Feasibility study of computational occupational dosimetry: evaluating a proof-of-concept in an endovascular and interventional cardiology setting.

Individual monitoring of radiation workers is essential to ensure compliance with legal dose limits and to ensure that doses are As Low As Reasonably Achievable. However, large uncertainties still exist in personal dosimetry and there are issues with compliance and incorrect wearing of dosimeters. The objective of the PODIUM (Personal Online Dosimetry Using Computational Methods) project was to improve personal dosimetry by an innovative approach: the development of an online dosimetry application based on computer simulations without the use of physical dosimeters. Occupational doses were calculated based on the use of camera tracking devices, flexible individualised phantoms and data from the radiation source. When combined with fast Monte Carlo simulation codes, the aim was to perform personal dosimetry in real-time. A key component of the PODIUM project was to assess and validate the methodology in interventional radiology workplaces where improvements in dosimetry are needed. This paper describes the feasibility of implementing the PODIUM approach in a clinical setting. Validation was carried out using dosimeters worn by Vascular Surgeons and Interventional Cardiologists during patient procedures at a hospital in Ireland. Our preliminary results from this feasibility study show acceptable differences of the order of 40% between calculated and measured staff doses, in terms of the personal dose equivalent quantity Hp(10), however there is a greater deviation for more complex cases and improvements are needed. The challenges of using the system in busy interventional rooms have informed the future needs and applicability of PODIUM. The availability of an online personal dosimetry application has the potential to overcome problems that arise from the use of current dosimeters. In addition, it should increase awareness of radiation protection among staff. Some limitations remain and a second phase of development would be required to bring the PODIUM method into operation in a hospital setting. However, an early prototype system has been tested in a clinical setting and the results from this two-year proof-of-concept PODIUM project are very promising for future development.

Fast Monte Carlo codes for occupational dosimetry in interventional radiology.

PURPOSE: Interventional radiology techniques cause radiation exposure both to patient and personnel. The radiation dose to the operator is usually measured with dosimeters located at specific points above or below the lead aprons. The aim of this study is to develop and validate two fast Monte Carlo (MC) codes for radiation transport in order to improve the assessment of individual doses in interventional radiology. The proposed methodology reduces the number of required dosemeters and provides immediate dose results. METHODS: Two fast MC simulation codes, PENELOPE/penEasyIR and MCGPU-IR, have been developed. Both codes have been validated by comparing fast MC calculations with the multipurpose PENELOPE MC code and with measurements during a realistic interventional procedure. RESULTS: The new codes were tested with a computation time of about 120 s to estimate operator doses while a standard simulation needs several days to obtain similar uncertainties. When compared with the standard calculation in simple set-ups, MCGPU-IR tends to underestimate doses (up to 5%), while PENELOPE/penEasyIR overestimates them (up to 18%). When comparing both fast MC codes with experimental values in realistic set-ups, differences are within 25%. These differences are within accepted uncertainties in individual monitoring. CONCLUSION: The study highlights the fact that computational dosimetry based on the use of fast MC codes can provide good estimates of the personal dose equivalent and overcome some of the limitations of occupational monitoring in interventional radiology. Notably, MCGPU-IR calculates both organ doses and effective dose, providing a better estimate of radiation risk.

PERSONAL DOSIMETRY USING MONTE-CARLO SIMULATIONS FOR OCCUPATIONAL DOSE MONITORING IN INTERVENTIONAL RADIOLOGY: THE RESULTS OF A PROOF OF CONCEPT IN A CLINICAL SETTING.

Exposure levels to staff in interventional radiology (IR) may be significant and appropriate assessment of radiation doses is needed. Issues regarding measurements using physical dosemeters in the clinical environment still exist. The objective of this work was to explore the prerequisites for assessing staff radiation dose, based on simulations only. Personal dose equivalent, Hp(10), was assessed using simulations based on Monte Carlo methods. The position of the operator was defined using a 3D motion tracking system. X-ray system exposure parameters were extracted from the x-ray equipment. The methodology was investigated and the simulations compared to measurements during IR procedures. The results indicate that the differences between simulated and measured staff radiation doses, in terms of the personal dose equivalent quantity Hp(10), are in the order of 30-70 %. The results are promising but some issues remain to be solved, e.g. an automated tracking of movable parts such as the ceiling-mounted protection shield.

A MODEL FOR EVALUATING THE USE OF IMAGING IN IMAGE-GUIDED INTERVENTIONAL PROCEDURES-POSSIBLE IMPLICATIONS ON OPTIMISATION OF RADIATION PROTECTION.

The present study focuses on introducing the concept of optimisation and proposing a model, including evaluation of image quality, to be used in the clinical routines where image-guided intervention is being performed. The overall aim of the study was to develop a model for evaluating the use of imaging in X-ray-guided interventional procedures and its possible implications on optimisation of radiation protection. In the search for an adequate evaluation model, data from endovascular interventions of the aorta (EVAR procedures) were used. The procedure was schematically described in steps. Every imaging event was connected to the steps in the medical procedure and was also described with the purpose of the imaging event. Available technical, as well as procedural parameters, were studied and analysed. Data were collected from the X-ray equipment for 70 EVAR procedures and, out of these, 12 procedures were randomly selected to be recorded on video to understand the procedure better. It was possible to describe the EVAR procedures in a general way with explanations of the clinical purpose connected to each imaging event. Possible quality parameters of the procedure were identified for the imaging events (radiation dose, image quality). The model method still needs to be refined and will then be applied to clinical data and to other clinical procedures to test the validity.

Challenges assessing radiation risk in image-guided treatments-implications on optimisation of radiological protection.

The present work explores challenges when assessing organ dose and effective dose concerning image-guided treatments. During these treatments considerable x-ray imaging is employed using technically advanced angiographic x-ray equipment. Thus, the radiation dose to organs and the related radiation risk are relatively difficult to assess. This has implications on the optimisation process, in which assessing radiation dose is one important part. In this study, endovascular aortic repair treatments were investigated. Organ dose and effective dose were assessed using Monte Carlo calculations together with a detailed specification of the exposure situation and patient size. The resulting normalised organ dose and effective dose with respect to kerma-area product for patient sizes and radiation qualities representative for the patient group were evaluated. The variability and uncertainty were investigated and their possible impact on optimisation of radiation protection was discussed. Exposure parameters, source to detector distances etc varied between treatments and also varied between image acquisitions during one treatment. Thus the derived normalised organ dose and effective dose exhibited a large range of values depending greatly on used exposure parameters and patient configuration. The derived normalised values for effective dose varied approximately between 0.05 and 0.30 mSv per Gy·cm2 when taking patient sizes and exposure parameters into consideration, the values for organ doses exhibited even larger variation. The study shows a possible systematic error for derived organ doses and effective dose up to a factor of 7 if detailed exposure or patient characteristics are not known and/or not taken into consideration. The intra-treatment variability was also substantial and the normalised dose values varied up to a factor of 2 between image acquisitions during one treatment. The study shows that the use of conversion factors that are not adapted to the clinic can cause the radiation dose to be exaggerated or underestimated considerably. A conclusion from the present study is that the systematic error could be large and should be estimated together with random errors. A large uncertainty makes it difficult to detect true differences in radiation dose between methods and technology-a prerequisite for optimising radiation protection for image-guided treatments.

Radiological protection of foetuses and breast-fed children of occupationally exposed women in nuclear medicine - Challenges for hospitals.

This paper describes issues of concern for protecting foetuses and breast-fed children of occupationally exposed women in nuclear medicine from unnecessary exposure of ionising radiation. The protection principle is to ensure the same level of protection for the foetus and child as for the general public. Therefore international radiation protection standards recommend a dose constraint of 1mSv to a foetus during the remaining time of pregnancy after it is known/declared and a yearly dose constraint of 1mSv to a breast-fed child. It is not self-evident how to guarantee this level of radiation protection. The exposure situation in nuclear medicine is complex. Exploring existing reported occupational exposure levels suggests great variability between work tasks and facilities. The standards and guidelines found give no detailed advice. Therefore each facility needs to systematically review external and internal exposure levels in order to plan appropriate protection measures and issue their own guidelines and rules. One strategy might be that each facility defines tasks that do not require any restrictions and lists such duties that are not suitable to do when pregnant or breastfeeding, taking also potential exposure levels into consideration. This paper gives examples of such types of work. Information to the staff about the necessity of declaring pregnancy or breastfeeding is of fundamental importance. The internal policies issued by the hospital management should make clear the basis for taking care of pregnant and breastfeeding employees.

Eye dosimetry and protective eyewear for interventional clinicians.

Doses to the eyes of interventional clinicians can exceed 20 mSv. Various protective devices can afford protection to the eyes with the final barrier being protective eyewear. The protection provided by lead glasses is difficult to quantify, and the majority of dosimeters are not designed to be worn under lead glasses. This study has measured dose reduction factors (DRFs) equal to the ratio of the dose with no protection, divided by that when lead glasses are worn. Glasses have been tested in X-ray fields using anthropomorphic phantoms to simulate the patient and clinician. DRFs for X-rays incident from the front vary from 5.2 to 7.6, while values for orientations reminiscent of clinical practice are between 1.4 and 5.2. Results suggest that a DRF of two is a conservative factor that could be applied to personal dosimeter measurements to account for the dose reduction provided by most types of lead glasses.

A management system integrating radiation protection and safety supporting safety culture in the hospital.

Quality assurance has been identified as an important part of radiation protection and safety for a considerable time period. A rational expansion and improvement of quality assurance is to integrate radiation protection and safety in a management system. The aim of this study was to explore factors influencing the implementing strategy when introducing a management system including radiation protection and safety in hospitals and to outline benefits of such a system. The main experience from developing a management system is that it is possible to create a vast number of common policies and routines for the whole hospital, resulting in a cost-efficient system. One of the key benefits is the involvement of management at all levels, including the hospital director. Furthermore, a transparent system will involve staff throughout the organisation as well. A management system supports a common view on what should be done, who should do it and how the activities are reviewed. An integrated management system for radiation protection and safety includes key elements supporting a safety culture.

Optimisation of occupational radiological protection in image-guided interventions: potential impact of dose rate measurements.

The optimisation of occupational radiological protection is challenging and a variety of factors have to be considered. Physicians performing image-guided interventions are working in an environment with one of the highest radiation risk levels in healthcare. Appropriate knowledge about the radiation environment is a prerequisite for conducting the optimisation process. Information about the dose rate variation during the interventions could provide valuable input to this process. The overall purpose of this study was to explore the prerequisite and feasibility to measure dose rate in scattered radiation and to assess the usefulness of such data in the optimisation process.Using an active dosimeter system, the dose rate in the unshielded scattered radiation field was measured in a fixed point close to the patient undergoing an image-guided intervention. The measurements were performed with a time resolution of one second and the dose rate data was continuously timed in a data log. In two treatment rooms, data was collected during a 6 month time period, resulting in data from 380 image-guided interventions and vascular treatments in the abdomen, arms and legs. These procedures were categorised into eight types according to the purpose of the treatment and the anatomical region involved.The dose rate varied substantially between treatment types, both regarding the levels and the distribution during the procedure. The maximum dose rate for different types of interventions varied typically between 5 and 100 mSv h(-1), but substantially higher and lower dose rates were also registered. The average dose rate during a complete procedure was however substantially lower and varied typically between 0.05 and 1 mSv h(-1). An analysis of the distribution disclosed that for a large part of the treatment types, the major amount of the total accumulated dose for a procedure was delivered in less than 10% of the exposure time and in less than 1% of the total procedure time.The present study shows that systematic dose rate measurements are feasible. Such measurements can be used to give a general indication of the exposure level to the staff and could serve as a first risk assessment tool when introducing new treatment types or x-ray equipment in the clinic. For example, it could provide an indication for when detailed eye dose measurements are needed. It also gives input to risk management considerations and the development of efficient routines for other radiological protection measures.

Evaluation of the impact of a system for real-time visualisation of occupational radiation dose rate during fluoroscopically guided procedures.

Optimisation of radiological protection for operators working with fluoroscopically guided procedures has to be performed during the procedure, under varying and difficult conditions. The aim of the present study was to evaluate the impact of a system for real-time visualisation of radiation dose rate on optimisation of occupational radiological protection in fluoroscopically guided procedures. Individual radiation dose measurements, using a system for real-time visualisation, were performed in a cardiology laboratory for three cardiologists and ten assisting nurses. Radiation doses collected when the radiation dose rates were not displayed to the staff were compared to radiation doses collected when the radiation dose rates were displayed. When the radiation dose rates were displayed to the staff, one cardiologist and the assisting nurses (as a group) significantly reduced their personal radiation doses. The median radiation dose (Hp(10)) per procedure decreased from 68 to 28 µSv (p = 0.003) for this cardiologist and from 4.3 to 2.5 µSv (p = 0.001) for the assisting nurses. The results of the present study indicate that a system for real-time visualisation of radiation dose rate may have a positive impact on optimisation of occupational radiological protection. In particular, this may affect the behaviour of staff members practising inadequate personal radiological protection.

Unsolved or unsolvable problems with diagnostic reference levels.

Diagnostic reference levels (DRLs) for X-ray examinations have been introduced in many countries, among others in most European countries, as a consequence of the Directive on Medical Exposures from 1997. The concept seems to be straight forward, but when implementing it into practice a number of problems arise. The results are dependent on how the dose data are measured and assessed. The interpretation of the results must be performed with great care giving due consideration to the fact that X-ray examinations are very complex and hence not very well characterised by a single figure. In this presentation the various parameters influencing the patient dose are discussed and suggestions are given on how these are managed so as to get the best outcome. When these intentions are followed diagnostic reference levels will be a powerful tool for the optimisation process.

The use of reference image criteria in X-ray diagnostics: an application for the optimisation of lumbar spine radiographs.

To ensure that sufficient image quality is obtained in diagnostic radiology, the image quality of clinical radiographs has to be evaluated. We present two methods herein for evaluating antero-posterior (AP) radiographs of the lumbar spine. One was using image criteria, including six anatomical details (absolute method). In the other, the visibility of anatomical details relative to a reference radiograph was evaluated (visual grading analysis). In total, 14 technique groups were evaluated. The technique groups differed in tube voltage and detector system characteristics. Six different gradients of the H&D curves were simulated. The visual grading analysis showed larger differences in image quality compared with the absolute method. The influence on the image quality due to a variation in tube voltage was easier to detect than the influence on the image quality from the detector characteristics. The visibility of the anatomical details was significantly dependent on the location in the spine. The visual grading analysis was found to be the preferable evaluation method in studies such as the present; however, it is necessary to guide and train the observer before the evaluation is performed.

Influence of the characteristic curve on the clinical image quality of lumbar spine and chest radiographs.

The "European Guidelines on Quality Criteria for Diagnostic Radiographic Images" do not address the choice of the film characteristic (H&D) curve, which is an important parameter for the description of a radiographic screen-film system. The image contrast of clinical lumbar spine and chest radiographs was altered by digital image processing techniques, simulating images with different H&D curves, both steeper and flatter than the original. The manipulated images were printed on film for evaluation. Seven experienced radiologists evaluated the clinical image quality by analysing the fulfilment of the European Image Criteria (ICS) and by visual grading analysis (VGA) of in total 224 lumbar spine and 360 chest images. A parallel study of the effect of the H&D curve has also been made using a theoretical model. The contrast (DeltaOD) of relevant anatomical details was calculated, using a Monte Carlo simulation-model of the complete imaging system including a 3D voxel phantom of a patient. Correlations between the calculated contrast and the radiologists' assessment by VGA were sought. The results of the radiologists' assessment show that the quality in selected regions of lumbar spine and chest images can be significantly improved by the use of films with a steeper H&D curve compared with the standard latitude film. Significant (p<0.05) correlations were found between the VGA results and the calculations of the contrast of transverse processes and trabecular details in the lumbar spine vertebrae, and with the contrast of blood vessels in the retrocardiac area of the chest.

The influence of different technique factors on image quality of chest radiographs as evaluated by modified CEC image quality criteria.

The Commission of the European Communities (CEC) research project "Predictivity and optimisation in medical radiation protection" addressed fundamental operational limitations in existing radiation protection mechanisms. The first part of the project aimed at investigating (1) whether the CEC image quality criteria could be used for optimization of a radiographic process and (2) whether significant differences in image quality based on these criteria could be detected in a controlled project with well known physical and technical parameters. In the present study, chest radiographs on film were produced using healthy volunteers. Four physical/technical parameters were varied in a carefully controlled manner: tube voltage (102 kVp and 141 kVp), nominal speed class (160 and 320), maximum film density (1.3 and 1.8) and method of scatter reduction (grid (R=12) and air gap). The air kerma at the entrance surface was measured for all patients and the risk-related dose H(Golem), based on calculated organ-equivalent dose conversion coefficients and the measured entrance air kerma values, was calculated. Image quality was evaluated by a group of European expert radiologists using a modified version of the CEC quality criteria. For the two density levels, density level 1.8 was significantly better than 1.3 but at the cost of a higher patient radiation exposure. The correlation between the number of fulfilled quality criteria and H(Golem) was generally poor. An air gap technique resulted in lower doses than scatter reduction with a grid but provided comparable image quality. The criteria can be used to highlight optimum radiographic technique in terms of image quality and patient dose, although not unambiguously. A recommendation for good radiographic technique based on a compromise between image quality and risk-related radiation dose to the patient is to use 141 kVp, an air gap, a screen-film system with speed 320 and an optical density of 1.8.

Demonstration of correlations between clinical and physical image quality measures in chest and lumbar spine screen-film radiography.

The ability to predict clinical image quality from physical measures is useful for optimization in diagnostic radiology. In this work, clinical and physical assessments of image quality are compared and correlations between the two are derived. Clinical assessment has been made by a group of expert radiologists who evaluated fulfillment of the European image criteria for chest and lumbar spine radiography using two scoring methods: image criteria score (ICS) and visual grading analysis score (VGAS). Physical image quality measures were calculated using a Monte Carlo simulation model of the complete imaging system. This model includes a voxelized male anatomy and was used to calculate contrast and signal-to-noise ratio of various important anatomical details and measures of dynamic range. Correlations between the physical image quality measures on the one hand and the ICS and VGAS on the other were sought. 16 chest and 4 lumbar spine imaging system configurations were compared in frontal projection. A statistically significant correlation with clinical image quality was found in chest posteroanterior radiography for the contrast of blood vessels in the retrocardiac area and a measure of useful dynamic range. In lumbar spine anteroposterior radiography, a similar significant correlation with clinical image quality was found between the contrast and signal-to-noise ratio of the trabecular structures in the L1-L5 vertebrae. The significant correlation shows that clinical image quality can, at least in some cases, be predicted from appropriate measures of physical image quality.

The influence of different technique factors on image quality of lumbar spine radiographs as evaluated by established CEC image criteria.

In this study we have investigated the image quality of lumbar spine radiographs taken after recording technical and physical parameters. Two technical parameters were altered, tube voltage (70 kV and 90 kV for the anteroposterior (AP) projection and 77 kV and 95 kV for the lateral projection) and sensitivity of the film-screen system (sensitivity class 400 and 600). In total, 85 images were included in the study. Entrance surface dose (ESD) was measured using thermoluminescent dosemeters. The mean value of ESD for the different technique groups varied between 1.9 mGy (90 kV, sensitivity class 400) and 4.6 mGy (70 kV, sensitivity class 400) for the AP projection, and between 6.4 mGy (95 kV, sensitivity class 600) and 20.4 mGy (70 kV, sensitivity class 400) for the lateral projection. Image criteria given in the "European Guidelines on Quality Criteria for Radiographic Images" were used to assess image quality. Two evaluation methods have been employed. A straightforward scoring of fulfilled image criteria, and visual grading analysis using the structures defined in the image criteria. The latter method provided a sharper distinction between groups of images taken using different radiographic techniques. The average number of fulfilled image criteria for the AP projections varied between 0.74 (90 kV, sensitivity class 400) and 0.87 (70 kV, sensitivity class 400). For the lateral projection this number varied between 0.79 (95 kV, sensitivity class 600) and 0.84 (77 kV, sensitivity class 600). This study shows that image criteria are useful tools in clinical studies of image quality.

Simple methods for the estimation of dose distributions, organ doses and energy imparted in paediatric radiology.

The energy imparted and the effective dose can both be used to describe the risk to the patient in diagnostic radiology. Simple methods must be employed to determine these quantities in clinical situations. Methods using measured relative depth-dose distributions are presented and evaluated here. Measurements of depth-dose distributions for x-ray beams were performed with an ionization chamber, a diode and a number of TL dosimeters. The energy imparted was calculated from measurements with both phantoms and patients. The method of calculating the mean absorbed dose to organs was applied to pelvis and lumbar spine examinations. TL dosimeters were found to be an appropriate detector for measuring depth-dose distributions. When calculating the energy imparted the entrance beam area must be accurately known. The mean absorbed dose to organs can be derived from measured relative depth-dose curves if accurate information on entrance beam position and area is available for the particular examination technique used. The advantage of these methods is that the dose distribution is measured for the photon beam used for the examination of the patients.

Examination technique, image quality, and patient dose in paediatric radiology. A survey including 19 Swedish hospitals.

PURPOSE: Investigation of examination technique, image quality, and absorbed dose to the patients in paediatric radiology. MATERIAL AND METHODS: In total, 19 Swedish hospitals participated in the study. Using a questionnaire, the hospitals described their examination technique for the pelvis, urinary tract, colon, scoliosis, and lung. The image quality and patient dose were experimentally studied for the simulated pelvis examination of a 1-year-old child. This examination was carried out with a test object containing a contrast-detail phantom. TL dosimeters were used to determine the absorbed dose on the surface of the phantom, approximately corresponding to the absorbed dose on the surface of the patient. RESULTS: Examination techniques varied considerably among the hospitals, the most striking difference concerning the film-screen sensitivity. Consequently, there was a variation in the absorbed dose on the surface of the phantom, from 0.09 mGy to 1.7 mGy (mean 0.65 mGy). For a large range of doses, 0.4-1.7 mGy, the image quality was not significantly different. CONCLUSION: The unharmonized, and in many places unoptimized, examination techniques led to a great variation in the absorbed dose to the children examined.

Dose distribution at radiographic examination of the spine in pediatric radiology.

STUDY DESIGN: The radiation dose distribution at radiographic examinations of the lumbar spine and the thoracic spine and at examinations for scoliosis has been investigated in pediatric patients. OBJECTIVES: To derive the conversion factors between the entrance surface dose and the mean absorbed dose to the most radiation-sensitive organs, and to determine the dose level and its variation for patients. SUMMARY OF BACKGROUND DATA: Fifty-five patients participated in the survey. Mathematically describable phantoms were used to simulate the body of children of various ages. METHODS: Relative absorbed dose in the body was determined with measurements. The entrance surface dose and the dose-area product were measured for a number of patients. The effective dose and its variation between patients were estimated. RESULTS: The mean entrance surface doses were highest for the lumbar spine examinations (2.6 mGy, anteroposterior projection; 6.7 mGy, lateral projection). The female gonads received the highest dose. The mean entrance surface doses for the thoracic spine examinations were 2.1 mGy for the frontal view and 6.1 mGy for the lateral view. The breasts, thyroid, lungs, and esophagus received the highest absorbed dose. Mean effective dose for one frontal and one lateral view was slightly higher for lumbar spine examinations than for thoracic spine examinations. An optimized scoliosis examination gives a much lower effective dose, about 0.05 mSv (frontal view). If an additional exposure determining the "skeletal age" is performed, the effective dose rises with 0.12 mSv. CONCLUSIONS: Thoracic spine and lumbar spine examinations lead to relatively high-absorbed doses to radiation-sensitive organs. Because the scoliosis investigation is repeated several times, the total effective dose could be considerable.

The radiation dose to children from X-ray examinations of the pelvis and the urinary tract.

X-ray examinations of the pelvis and the urinary tract are frequent examinations of children, in which a large part of the trunk is irradiated. The irradiated volume contains many of the most radiation sensitive organs and tissues. The absorbed dose to children during the examination was estimated from measurements with a dose-area product meter and thermoluminescent dosemeters (TLDs). Entrance surface dose and the dose-area product results are presented. Conversion factors between the entrance surface dose and the organ dose were derived. The energy imparted, organ dose and effective dose were determined. The entrance surface dose for one single exposure varied between 0.32 mGy and 8.6 mGy for the urinary tract examination and between 0.26 mGy and 2.89 mGy per exposure for the pelvis examination. These variations are mainly influenced by the body size of the patient. The number of images taken during one examination varied. For the urinary tract investigation, the average number of exposures was six, while the corresponding number for the pelvis examination was two. The average effective dose for a typical urinary tract investigation ranged from 0.9 mSv to 8.5 mSv and from 0.3 mSv to 1.4 mSv for the pelvis examination. The radiation dose depends greatly on the body size. The recommendations to present the results in relation to age have been followed; however, the variation of body size even within each specified age range is significant. It is suggested that doses should be quoted in relation to a more critical parameter than age.