-ended tube. The highest effective dose using the ICRP 60 weighting factors was 4 microSv for the Oralix 50, the lowest 1 microSv for the X-ray sets with a rectangular open-ended tube. The probability of stochastic effects was calculated as at the most 0.18 x 10(-6) using a nominal probability coefficient of 165 x 10(-4) Sv-1 (ICRP 26); when using the ICRP 60 recommendations (where the nominal probability coefficient for stochastic effects including non-fatal cancer is 730 x 10(-4) Sv-1) the maximum probability was 0.25-0.31 x 10(-6). The maximum probability of fatal cancer induction was calculated as 0.18 x 10(-4) for both fatal probability coefficients, 125 x 10(-4) Sv-1 in ICRP 26 and 500 x 10(-4) Sv-1 in ICRP 60. The calculated probability of the total stochastic effects is nearly twice as high when using the new recommendations, whilst the estimated probability of fatal cancer induction is of the same order of magnitude with both.     

Radiation dose and risk of patients through nuclear medical procedures in the GDR. A comparison of 1978 and 1981

For the years 1978 and 1981 we compared the radiation dose for the patients examined by in vivo methods after administration of radiopharmaceuticals (27 procedures). The somatic effective dose equivalent, the effective collective dose, the somatic radiation risk, and the number of induced malignancies were calculated according to ICRP publication No. 26. All the procedures give rise to a radiation-induced somatic risk from the 4th up the 7th order. In recent years we have seen an increase of the application of 99mTc compounds and a decrease in the use of 131I-sodium iodide. A comparison of the results for the two years shows the expected reduction of radiation dose and risk.     

有效剂量与有效剂量当量的异同

本文作者主要讨论ＩＣＲＰ６０号出版物中提出的有效剂量，与ＩＣＲＰ２６号出版物中的有效剂量当量，在概念和使用上比较它们的异同。     

Bestimmung von Organdosen und effektiven Dosen in der Radioonkologie

BACKGROUND AND PURPOSE: With an increasing chance of success in radiooncology, it is necessary to estimate the risk from radiation scatter to areas outside the target volume. The cancer risk from a radiation treatment can be estimated from the organ doses, allowing a somewhat limited effective dose to be estimated and compared. MATERIAL AND METHODS: The doses of the radiation-sensitive organs outside the target volume can be estimated with the aid of the PC program PERIDOSE developed by van der Giessen. The effective doses are determined according to the concept of ICRP, whereby the target volume and the associated organs related to it are not taken into consideration. RESULTS: Organ doses outside the target volume are generally < 1% of the dose in the target volume. In some cases, however, they can be as high as 3%. The effective doses during radiotherapy are between 60 and 900 mSv, depending upon the specific target volume, the applied treatment technique, and the given dose in the ICRU point. CONCLUSION: For the estimation of the radiation risk, organ doses in radiooncology can be calculated with the aid of the PC program PERIDOSE. While evaluating the radiation risk after ICRP, for the calculation of the effective dose, the advanced age of many patients has to be considered to prevent that, e.g., the high gonad doses do not overestimate the effective dose.     

Estimation of paediatric organ and effective doses from dental cone beam CT using anthropomorphic phantoms

Objectives

Cone beam CT (CBCT) is an emerging X-ray technology applied in dentomaxillofacial imaging. Previous published studies have estimated the effective dose and radiation risks using adult anthropomorphic phantoms for a wide range of CBCT units and imaging protocols.

Methods

Measurements were made five dental CBCT units for a range of imaging protocols, using 10-year-old and adolescent phantoms and thermoluminescent dosimeters. The purpose of the study was to estimate paediatric organ and effective doses from dental CBCT.

Results

The average effective doses to the 10-year-old and adolescent phantoms were 116 μSv and 79 μSv, respectively, which are similar to adult doses. The salivary glands received the highest organ dose and there was a fourfold increase in the thyroid dose of the 10-year-old relative to that of the adolescent because of its smaller size. The remainder tissues and salivary and thyroid glands contributed most significantly to the effective dose for a 10-year-old, whereas for an adolescent the remainder tissues and the salivary glands contributed the most significantly. It was found that the percentage attributable lifetime mortality risks were 0.002% and 0.001% for a 10-year-old and an adolescent patient, respectively, which are considerably higher than the risk to an adult having received the same doses.

Conclusion

It is therefore imperative that dental CBCT examinations on children should be fully justified over conventional X-ray imaging and that dose optimisation by field of view collimation is particularly important in young children.Cone beam CT (CBCT) is an advancement of CT technology that has found wide application in dentomaxillofacial imaging. The ability of the CBCT systems to produce three-dimensional high-resolution images with diagnostic reliability has resulted in a significant increase in CBCT examinations in areas such as orthodontics, endodontics, periodontics, implantology, restorative dentistry, and dental and maxillofacial surgery [1-12]. However, CBCT imaging is associated with a higher radiation dose to the patient than panoramic and intra-oral imaging but a lower patient dose than conventional single and multislice CT [13-16]. Although radiation dose from CBCT is low relative to conventional CT, the radiation risk to the patient should be assessed and quantified. The radiation risk can be estimated by calculating the effective dose, which is a radiation quantity proposed by the International Commission on Radiological Protection (ICRP) [17].Several studies have estimated the effective dose for a range of CBCT units and imaging protocols [13-16,18-24]. The organ doses were measured with anthropomorphic phantoms and thermoluminescent dosimeters (TLDs). The ICRP 103 [25] tissue weighting factors were applied to organ doses to account for the tissue radiosensitivity. The ICRP 60 [17] and the revised ICRP 103 [25] tissue weighting factors have been used for studies before and after 2006, respectively. For the head and neck region, the ICRP 103 [25] factors include the salivary glands, oral mucosa and lymph nodes as radiosensitive organs that were not included in ICRP 60 [17]. In addition, the weighting factor of the remainder tissues was increased from 0.05 to 0.12. The published effective doses range from a few tens to several hundreds of microsieverts depending on the CBCT unit, the field of view and the position of the radiation field with respect to the radiosensitive organs.To the best knowledge of the authors, all the published studies on dental CBCT dosimetry have focused on effective doses to adult patients for a range of CBCT units and imaging protocols but none has estimated the organ and effective doses to paediatric patients. Children are more sensitive to radiation than adults because the number of dividing cells promoting DNA mutagenesis is higher and they have more time to express any radiation-induced effects, such as cancer. There is an order of magnitude increase in cancer risk between children and adults, and there is also a significant difference between boys and girls, with the latter being more radiosensitive [26,27]. Furthermore, a substantial proportion of dental X-ray procedures are performed in the paediatric group, notably in relation to orthodontics.The aim of this study was to measure paediatric organ doses and, hence, derive effective doses using two anthropomorphic phantoms and TLDs for a range of CBCT units and for standard imaging protocols.     

Evaluation of effective dose from a RANDO phantom in videofluorography diagnostic procedures for diagnosing dysphagia

Objectives

Videofluorography (VF) is useful for diagnosing dysphagia; however, few reports have investigated appropriate effective doses for VF. The present study aimed to estimate the effective radiation dose in VF for diagnosis of dysphagia.

Methods

Radiation doses to tissues and organs were measured using the anthropomorphic RANDO woman phantom as an equivalent to the human body. Effective doses were estimated according to the recommendations of the International Commission on Radiological Protection (ICRP) 60 in 1990 and IRCP 103 in 2007. The tissues measured were those recommended by ICRP 60 and ICRP 103 including gonads (ovaries and testes), red bone marrow and tissues in which excessive radiation commonly causes malignant tumours including lung, thyroid gland, stomach, large intestine, liver, oesophagus, bladder, breast, bone marrow, skin, brain and salivary gland. Skin dose was also measured using thermoluminescent dosimeters.

Results

Using ICRP 103, the effective dose was estimated as 118.1 μSv at a tube voltage of 50 kV and 82.4 μSv at 45 kV. However, using ICRP 60 the effective dose for 1 min of VF was estimated at 62.4 μSv and 47.2 μSv under the same exposure conditions.

Conclusions

Using ICRP 103, the effective dose for VF per examination at a total estimation time of 1 min was estimated as approximately 2.5–8.3 times that observed for digital panoramic radiography and 1/12 to 3 times depending on the measurement device for cone beam CT (CBCT). This value can be decreased in the future using a smaller irradiation field and decreased time for examination in VF in the future.     

Heel spur radiotherapy and radiation carcinogenesis risk estimation

Purpose Radiotherapy is a nonsurgical alternative therapy of painful heel spur patients. Nonetheless, cancer induction is the most important somatic effect of ionizing radiation. This study was designed to evaluate the carcinogenesis risk factor in benign painful heel spur patients treated by radiotherapy. Materials and methods Between 1974 and 1999, a total of 20 patients received mean 8.16 Gy total irradiation dose in two fractions. Thermoluminescent dosimeters (TLD₁₀₀) were placed on multiple phantom sites in vivo within the irradiated volume to verify irradiation accuracy and carcinogenesis risk factor calculation. The 20 still-alive patients, who had a minimum 5-year and maximum 29-year follow-up (mean 11.9 years), have been evaluated by carcinogenic radiation risk factor on the basis of tissue weighting factors as defined by the International Commission on Radiological Protection Publication 60. Results Reasonable pain relief has been obtained in all 20 patients. The calculated mean carcinogenesis risk factor is 1.3% for radiation portals in the whole group, and no secondary cancer has been clinically observed. Conclusion Radiotherapy is an effective treatment modality for relieving pain in calcaneal spur patients. The estimated secondary cancer risk factor for irradiation of this benign lesion is not as high as was feared.     

Estimation and comparison of effective dose (E) in standard chest CT by organ dose measurements and dose-length-product methods and assessment of the influence of CT tube potential (energy dependency) on effective dose in a dual-source CT

Purpose

To determine effective dose (E) during standard chest CT using an organ dose-based and a dose-length-product-based (DLP) approach for four different scan protocols including high-pitch and dual-energy in a dual-source CT scanner of the second generation.

Materials and methods

Organ doses were measured with thermo luminescence dosimeters (TLD) in an anthropomorphic male adult phantom. Further, DLP-based dose estimates were performed by using the standard 0.014 mSv/mGycm conversion coefficient k. Examinations were performed on a dual-source CT system (Somatom Definition Flash, Siemens). Four scan protocols were investigated: (1) single-source 120 kV, (2) single-source 100 kV, (3) high-pitch 120 kV, and (4) dual-energy with 100/Sn140 kV with equivalent CTDIvol and no automated tube current modulation. E was then determined following recommendations of ICRP publication 103 and 60 and specific k values were derived.

Results

DLP-based estimates differed by 4.5–16.56% and 5.2–15.8% relatively to ICRP 60 and 103, respectively. The derived k factors calculated from TLD measurements were 0.0148, 0.015, 0.0166, and 0.0148 for protocol 1, 2, 3 and 4, respectively. Effective dose estimations by ICRP 103 and 60 for single-energy and dual-energy protocols show a difference of less than 0.04 mSv.

Conclusion

Estimates of E based on DLP work equally well for single-energy, high-pitch and dual-energy CT examinations. The tube potential definitely affects effective dose in a substantial way. Effective dose estimations by ICRP 103 and 60 for both single-energy and dual-energy examinations differ not more than 0.04 mSv.     

Dose reduction in multislice CT by means of bismuth shields: results of in vivo measurements and computed evaluation

Purpose

We investigated the amount of patient dose reduction in the thyroid, lens of the eye and the breast when using bismuth protections in multislice computed tomography (CT) exams as well as their influence on the quality of diagnostic images.

Materials and methods

The radiation dose was measured by using thermoluminescence dosimeters. The study was conducted on the two CT scanners installed in our radiology department (64 and eight slices). The shield effects on the CT image were evaluated by measuring the signal-to-noise ratio in a phantom and in vivo, and by verifying the presence of artefacts on patients’ images. The obtained organ-dose reduction factors were used to evaluate the effects of shielding on the effective dose.

Results

The shielding attenuation ranged from 30% to 60% depending on the CT scan protocols and organs. The difference between shielded and unshielded signal-to-noise ratio was statistically significant but within the standard requirements for quality assurance. Results were in agreement with the radiologists’ perception of image quality. The use of the shields allowed up to 38% reduction of effective dose.

Conclusions

Use of bismuth shields significantly decreases both organ and effective radiation dose, with a consequent reduction in health risk for the patient, quantified in 1.4 fewer cases of radiation-induced tumours every 5 years in our centre (12,100 exams/year), in agreement with the risk factors proposed by Publication 60 of the International Commission on Radiological Protection (ICRP). The relative inexpensiveness of these protections, their easy application and their substantial lack of influence on image quality suggest their massive introduction into routine clinical practice.     

Issues in the control of low‐level radiation exposure*

The carcinogenic risks of exposure to low level ionizing radiation used by the International Commission on Radiation Protection (ICRP) have been challenged as being, at the same time, both too high and too low. This paper explains that the epidemiological evidence will always be limited at low doses, so that understanding the cellular mechanisms of carcinogenesis is increasingly important to assess the biological risks. An analysis is then given of the reasons why the challenges to ICRP, especially about the linear non‐threshold response model, have arisen. As a result of considering the issues, the Main Commission of ICRP is now consulting on a revised, simpler, approach based on an individual oriented philosophy. This represents a potential shift by the Commission from the past emphasis on societal‐oriented criteria. These proposals have been promulgated through the International Radiation Protection Association (IRPA) and an open literature publication was published in the Journal of Radiological Protection ¹ in June 1999. On the basis of comments received and the observations presented at the IRPA 10 Conference, the Commission will begin to develop the outline of the next Recommendations. It is now more than ten years since ICRP distributed, for comment, a draft of what was to become the publication of the 1990 Recommendations. The Commission plans to develop its new Recommendations on a time scale of the next four or five years. In this paper, many of the issues that will need to be addressed in the development of the recommendations will be identified. These issues will cover biological effects, dosimetric quantities and the establishment of those levels of dose at which different protection requirements will be put into place. Concepts of exclusion and exemption will need to be clarified as well as the meaning of how to achieve what the proposal identifies as ‘As Low as Reasonably Practicable’ (ALARP). Finally, the Commission has decided to develop an environmental radiation protection philosophy that will need to be developed as part of the new Recommendations.     

Dosimetry of two extraoral direct digital imaging devices: NewTom cone beam CT and Orthophos Plus DS panoramic unit

OBJECTIVES: This study provides effective dose measurements for two extraoral direct digital imaging devices, the NewTom 9000 cone beam CT (CBCT) unit and the Orthophos Plus DS panoramic unit. METHODS: Thermoluminescent dosemeters were placed at 20 sites throughout the layers of the head and neck of a tissue-equivalent RANDO phantom. Variations in phantom orientation and beam collimation were used to create three different CBCT examination techniques: a combined maxillary and mandibular scan (Max/Man), a maxillary scan and a mandibular scan. Ten exposures for each technique were used to ensure a reliable measure of radiation from the dosemeters. Average tissue-absorbed dose, weighted equivalent dose and effective dose were calculated for each major anatomical site. Effective doses of individual organs were summed with salivary gland exposures (E(SAL)) and without salivary gland exposures (E(ICRP60)) to calculate two measures of whole-body effective dose. RESULTS: The effective doses for CBCT were: Max/Man scan, E(ICRP60)=36.3 micro Sv, E(SAL)=77.9 micro Sv; maxillary scan, E(ICRP60)=19.9 micro Sv, E(SAL)=41.5 micro Sv; and mandibular scan, E(ICRP60)=34.7 micro Sv, E(SAL)=74.7 micro Sv. Effective doses for the panoramic examination were E(ICRP60)=6.2 micro Sv and E(SAL)=22.0 micro Sv. CONCLUSION: When viewed in the context of potential diagnostic yield, the E(ICRP60) of 36.3 micro Sv for the NewTom compares favourably with published effective doses for conventional CT (314 micro Sv) and film tomography (2-9 micro Sv per image). CBCT examinations resulted in doses that were 3-7 (E(ICRP60)) and 2-4 (E(SAL)) times the panoramic doses observed in this study.     

Symposium of ICRP Committee I

Since the publication of ICRP new recommendation,Chinese scientists in the field of radiation protection,radiobiology. Health physics and radiation dosimetry have paid great attention tO it Series of symposiums and workshops were held to introduce and discuss the new recommendation Of course,many questions were raised which focused on the quantitative estimation of radiation risk and the biological bases of the new dose limits,es. Peciatly the dose response model,risk projection model in life span time,transfer of risk from one population to another,the extrapolation from high dose and high dose rate to low dose and low dose rate,the adoption of DDREF,and some new terms of quantity and unit were discussed more extensively and deeply.     

Radiation Protection Guidelines for the Skin

Summary

The past recommendations of the ICRP about dose limits to the skin are reviewed. Recently, an ICRP Task Group has been revisiting the old arguments and setting them against new data. With the exception of the function of cells in the skin associated with immunocompetence, non-stochastic effects have been well characterized and threshold doses are known with a precision appropriate for setting radiation protection standards. The current dose limitations of 0·5 Sv per year and a working lifetime dose limit of 20 Sv should protect the worker population against deterministic effects. When the ICRP made its recommendations in 1977 for dose limits there was no appreciation of the importance of the interaction of ultraviolet radiation (UVR) and X-rays. Both clinical and experimental data show that the risk of ionizing-radiation-induced cancer is significantly increased by subsequent exposures to UVR. Therefore, risks for sun-exposed areas of skin differ from those that are shielded. The risk estimate for skin cancer is very dependent on the selection of the projection model and on the mortality rate assumed. Based on the relative risk model a mortality rate of 0·2 per cent and summing risks for both UVR exposed and shielded skin the risk is about twice (1·94 × 10^?4 Sv^?1) that which ICRP derived in 1977. With the absolute model the risk is considerably less, about 0·5 × 10^?4 Sv^?1. There is still insufficient understanding of the effects of multiple or protracted exposures on the risk of skin cancer induction. Experimental results suggest that exposures, at least to relatively high total doses, that are protracted over a long period are more carcinogenic than a small number of expsoures over a short period.     

Nuclear epidemiologic studies and the estimation of DREF

Purpose: Estimating cancer risks for continuous radiation exposures based upon data from acute exposures has been an important public health problem. A dose and dose rate effectiveness factor (DDREF) is typically used to estimate cancer risks for chronic exposures based upon risk estimates from acute exposures. A value of 2 for a DDREF has most often been used as proposed by the ICRP in ICRP60; however, an influential analysis of several cohorts concluded that there is no risk difference between acute and chronic exposures. It is the purpose of this article to analyze the recent nuclear worker studies and estimate the dose rate effectiveness factor, DREF, for solid cancers.

Materials and methods: Twelve mortality studies were identified each with at least 100 cancer deaths and a meta-analysis was then carried out using their individual ratio of low dose rate cancer effect (LDR) to the corresponding high dose rate effect from the A-bomb cohort (LSS). The ratio is denoted by Q and its reciprocal is then an estimate of the DREF.

Results: The result was Q=?0.36 (95% CI?=?0.11, 0.60) and DREF?=?2.63 (95% confidence interval [CI]?=?1.61, 7.14). Clearly, this estimate is more consistent with a DREF of 2 than with a DREF of 1. The difficulty with the estimate Q?=?0.36 is that it is driven by only two large and dissimilar worker studies, the INWORKS study (q₁?=?1.14) and the Mayak worker cohort (q₃?=?0.30). The higher exposures for these nuclear workers were often in the early years (e.g. before 1960) with exposures from neutrons and internal emitters that are not included in the risk analyses resulting in likely overestimation of cancer effects per dose which would increase the estimate of the DREF. The Mayak study did, however, adjust for plutonium exposures. Finally, consideration is given to other cohort studies where DREF values may possibly be determined, such as the environmental exposures to the Techa River area residents and the Chernobyl cleanup workers as well as medical X-ray workers. Although dissimilar an overall meta-analysis yielded a Q?=?0.45 (95% CI?=?0.24, 0.66).

Conclusions: It is concluded that the best estimate of a DREF is still about 2. However, because of the various problems with the epidemiology studies, especially their dosimetry, it is concluded that a DREF of about 2 should be accepted with considerable caution since it is driven solely by the Mayak study.     

Equivalent dose and efficient dose in risk assessment of stochastic effects in diagnostic radiology

INTRODUCTION: Purpose. The equivalent dose absorbed during a radiological examination and the resulting effective dose correlate with the probability of late stochastic effects, of which the ICRP 60 has determined the nominal coefficients normalized to 1 Sv. We have normalized the risk coefficients to 1 mSv for better simulation of working conditions. We propose a simple method for estimating the radiological stochastic risk by correctly using both the equivalent dose and the effective dose concepts. MATERIAL AND METHODS: The effective dose depends on the irradiated body volume; thus, we calculated the stochastic risk in three hypothetical radiological examinations. The equivalent dose in the volume irradiated by the main beam was assumed to be 10 mSv and homogeneous; the equivalent dose in adjacent volumes was assumed to decrease by two different dose gradients. In our models, the sum of the equivalent dose absorbed by various tissues multiplied by the different weight-tissue values gives three effective dose values. Finally, the stochastic risk is estimated by multiplying the effective dose values by the nominal risk coefficient determined by ICRP 60. RESULTS: The effective dose is highest when the volume irradiated by the main beam is largest and the dose gradient in adjacent volumes is slowest. With a slow gradient, the effective dose is 10 mSv for total body examinations, 6.25 mSv for abdominopelvic examinations and 1.4 mSv for head and neck examinations. With a fast gradient, the effective dose is 10 mSv, 5.99 mSv and 1.10 mSv, respectively. The lethal tumor probability over the entire life-span is 65/10(6) for head and neck examinations, 300/10(6) for abdominopelvic examinations and 500/10(6) for total body examinations. CONCLUSIONS: The risk of stochastic effects in diagnostic radiology is low, inasmuch as it is projected over the entire life-span of the subject. Nevertheless, it must not be overlooked. Our calculation method aims to explain the correct use of equivalent dose and effective dose concepts, particularly relative to that great majority of radiological examinations which involve limited body volumes. In these cases it is important to estimate correctly the dose gradient from the examined volume towards the adjacent volumes. Close collaboration between physicist and radiologist is therefore essential, as their respective specialist tasks must necessarily be integrated.     

Principles of the International Commission on Radiological Protection system of dose limitation

The current recommendations of the International Commission on Radiological Protection (ICRP), published in 1977, identify two types of effect against which protection is required. "Stochastic" effects are those for which the probability of an effect occurring, rather than its severity, is regarded as a function of dose without threshold, whereas "non-stochastic" effects are those for which the severity varies with the dose and for which a threshold may occur. The system of dose limitation recommended by the ICRP is based on the prevention of non-stochastic effects and limitation of the probability of stochastic effects to levels deemed to be acceptable. The prevention of non-stochastic effects is achieved by setting dose-equivalent limits at values such that no threshold dose would be reached, even following exposure for the whole of a lifetime or for the total period of a working life. The limitation of stochastic effects is achieved by keeping all justifiable exposures as low as is reasonably achievable, economic and social factors being taken into account, subject to the constraint that reductions in collective exposure do not cause unacceptably large individual exposures. The formulation of a quantitative system of dose limitation based on these principles requires that judgments be made on several factors including: relationships between radiation dose and the induction of deleterious effects for a variety of endpoints and radiation types; acceptable levels of risk for radiation workers and members of the public; and methods of assessing whether the cost of introducing protective measures is justified by the reduction in radiation detriment which they will provide. In the case of patients deliberately exposed to ionising radiations, the objectives of radiation protection differ somewhat from those applying to radiation workers and members of the public. For patients, risks and benefits relate to the same person and upper limits on acceptable risks may differ grossly from those appropriate to normal individuals. For these reasons, and because of its historical relationship with the International Congress of Radiology, the ICRP has given special consideration to radiation protection in medicine and has published reports on protection of the patient in diagnostic radiology and in radiation therapy.