B F Cronin

Evaluation of (18)fluorodeoxyglucose positron emission tomography ((18)FDG PET) in the detection of malignant peripheral nerve sheath tumours arising from within plexiform neurofibromas in neurofibromatosis 1

OBJECTIVES The ability of 18fluorodeoxyglucose positron emission tomography (18FDG PET) to detect malignant change in plexiform neurofibromas from patients with neurofibromatosis 1 (NF1) was evaluated. METHODS Eighteen NF1 patients who presented with pain, increase in size, or neurological deficit associated with a plexiform neurofibroma were assessed. Magnetic resonance imaging determined the site and extent of the lesion. Qualitative18FDG PET was performed and the standard uptake value (SUV) measured the regional glucose metabolism. Histological confirmation of the diagnosis was obtained in 10 patients. RESULTS Twenty three plexiform neurofibromas were detected in 18 patients. Seven malignant peripheral nerve sheath tumours, four high grade and three low grade tumours, occurred in five patients. In one patient the clinical and radiological characteristics of the tumour suggested malignancy, but histology was inconclusive. Fifteen benign plexiform neurofibromas were identified in 12 patients and these findings were confirmed histologically in five lesions from four patients. Ten plexiform neurofibromas occurring in eight patients were considered benign on18FDG PET and the patients did not undergo surgery. They remained stable or their symptoms improved on clinical follow up (median 9 months). The results of qualitative 18FDG PET were interpreted as indicating that 13 plexiform neurofibromas were benign and 10 were malignant. No malignant tumours were classified as benign, but two benign tumours were reported as malignant. The SUV was calculated for 20 tumours and was significantly higher in five malignant tumours 5.4 (SD 2.4), than in 15 benign tumours 1.54 (SD 0.7), p=0.002. There was an overlap between benign and malignant tumours in the SUV range 2.7–3.3. CONCLUSIONS 18FDG PET is helpful in determining malignant change in plexiform neurofibromas in NF1. Increased separation between benign and malignant lesions could be obtained by calculating the SUV at about 200 minutes after injection of 18FDG, when the peak activity concentration is obtained in malignant tumours.

Quantification of Skeletal Kinetic Indices in Paget's Disease Using Dynamic18F‐Fluoride Positron Emission Tomography

The purpose of this study was to quantify indices of regional bone metabolism in Pagets disease and to compare these indices with normal bone using dynamic18F‐fluoride positron emission tomography (PET). Seven patients with vertebral Pagets disease had 1 h dynamic18F‐fluoride PET scans performed. The scans included a diseased vertebra and an adjacent normal vertebra. Arterial plasma input functions were also measured. A three‐compartment, four‐parameter model was used with nonlinear regression analysis to estimate bone kinetic variables. Compared with normal bone, pagetic bone demonstrated higher values of plasma clearance to bone mineral (Ki; 1.03 × 10−1 vs. 0.36 × 10−1 ml/min per milliliter; p = 0.018) and clearance to total bone tissue (K1; 2.38 × 10−1 vs. 1.25 × 10−1 ml/min per milliliter; p = 0.018), reflecting increased mineralization and blood flow, respectively. Release of18F‐fluoride from bone mineral (k4) was lower in pagetic bone (p = 0.022), suggesting tighter binding of18F‐fluoride to bone mineral. The notional volume of the extravascular bone compartment (K1/k2) was greater in pagetic bone (p = 0.018). Although the unidirectional extraction efficiency from the extravascular space to bone mineral (Ki/K1) was greater in pagetic bone (p = 0.018), a lower pagetic value of k2 (p = 0.028), describing the rate of transfer from the bone extravascular compartment to plasma, suggests that the18F‐fluoride that enters the relatively fibrotic marrow space of pagetic bone may be less accessible for return to plasma. These findings confirm some of the known pathophysiology of Pagets disease, introduce some new observations, and show how dynamic18F‐fluoride PET may be of value in the measurement of regional metabolic parameters in focal bone disorders.

Radiation dose rates from patients undergoing PET: implications for technologists and waiting areas.

Abstract.Increasingly hospitals are showing an interest in developing their imaging services to include positron emission tomography (PET). There is therefore a need to be aware of the radiation doses to critical groups. To assess the effective whole-body dose received by technologists within our dedicated PET centre, each staff member was issued with a dose rate meter, and was instructed to record the time spent in contact with any radioactive source, the dose received per working day and the daily injected activity. On average each technologist administered 831 MBq per day. The mean whole-body dose per MBq injected was 0.02 µSv/MBq–1. The average time of close contact (<2.0 m) with a radioactive source per day was 32 min. The average effective dose per minute close contact was 0.5 µSv/min–1, which resulted in a mean daily effective dose of 14.4 µSv. No technologist received greater than 60 µSv (the current UK limit for non-classified workers) in any one day, and in general doses received were less than 24 µSv, the daily dose corresponding to the proposed new annual limit for non-classified workers of 6.0 mSv per annum. However, we recognise that the layout of nuclear medicine departments will not mirror our own. We therefore measured the instantaneous dose rates at 0.1, 0.5, 1.0 and 2.0 m from the mid-thorax on 115 patients immediately after injection, to provide estimates of the likely effective doses that might be received by technologists operating dual-headed coincidence detectionsystems, and others coming into contact in the waiting room with patients who have been injected with fluorine-18 fluorodeoxyglucose. The mean (95th percentile) dose rates measured at the four aforementioned distances were 391.7 (549.5), 127.0 (199.8), 45.3 (70.0) and 17.1 (30.0) µSv/h–1, respectively. A number of situations have been modelled showing that, with correct planning, FDG studies should not significantly increase the effective doses to technologists. However, one possible area of concern is that, depending on the number of patients in a waiting area at any one time, accompanying persons may approach the limits set by the new UK IRR 1999 regulations for members of the public.

Effect of corrections for blood glucose and body size on [18F]FDG PET standardised uptake values in lung cancer

Abstract. Standardised uptake values (SUVs) are commonly used as a semi-quantitative index of 2-[18F]fluoro-2-deoxy-D-glucose (FDG) tracer uptake in positron emission tomography (PET). Studies have shown that SUVs may depend on body size and blood glucose concentration and corrections for these effects have been proposed in the literature. This retrospective study investigated the effect of the proposed corrections on SUVs from a group of 154 patients with lung cancer who had scans on a dedicated PET scanner. A total of 252 SUVs were requested as an aid to staging during consideration for surgical resection. SUVs were calculated normalised to body weight (SUVW), lean body mass (SUVLBM) and body surface area (SUVBSA). The following correlations were examined: SUV with height, weight and body surface area for the different body size normalisations; SUVW and SUVW × blood glucose (SUVBG) with blood glucose; SUVW with scan time post injection; and SUVW with apparent lesion diameter. Significant correlations were only observed between: SUVLBM and height (P=0.007); SUVW and scan time (P=0.007); SUVW and lesion diameter (P=0.0005); and SUVBG and blood glucose (P<0.00001). The correlation between SUVLBM and height suggests that lean body mass as a function of height alone should not be used to normalise SUVs; however, the lean body mass calculated from a height and weight nomogram did not show this effect. The strong correlation between SUVBG and blood glucose concentration suggests that for non-diabetic fasted patients, lung tumour SUVs should not be adjusted for blood glucose.

Evaluation of Myocardial Perfusion Using Positron Emission Tomography in Infants Following a Neonatal Arterial Switch Operation

Abstract. This study was performed to examine the use of positron emission tomography (PET) as a method of evaluating myocardial perfusion after the arterial switch operation for correction of transposition of the great arteries. Eleven asymptomatic patients (median age 2.3 years, range 1.3–4.3 years) post successful neonatal arterial switch repair for transposition underwent cardiac PET scanning using N13 ammonia before and after dipyridamole infusion. Reconstructed data from static scans were analyzed for regional perfusion defects before and after pharmacological stress. Simultaneous assessment of coronary flow before and after stress was performed using a Patlak graphical analysis of data from dynamic scans. Results obtained from PET scanning were correlated with patterns of coronary artery anatomy, electrocardiogram (ECG) recordings, and echocardiographic evaluation. PET scanning demonstrated normal distribution of myocardial perfusion before and after stress in all but one patient, who was found to have a discrete inferior transmural perfusion defect. The defect was well correlated with perioperative ECG changes and a complicated postoperative course. Myocardial blood flow before dipyridamole (0.690 ml/min/g) was similar to reported adult rest values. There was a small but significant (p < 0.002) increase in myocardial blood flow after dipyridamole stress with a mean coronary flow reserve of 1.19 (±0.103). Echocardiographic evaluation failed to demonstrate significant wall motion abnormalities in any of the patients. Cardiac PET scanning is a reliable noninvasive method for evaluation of myocardial perfusion in small children. In this study, the incidence of myocardial perfusion defects after the arterial switch operation is lower than previously reported. The data obtained concerning coronary flow and coronary flow reserve after the arterial switch need to be interpreted with caution because normal data in children are not available.