[18F]fluorothymidine PET imaging in the diagnosis of leptomeningeal involvement with diffuse large B-cell lymphoma

PET/CT in leukemia: utility and future directions

2-Deoxy-2-[ 18 F]fluoro- d -glucose PET/computed tomography ([ 18 F]FDG PET/CT) has proven to be a sensitive method for the detection and evaluation of hematologic malignancies, especially lymphoma. The increasing incidence and mortality rates of leukemia have raised significant concerns. Through the utilization of whole-body imaging, [ 18 F]FDG PET/CT provides a thorough assessment of the entire bone marrow, complementing the limited insights provided by biopsy samples. In this regard, [ 18 F]FDG PET/CT has the ability to assess diverse types of leukemia The utilization of [ 18 F]FDG PET/CT has been found to be effective in evaluating leukemia spread beyond the bone marrow, tracking disease relapse, identifying Richter's transformation, and assessing the inflammatory activity associated with acute graft versus host disease. However, its role in various clinical scenarios in leukemia remains unacknowledged. Despite their less common use, some novel PET/CT radiotracers are being researched for potential use in specific scenarios in leukemia patients. Therefore, the objectives of this review are to provide a thorough assessment of the current applications of [ 18 F]FDG PET/CT in the staging and monitoring of leukemia patients, as well as the potential for an expanding role of PET/CT in leukemia patients.

Whole-body 18-F-FDG-PET in patients with leptomeningeal disease and correlation with MRI

OBJECTIVE

Studies evaluating leptomeningeal disease on whole-body 18F-FDG PET are lacking. The purpose was to evaluate PET imaging of leptomeningeal disease and investigate the incremental utility of newer PET reconstructions in leptomeningeal disease.

METHODS

PET imaging of 56 patients with leptomeningeal disease detected initially on MRI (n = 53) or cytopathology (n = 35) were retrospectively reviewed. Regular 3-dimensional iterative reconstruction (3D IR, n = 56) and advanced reconstruction (AdvRecon, n = 41) PET images were evaluated by readers blinded to clinical and MRI findings for uptake involving cauda equina, posterior fossa and spinal cord. Spinal cord uptake pattern was classified as normal (uptake < liver), uptake = liver, conus uptake > liver, conus and cervical cord uptake > liver and multifocal/diffuse uptake > liver. SUVmax ratios of conus/liver, conus/left atrium and conus/cervical cord were compared between 3D IR and AdvRecon datasets.

RESULTS

Cauda equina uptake was seen in 64% and 78% on 3D IR and AdvRecon; posterior fossa uptake was seen in 52% and 54% on 3D IR and AdvRecon, respectively. Twelve percent had cauda equina or posterior fossa uptake visible only on AdvRecon. On 3D IR, normal spinal cord uptake was most common (27%); on AdvRecon, conus and cervical cord uptake > liver was most common (32%). Seven of 11 patients with normal spinal cord uptake on 3D IR were upgraded to increased uptake on AdvRecon. AdvRecon showed significantly higher conus/liver, conus/blood pool and conus/cervical cord SUVmax ratios (P < 0.0001).

CONCLUSION

Abnormal uptake in cauda equina, posterior fossa and spinal cord uptake are visible on FDG PET in leptomeningeal disease with increased conspicuity advanced PET reconstructions.

Applications of PET in Diagnosis and Prognosis of Leukemia

As a malignant hematopoietic stem cell disease, leukemia remains life-threatening due to its increasing incidence rate and mortality rate. Therefore, its early diagnosis and treatment play a very important role. In the present work, we systematically reviewed the current applications and future directions of positron emission tomography (PET) in patients with leukemia, especially ¹⁸F-FDG PET/CT. As a useful imaging approach, PET significantly contributes to the diagnosis and treatment of different types of leukemia, especially in the evaluation of extramedullary infiltration, monitoring of leukemia relapse, detection of Richter’s transformation (RT), and assessment of the inflammatory activity associated with acute graft versus host disease. Future investigations should be focused on the potential of PET/CT in the prediction of clinical outcomes in patients with leukemia and the utility of novel radiotracers.

Imaging haemopoietic stem cells and microenvironment dynamics through transplantation

Understanding the subclinical pathway to cellular engraftment following haemopoietic stem cell transplantation (HSCT) has historically been limited by infrequent marrow biopsies, which increase the risk of infections and might poorly represent the health of the marrow space. Nuclear imaging could represent an opportunity to evaluate the entire medullary space non-invasively, yielding information about cell number, proliferation, or metabolism. Because imaging is not associated with infectious risk, it permits assessment of neutropenic timepoints that were previously inaccessible. This Viewpoint summarises the data regarding the use of nuclear medicine techniques to assess the phases of HSCT: pre-transplant homoeostasis, induced aplasia, early settling and engraftment of infused cells, and later recovery of lymphocytes that target cancers or mediate tolerance. Although these data are newly emerging and preliminary, nuclear medicine imaging approaches might advance our understanding of HSCT events and lead to novel recommendations to enhance outcomes.

In vivo imaging of cell proliferation in meningioma using 3'-deoxy-3'-[18F]fluorothymidine PET/MRI

PURPOSE

Positron emission tomography (PET) with 3'-deoxy-3'-[¹⁸F]fluorothymidine ([¹⁸F]FLT) provides a noninvasive assessment of tumour proliferation in vivo and could be a valuable imaging modality for assessing malignancy in meningiomas. We investigated a range of static and dynamic [¹⁸F]FLT metrics by correlating the findings with cellular biomarkers of proliferation and angiogenesis.

METHODS

Seventeen prospectively recruited adult patients with intracranial meningiomas underwent a 60-min dynamic [¹⁸F]FLT PET following surgery. Maximum and mean standardized uptake values (SUV_max, SUV_mean) with and without normalization to healthy brain tissue and blood radioactivity obtained from 40 to 60 min summed dynamic images (PET_40-60) and ~ 60-min blood samples were calculated. Kinetic modelling using a two-tissue reversible compartmental model with a fractioned blood volume (V_B) was performed to determine the total distribution volume (V_T). Expressions of proliferation and angiogenesis with key parameters including Ki-67 index, phosphohistone-H3 (phh3), MKI67, thymidine kinase 1 (TK1), proliferating cell nuclear antigen (PCNA), Kirsten RAt Sarcoma viral oncogene homolog (KRAS), TIMP metallopeptidase inhibitor 3 (TIMP3), and vascular endothelial growth factor A (VEGFA) were determined by immunohistochemistry and/or quantitative polymerase chain reaction.

RESULTS

Immunohistochemistry revealed 13 World Health Organization (WHO) grade I and four WHO grade II meningiomas. SUV_max and SUV_mean normalized to blood radioactivity from PET_40-60 and blood sampling, and V_T were able to significantly differentiate between WHO grades with the best results for maximum and mean tumour-to-whole-blood ratios (sensitivity 100%, specificity 94-95%, accuracy 99%; P = 0.003). Static [¹⁸F]FLT metrics were significantly correlated with proliferative biomarkers, especially Ki-67 index, phh3, and TK1, while no correlations were found with VEGFA or V_B. Using Ki-67 index with a threshold > 4%, the majority of [¹⁸F]FLT metrics showed a high ability to identify aggressive meningiomas with SUV_mean demonstrating the best performance (sensitivity 80%, specificity 81%, accuracy 80%; P = 0.024).

CONCLUSION

[¹⁸F]FLT PET could be a useful imaging modality for assessing cellular proliferation in meningiomas.

Usefulness and pitfalls of F-18-FDG PET/CT for diagnosing extramedullary acute leukemia

PURPOSE

It is very important to identify whether there is extramedullary involvement in acute leukemia (AL), especially in those with recurrent disease. This retrospective study aimed to assess the role of (18)F-FDG PET/CT for diagnosing extramedullary AL.

MATERIALS AND METHODS

PET/CT examinations were performed in 9 patients with newly diagnosed AL, and 70 patients suspected to have recurrent AL. All the patients were diagnosed with AL by bone marrow biopsy. The diagnosis of extramedullary lesions was established according to the combination of pathology, physical examination, and imaging techniques including magnetic resonance imaging (MRI) and PET/CT, and/or cerebrospinal fluid (CSF) cytologic testing, and clinical follow-up.

RESULTS

Of the 79 patients, including 34 acute lymphocytic leukemia (ALL) and 45 acute myeloid leukemia (AML) cases, 30 patients were diagnosed with extramedullary AL. (18)F-FDG PET/CT demonstrated (18)F-FDG positive lesions in the extramedullary regions in 42 patients. Among them, 28 patients were diagnosed to have extramedullary AL and the other 14 were diagnosed with non-hematological malignancies (false positive disease). The sensitivity, specificity, and accuracy of (18)F-FDG PET/CT in diagnosing extramedullary involvement of AL were 93.3% (28/30), 71.4% (35/49), and 79.7%, respectively. The (18)F-FDG uptake of lesions was not significantly different between extramedullary AL and false positive cases (SUVmax: 6.66 ± 2.65 vs. 5.85 ± 1.88, t=1.275, P=0.206). The FDG uptake of extramedullary AL between ALL and AML were also not significantly different (SUVmax: 7.01 ± 2.82 vs. 6.10 ± 2.29, t=1.332, P=0.188). The predominant locations of extramedullary AL were the spleen, soft tissue, lymph nodes, central nerve system, liver, testis, and kidney. A total of 48.2% (27/56) of extramedullary AL lesions presented as diffuse FDG uptake compared with 6.25% (1/16) in the false positive lesions (χ(2)=9.221, P=0.002).

CONCLUSION

(18)F-FDG PET/CT is a sensitive, but not specific imaging modality for diagnosing extramedullary AL. Diffuse (18)F-FDG uptake in extramedullary lesions may indicate leukemia involvement.

Fluorothymidine PET/CT in neurolymphomatosis

Neurolymphomatosis, a rare extranodal lymphoma, may involve the cranial nerves, peripheral nerves, spinal nerve roots, or nerve plexus. F-FDG PET/CT however shows increased FDG uptake in the nerves but is not specific for malignancy. We report a patient with neurolymphomatosis who underwent F-FDG PET/CT and F-fluorothymidine PET/CT where both the scans showed increased tracer uptake in the right sciatic nerve, proven later as neurolymphomatosis on nerve biopsy. Furthermore, the F-FDG avid mediastinal lymph nodes did not show F-fluorothymidine avidity, suggesting an inflammatory etiology.

PET imaging of proliferation with pyrimidines

Several new tracers are being developed for use with PET to assess pathways that are altered in cancers, including energy use, cellular signaling, transport, and proliferation. Because increased proliferation is a hallmark of many cancers, several tracers have been tested to track the DNA synthesis pathway. Thymidine, which is incorporated into DNA but not RNA, has been used in laboratory studies to measure tumor growth. Because thymidine labeled with (11)C undergoes rapid biologic degradation and has a short physical half-life, tracers labeled with (18)F have been preferred in PET imaging. One such tracer is (18)F-labeled 3'-deoxy-3'-fluorothymidine ((18)F-FLT). (18)F-FLT is trapped after phosphorylation by thymidine kinase 1, whose expression is increased in replicating cells. Several studies on breast, lung, and brain tumors have demonstrated that retention of (18)F-FLT correlated with tumor proliferation. Although (18)F-FLT has been used to image and stage several tumor types, the standardized uptake value is generally lower than that obtained with (18)F-FDG. (18)F-FLT can be used to image many areas of the body, but background uptake is high in the liver, marrow, and renal system, limiting use in these organs. (18)F-FLT PET imaging has primarily been studied in the assessment of treatment response. Rapid declines in (18)F-FLT retention within days to weeks have been demonstrated in several tumor types treated with cytotoxic drugs, targeted agents, and radiotherapy. Further work is ongoing to validate this approach and determine its utility in the development of new drugs and in the clinical evaluation of standard treatment approaches.