Protection of patients

Unlike staff and members of the public, there are no dose limits prescribed by any international or national organizations for patients. This does not mean that any amount of radiation dose can be delivered to patients in medical examinations and procedures. There are very clear requirements on justification, optimization and reference levels. In addition to generic justification of the use of PET/CT for defined clinical conditions, justification at an individual level is recommended. Individual justification assesses the need of the PET/CT examination for the particular patient. If the justification principle is suitably applied, many unnecessary examinations can be avoided. Unfortunately, in practice, there is substantial need to improve situation with regard to justification.

Optimization requires that once an examination is justified, the exposure should be kept as low as reasonably achievable for the required image quality [9.2]. There has been lot of emphasis on this and has resulted in a significant reduction of patient doses through the process of optimization as evidenced by a large number of publications.

The concept of diagnostic reference level (DRL) is a powerful tool for optimization in patient protection as it helps in evaluating whether the patient dose (with respect to stochastic effects) is unusually high or low for a particular medical imaging procedure. DRLs are not dose limits as they are established based on contemporary technology and practice. In Publication 60 of the ICRP [9.1], diagnostic reference levels were described as values of measured quantities above which some specified action or decision should be taken. They should be applied with flexibility, to allow higher doses where indicated by sound clinical judgment.

The most suitable measured quantity in nuclear medicine is the injected radioactivity and, in the case of CT, a computed tomography dose index (CTDI) has been the accepted quantity. Very often, the non-measurable quantity of

‘effective dose’ is used to indicate patient dose. The ICRP and UNSCEAR have cautioned against the use of effective dose to estimate detriment (to individual or specific populations). Such estimates suffer from uncertainties arising from potential demographic differences (in terms of health status, age and sex) between a specific population of patients and the general populations for whom the ICRP derived the risk coefficient. It has been suggested, for example, that effective dose could broadly underestimate the detriment from diagnostic exposure of younger patients by a factor of about 2 and, conversely, could overestimate the detriment from the exposure of older patients by a factor of at least 5. Thus, rigorous analysis of radiation risk from diagnostic medical exposure requires detailed knowledge of organ doses and the age and sex of patients.

Despite these limitations, both the ICRP and UNSCEAR have used effective dose as the quantity to represent risk in the absence of a better quantity, but with an understanding of its limitations. This report follows this approach.

9.5.1. Internal exposure

The effective dose E_int resulting from intravenous administration of an activity A can be estimated from:

E_int = ·A (9.1)

where  is a dose coefficient computed for the adult hermaphrodite MIRD phantom. For ¹⁸F labelled FDG and ⁸²Rb, the dose coefficients are 19 and 3.4 Sv/MBq, respectively [9.13]. These dose coefficients hold for standard patients with a body weight of about 70 kg and are generic rather than patient specific since age, gender of patients and individual pharmacokinetics are not taken into account. In fact, the radiation risk is somewhat higher for females and for younger patients when compared to male and older patients. Age and gender specific dose coefficients can be found in the ICRP report.

filter, geometry and acquisition algorithm used) [9.14,9.15]. Thus, values for patient dose vary considerably between centres and machines. Oversimplified approaches have correlated mAs with patient dose, assuming that kVp and other parameters in a particular CT scanner are kept constant. This has also led to the unsatisfactory practice of using mAs as dose indicator when comparing different scanners.

Various software packages have been developed to address these problems.

For example, with whole body CT scans, Brix et al. [9.16] present a simple approach to providing a rough estimate of organ doses and effective dose. The organ dose, D_T, can be roughly estimated as:

D_T = _T^CTCTDI_vol (9.2)

where _T^CT is an organ specific dose coefficient that relates the volume CT dose index, CTDI_vol, to organ dose [9.16]. The organ doses can be combined with weighting factors in the normal way, to give effective dose. For a whole body CT (thyroid to the symphysis), Brix et al. use 1.47 dose coefficient together with the CTDI_vol to calculate an effective dose in mSv [9.16].

9.5.3. Combined exposure in PET/CT

The effective dose from a combined PET/CT examination is the sum of the effective doses arising from all scan components, and thus depends on the range of acquisition parameters mentioned above. The total effective dose for the whole body FDG-PET/CT depends upon protocol, but can be around 25 mSv [9.16]. Up to 70% of this is contributed by the CT scan elements; and 85% of the CT contribution (two-thirds of the total) may arise from the final diagnostic scan (8.6), while the remaining 15% comes from the scout and CT acquisition done for attenuation correction purposes. An alternative approach, which is commonly used, is to perform the PET/CT only from cerebellum to mid-thigh. The dose in this case would be approximately in the range 15–20 mSv.

Patient dose management in PET is less complex than in CT, provided one has a well designed facility, and one has control of the radioactivity that is administered. On the other hand, dose management in CT has continued to be a challenge. A significant part of this challenge arises from the fact that overexposure in CT is frequently not detected. In contrast to film based radiography where overexposure is evident in a dark images, increasing dose in CT and in other digital imaging techniques results in images with less noise (improved visual appearance) and fewer streak artefacts, although not necessarily with greater diagnostic information. It is widely believed that image quality in CT often exceeds the clinical requirements for diagnosis [9.17].

The ICRP has noted that technical and clinical developments in CT have not necessarily led to a reduction in patient dose per examination, and that there is a clear need for optimization of doses [9.14, 9.15]. In recent years, many papers have shown that adequate diagnostic information can be obtained with CT studies using lower doses [33–35]. All manufacturers have incorporated automated exposure control (AEC) into their systems. Further details about patient dose management are available in Refs [9.6, 9.14, 9.15].