Radiation dose to nuclear medicine technologists when operating PET/MR compared with PET/CT.

Since 2010, positron emission tomography/magnetic resonance (PET/MR) has been increasingly used as clinical routine in nuclear medicine departments. One advantage of PET/MR over PET/computed tomography (CT) is the lower dose of ionising radiation delivered to patients. However, data on the radiation dose delivered to staff operating PET/MR compared with the new generation of PET/CT equipment are still lacking. Our aim was to compare the radiation dose to nuclear medicine technologists performing routine PET/MR and PET/CT in the same department. We retrospectively measured the daily radiation dose received by PET technologists over 13 months by collecting individual dosimetry measurements (from electronic personal dosimeters). Data were analysed taking into account the total number of studies performed with each PET modality (PET/MR with Signa 3T, General Electric Healthcare versus PET/CT with Biograph mCT flow, Siemens), the type of exploration (brain versus whole-body PET), the18F activity injected per day and per patient as well as the time spent in contact with patients after tracer injection. Our results show a significantly higher whole-body exposure to technologists for PET/MR compared with PET/CT (10.3 ± 3.5 nSv versus 4.7 ± 1.2 nSv per18F injected MBq, respectively;p< 0.05). This difference was related to prolonged contact with injected patients during patient positioning with the PET/MR device and MR coil placement, especially in whole-body studies. For an equal injected activity, radiation exposure to PET technologists for PET/MR was twice that of PET/CT. To minimise the radiation dose to staff, efforts should be made to optimise patient positioning, even in departments with extensive PET/CT experience.

Radiation dose of nuclear medicine technicians performing PET/MR.

Since october 2015, PET/MR has been used extensively for clinical routine in the nuclear medicine department of the Pitié-Salpêtrière Hospital (Paris, France) with a throughput of 11 to 15 patients each day. While many studies have been conducted to investigate dose reduction strategies to patients with hybrid PET/MR devices, no study has focused on staff radiation safety. Knowing that patient positioning within the scanner takes longer in PET/MR than in PET/CT because of the placement of several local MR receive coils, a retrospective study was carried out to measure the radiation doses to nuclear medicine technologists from the patient. The analysis was conducted during one year on 1332 clinical PET/MR studies performed with the Signa PET/MR system (General Electric Healthcare) in our department. The whole-body exposure of the technologist staff was on average for all PET/MR exams10.3 ± 4 nSv per injected MBq of 18 F. When performing brain PET/MR exams only, the whole-body exposure was on average 8.7 ± 2 nSv per injected MBq of 18 F. Brain PET/MR provides lower radiation dose than whole-body examinations for cancer screening due to a lower injected activity (2 vs. 3 MBq kg-1) and shorter patient positioning (5 vs. 15 min). When starting PET/MR in a nuclear medicine department, an important step is to optimise patient positionning within the scanner to minimise radiation dose received by the technical staff from patients.

Feasibility of Liver Transplantation after 90Y Radioembolization: Lessons from a Radiation Protection Incident.

ABSTRACT: Radioembolization using 90Y is a growing procedure in nuclear medicine for treating hepatocellular carcinoma. Current guidelines suggest postponing liver transplantation or surgical resection for a period of 14 to 30 d after radioembolization to minimize surgeons' exposure to ionizing radiation. In light of a radiation protection incident, we reevaluated the minimum delay required between radioembolization and subsequent liver transplantation. A patient with a hepatocellular carcinoma underwent a liver transplantation 44 h after undergoing radioembolization using 90Y (860 MBq SIR-Spheres). No specific radioprotection measures were followed during surgery and pathological analysis. We subsequently (1) evaluated the healthcare professionals' exposure to ionizing radiation by conducting dose rate measurements from removed liver tissue and (2) extrapolated the recommended interval to be observed between radioembolization and surgery/transplantation to ensure compliance with the radiation dose limits for worker safety. The surgeons involved in the transplantation procedure experienced the highest radiation exposure, with whole-body doses of 2.4 mSv and extremity doses of 24 mSv. The recommended delay between radioembolization and liver transplantation was 8 d when using SIR-Spheres and 15 d when injecting TheraSphere. This delay can be reduced further when considering the specific 90Y activity administered during radioembolization. This dosimetric study suggests the feasibility of shortening the delay for liver transplantation/surgery after radioembolization from the 8th or 15th day after using SIR-Spheres or TheraSphere, respectively. This delay can be decreased further when adjusted to the administrated activity while upholding radiation protection standards for healthcare professionals.

Prognostic implications of volume-based measurements on FDG PET/CT in stage III non-small-cell lung cancer after induction chemotherapy.

PURPOSE: We sought to determine whether metabolic volume-based measurements on FDG PET/CT scans could provide additional information for predicting outcome in patients with stage III non-small-cell lung cancer (NSCLC) treated with induction chemotherapy. METHODS: Included in the study were 32 patients with stage III NSCLC who were treated with induction platinum-based chemotherapy followed in 21 by surgery. All patients had an FDG PET/CT scan before and after the induction chemotherapy. Tumours were delineated using adaptive threshold methods. The SUVmax, SUVpeak, SUVmean, tumour volume (TV), total lesion glycolysis (TLG), and volume and largest diameter on the CT images (CTV and CTD, respectively) were calculated. Index ratios of the primary tumour were calculated by dividing the follow-up measurements by the baseline measurements. The prognostic value of each parameter for event-free survival (EFS) was determined using Cox regression models. RESULTS: The median follow-up time was 19 months (range 6-43 months). Baseline PET and CT parameters were not significant prognostic factors. After induction therapy, only SUVmax, SUVpeak, SUVmean, TV, TLG and CTV were prognostic factors for EFS, in contrast to CTD. Of the index ratios, only TV and TLG ratios were prognostic factors for EFS. Patients with a TLG ratio <0.48 had a longer EFS than those with a TLG ratio >0.48 (13.9 vs. 9.2 months, p = 0.04). After adjustment for the effect of surgical treatment, all the parameters significantly correlated with EFS remained significant. CONCLUSION: SUV, metabolic volume-based indices, and CTV after induction chemotherapy give independent prognostic information in stage III NSCLC. However, changes in metabolic TV and TLG under induction treatment provide more accurate prognostic information than SUV alone, and CTD and CTV.

Comparison of PET metabolic indices for the early assessment of tumour response in metastatic colorectal cancer patients treated by polychemotherapy.

PURPOSE: To compare the performance of eight metabolic indices for the early assessment of tumour response in patients with metastatic colorectal cancer (mCRC) treated with chemotherapy. METHODS: Forty patients with advanced mCRC underwent two FDG PET/CT scans, at baseline and on day 14 after chemotherapy initiation. For each lesion, eight metabolic indices were calculated: four standardized uptake values (SUV) without correction for the partial volume effect (PVE), two SUV with correction for PVE, a metabolic volume (MV) and a total lesion glycolysis (TLG). The relative change in each index between the two scans was calculated for each lesion. Lesions were also classified as responding and nonresponding lesions using the Response Evaluation Criteria In Solid Tumours (RECIST) 1.0 measured by contrast-enhanced CT at baseline and 6-8 weeks after starting therapy. Bland-Altman analyses were performed to compare the various indices. Based on the RECIST classification, ROC analyses were used to determine how accurately the indices predicted lesion response to therapy later seen with RECIST. RESULTS: RECIST showed 27 responding and 74 nonresponding lesions. Bland-Altman analyses showed that the four SUV indices uncorrected for PVE could not be used interchangeably, nor could the two SUV corrected for PVE. The areas under the ROC curves (AUC) were not significantly different between the SUV indices not corrected for PVE. The mean SUV change in a lesion better predicted lesion response without than with PVE correction. The AUC was significantly higher for SUV uncorrected for PVE than for the MV, but change in MV provided some information regarding the lesion response to therapy (AUC >0.5). CONCLUSION: In these mCRC patients, all SUV uncorrected for PVE accurately predicted the tumour response on day 14 after starting therapy as assessed 4 to 6 weeks later (i.e. 6 to 8 weeks after therapy initiation) using the RECIST criteria. Neither correcting SUV for PVE nor measuring TLG improved the assessment of tumour response compared to SUV uncorrected for PVE. The change in MV was the least accurate index for predicting tumour response.

Ultra-low-dose in brain 18F-FDG PET/MRI in clinical settings.

We previously showed that the injected activity could be reduced to 1 MBq/kg without significantly degrading image quality for the exploration of neurocognitive disorders in 18F-FDG-PET/MRI. We now hypothesized that injected activity could be reduced ten-fold. We simulated a 18F-FDG-PET/MRI ultra-low-dose protocol (0.2 MBq/Kg, PETULD) and compared it to our reference protocol (2 MBq/Kg, PETSTD) in 50 patients with cognitive impairment. We tested the reproducibility between PETULD and PETSTD using SUVratios measurements. We also assessed the impact of PETULD for between-group comparisons and for visual analysis performed by three physicians. The intra-operator agreement between visual assessment of PETSTD and PETULD in patients with severe anomalies was substantial to almost perfect (kappa > 0.79). For patients with normal metabolism or moderate hypometabolism however, it was only moderate to substantial (kappa > 0.53). SUV ratios were strongly reproducible (SUVratio difference ± SD = 0.09 ± 0.08). Between-group comparisons yielded very similar results using either PETULD or PETSTD. 18F-FDG activity may be reduced to 0.2 MBq/Kg without compromising quantitative measurements. The visual interpretation was reproducible between ultra-low-dose and standard protocol for patients with severe hypometabolism, but less so for those with moderate hypometabolism. These results suggest that a low-dose protocol (1 MBq/Kg) should be preferred in the context of neurodegenerative disease diagnosis.

Dose Reduction in Brain [18F]FDG PET/MRI: Give It Half a Chance.

PURPOSE: Integrated positron emission tomography (PET)/magnetic resonance imaging (MRI) offers promising tools for evaluating brain disorders, including the minimization of exposure to ionizing radiation. Considering the length of scanning time with PET/MRI systems and their high sensitivity, we assumed that the activity could be reduced by one half compared with recommended activity for brain 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) PET exams without degrading image quality. PROCEDURES: We retrospectively simulated the reduction of injected activity (1 vs. 2 MBq/kg, [18F]FDG) in 100 patients assessed for cognitive impairment with simultaneous PET/MRI imaging. A list-mode acquisition was used to generate a 20-min image set as a reference (PETSTD) and to simulate a low-dose injection with a 10-min image (PETLD). We tested the reproducibility between PETLD and PETSTD with a blinded visual interpretation by two nuclear physicians asked to classify metabolic patterns, and a quantitative analysis conducted with regions-of-interest. Voxelwise comparisons between patients suggestive of Alzheimer's disease (AD) and frontotemporal dementia (FTD) were also conducted. RESULTS: The intra-operator agreement was high between the PETSTD and PETLD visual assessments for both readers (kappa 0.92 and 0.99). SUV ratios were strongly reproducible (intraclass correlation coefficient 0.95). The voxelwise and regional comparisons between AD vs. FTD metabolic profiles yielded very similar results with PETSTD and PETLD. CONCLUSIONS: A reduction of the [18F]FDG dose down to 1 MBq/kg is possible when performing 20-min brain PET/MRI without modifying diagnostic performance and quantitative assessments. The advantage is a significant reduction in the patient effective dose, which is non-negligible in longitudinal follow-up studies and in research protocols involving healthy volunteers.

Tumor texture analysis in 18F-FDG PET: relationships between texture parameters, histogram indices, standardized uptake values, metabolic volumes, and total lesion glycolysis.

UNLABELLED: Texture indices are of growing interest for tumor characterization in (18)F-FDG PET. Yet, on the basis of results published in the literature so far, it is unclear which indices should be used, what they represent, and how they relate to conventional indices such as standardized uptake values (SUVs), metabolic volume (MV), and total lesion glycolysis (TLG). We investigated in detail 31 texture indices, 5 first-order statistics (histogram indices) derived from the gray-level histogram of the tumor region, and their relationship with SUV, MV, and TLG in 3 different tumor types. METHODS: Three patient groups corresponding to 3 cancer types at baseline were studied independently: patients with metastatic colorectal cancer (72 lesions), non-small cell lung cancer (24 lesions), and breast cancer (54 lesions). Thirty-one texture indices were studied in addition to SUVs, MV, and TLG, and 5 indices extracted from histogram analysis were also investigated. The relationships between indices were studied as well as the robustness of the various texture indices with respect to the parameters involved in the calculation method (sampling schemes and tumor volume of interest). RESULTS: Regardless of the patient group, many indices were highly correlated (Pearson correlation coefficient |r| ≥ 0.80), making it desirable to focus on only a few uncorrelated indices. Three histogram indices were highly correlated with SUVs (|r| ≥ 0.84). Four texture indices were highly correlated with MV, and none was highly correlated with SUVs (|r| ≤ 0.55). The resampling formula used to calculate texture indices had a substantial impact, and resampling using at least 32 discrete values should be used for texture indices calculation. The texture indices change as a function of the segmentation method was higher than that of peak and maximum SUVs but less than mean SUV for 5 texture indices and was larger than that of MV for 14 texture indices and for the 5 histogram indices. All these results were extremely consistent across the 3 tumor types and explained many of the observations reported in the literature so far. CONCLUSION: None of the histogram indices and only 17 of 31 texture indices were robust with respect to the tumor-segmentation method. An appropriate resampling formula with at least 32 gray levels should be used to avoid introducing a misleading relationship between texture indices and SUV. Some texture indices are highly correlated or strongly correlate with MV whatever the tumor type. Such correlation should be accounted for when interpreting the usefulness of texture indices for tumor characterization, which might call for systematic multivariate analyses.