Prostate-specific membrane antigen-positron emission tomography (PSMA-PET) of prostate cancer: current and emerging applications

Prostate-specific membrane antigen-positron emission tomography (PSMA-PET) is transforming the management of patients with prostate cancer. In appropriately selected patients, PSMA-PET offers superior sensitivity and specificity compared to conventional imaging (e.g., computed tomography and bone scintigraphy) as well as choline and fluciclovine PET, with the added benefit of consolidating bone and soft tissue evaluation into a single study. Despite being a newly available imaging tool, PSMA-PET has established indications, interpretation guidelines, and reporting criteria, which will be reviewed. The prostate cancer care team, from imaging specialists to those delivering treatment, should have knowledge of physiologic PSMA radiotracer uptake, patterns of disease spread, and the strengths and limitations of PSMA-PET. In this review, current and emerging applications of PSMA-PET, including appropriateness use criteria as well as image interpretation and pitfalls, will be provided with an emphasis on clinical implications.

Current role of PSMA-PET imaging in the clinical management of prostate cancer

Despite the developments of the last few years, metastatic castration-resistant prostate cancer (PC) remains a deadly disease. Until recently, almost all guidelines recommended magnetic resonance imaging (MRI) or computed tomography (CT) for the initial staging and local/systematic recurrence. Positron emission tomography/computed tomography (PET/CT) with prostate-specific membrane antigen (PSMA) at the present stage, emerged as a promising diagnostic imaging tool for PC. PSMA PET/CT alone or in combination with multiparametric magnetic resonance imaging (mpMRI) can improve the detection of clinically significant PC, especially for Prostate Imaging Reporting & Data System (PI-RADS) = 3 lesions. In addition, PSMA PET/CT is more accurate than CT and bone scan for intermediate to high-risk disease at the initial staging. Contrariwise, a negative PET is not useful for surgeons to avoid a pelvic nodal dissection. PET-PSMA imaging is appropriate for prostate-specific antigen (PSA) persistence or PSA rise from undetectable level after radical prostatectomy or for PSA rise above nadir after definitive radiotherapy. Also, it is recommended for patients fit for curative salvage treatment. It should be noted that in patients, candidates for radionuclide therapy with Lutetium-177 (117Lu), a PSMA strong expression from PET/CT at baseline is considered necessary. This review summarizes the evolution of PSMA PET/CT and its current role in the management of PC.

Clinical practice in prostate PET imaging

Positron emission tomography (PET) imaging in prostate cancer has advanced significantly in the past decade with prostate cancer targeted radiopharmaceuticals now playing a growing role in diagnosis, staging, and treatment. This narrative review focuses on the most commonly used PET radiopharmaceuticals in the USA: prostate-specific membrane antigen (PSMA), fluciclovine, and choline. 18F-fluorodeoxyglucose (FDG) is used in many other malignancies, but rarely in prostate cancer. Previous literature is discussed regarding each radiopharmaceutical's utility in the settings of screening/diagnosis, initial staging, biochemical recurrence, advanced disease, and evaluation prior to targeted radiopharmaceutical therapy and radiation therapy. PET imaging has demonstrated utility over traditional imaging in various scenarios; however, there are few head-to-head studies comparing PET radiopharmaceuticals. PSMA radiopharmaceuticals are the newest tracers developed and have unique properties and uses, especially at low prostate-specific antigen (PSA) levels. However, each PET radiopharmaceutical has different properties which can affect image interpretation. Choline and fluciclovine have minimal urinary activity, whereas PSMA agents can have high urinary activity which may affect locoregional disease evaluation. Of the three radiopharmaceuticals, only PSMA is approved for both diagnostic and therapeutic indications with 177Lu-PSMA. A variety of diagnostic PET radiotracers for prostate cancer allows for increased flexibility, especially in the setting of supply chain and medication shortages. For the time being, keeping a diverse group of PET radiopharmaceuticals for prostate cancer is justifiable.

18F-Fluciclovine Uptake in an Incidentally Discovered Meningioma in a Patient With a History of Metastatic Prostate Cancer: A Case Report

Meningiomas are common, benign tumors found in the dural layers of the central nervous system (CNS) that are often found incidentally. 18F-fluciclovine is an amino acid radiotracer that is used to monitor the recurrence of prostate cancer due to its high uptake in prostate cancer cells. This case report outlines a patient with a retinoblastoma tumor suppressor gene (RB1) mutation and prostate cancer metastasis to the bone incidentally noted to have an enhancing, extra-axial mass on a screening MRI. On prior scans, the mass displayed increased 18F-fluciclovine uptake. Because prostate cancer can metastasize to the meninges, especially in older patients with advanced disease, consideration was given to the progression of his oncological disease. However, additional imaging validated the existence and size of the mass, making a meningioma the final diagnosis. Meningeal metastases can be virtually indistinguishable from other CNS tumors, including meningiomas appearing as single or multiple dural-based, enhancing masses, and without prior imaging, further investigation is warranted.

Molecular imaging of bone metastasis

Recent advances in molecularly targeted modular designs for in vivo imaging applications has thrusted open possibilities of investigating deep molecular interactions non-invasively and dynamically. The shifting landscape of biomarker concentration and cellular interactions throughout pathological progression requires quick adaptation of imaging agents and detection modalities for accurate readouts. The synergy of state of art instrumentation with molecularly targeted molecules is resulting in more precise, accurate and reproducible data sets, which is facilitating investigation of several novel questions. Small molecules, peptides, antibodies and nanoparticles are some of the commonly used molecular targeting vectors that can be applied for imaging as well as therapy. The field of theranostics, which encompasses joint application of therapy and imaging, is successfully leveraging the multifunctional use of these biomolecules [[1], [2]]. Sensitive detection of cancerous lesions and accurate assessment of treatment response has been transformative for patient management. Particularly, since bone metastasis is one of the dominant causes of morbidity and mortality in cancer patients, imaging can be hugely impactful in this patient population. The intent of this review is to highlight the utility of molecular positron emission tomography (PET) imaging in the context of prostate and breast bone metastatic cancer, and multiple myeloma. Furthermore, comparisons are drawn with traditionally utilized bone scans (skeletal scintigraphy). Both these modalities can be synergistic or complementary for assessing lytic- and blastic- bone lesions.

Landscape of prostate-specific membrane antigen heterogeneity and regulation in AR-positive and AR-negative metastatic prostate cancer

Tumor expression of prostate-specific membrane antigen (PSMA) is lost in 15-20% of men with castration-resistant prostate cancer (CRPC), yet the underlying mechanisms remain poorly defined. In androgen receptor (AR)-positive CRPC, we observed lower PSMA expression in liver lesions versus other sites, suggesting a role of the microenvironment in modulating PSMA. PSMA suppression was associated with promoter histone 3 lysine 27 methylation and higher levels of neutral amino acid transporters, correlating with 18F-fluciclovine uptake on positron emission tomography imaging. While PSMA is regulated by AR, we identified a subset of AR-negative CRPC with high PSMA. HOXB13 and AR co-occupancy at the PSMA enhancer and knockout models point to HOXB13 as an upstream regulator of PSMA in AR-positive and AR-negative prostate cancer. These data demonstrate how PSMA expression is differentially regulated across metastatic lesions and in the context of the AR, which may inform selection for PSMA-targeted therapies and development of complementary biomarkers.

Addressing the need for more therapeutic options in neuroendocrine prostate cancer

INTRODUCTION

Neuroendocrine prostate cancer (NEPC) is an aggressive form of prostate cancer frequently seen after prolonged treatment of castration resistant prostate cancer (CRPC). NEPC has become increasingly prevalent over the last 20 years, with a poor prognosis caused by a late diagnosis and limited treatment options. Recent advances in PET/CT imaging and targeted radioimmunotherapy are promising, but more research into additional treatment options is needed.

AREAS COVERED

The aim of this review is to analyze the current imaging and treatment options for NEPC, and to highlight future potential treatment strategies. A Pubmed search for 'Neuroendocrine Prostate Cancer' was performed and relevant articles were reviewed.

EXPERT OPINION

The recent FDA approval and success of 177 PSMA Lutetium in CRPC is promising, as 177 Lutetium could potentially be paired with a NEPC specific biomarker for targeted therapy. Recent laboratory studies pairing DLL3, which is overexpressed in NEPC, with 177 Lutetium and new PET agents have showed good efficacy in identifying and treating NEPC. The success of future development of NEPC therapies may depend on the availability of 177 Lutetium, as current supplies are limited. Further research into additional imaging and treatment options for NEPC is warranted.

Targeted Molecular Imaging as a Biomarker in Urologic Oncology

Urologic malignancies constitute a large portion of annually diagnosed cancers. Timely diagnosis, accurate staging, and assessment of tumor heterogeneity are essential to devising the best treatment strategy for individual patients. The high sensitivity of molecular imaging allows for early and sensitive detection of lesions that were not readily detectable using conventional imaging techniques. Moreover, molecular imaging enables the interrogation of molecular processes used in targeted cancer therapies and predicts cancer response to treatment. Here we review the current advancements in molecular imaging of urologic cancers, including prostatic, vesical, renal testicular, and ureteral cancers.

Prostate Cancer-PET Imaging Update

Prostate cancer is the most common non-dermatologic cancer in men, and one of the leading causes of cancer-related mortality. The incidence of prostate cancer increases precipitously after the age of 65 and demonstrates variable aggressiveness, depending on its grade and stage at diagnosis. Despite recent advancements in prostate cancer treatment, recurrence is seen in 25% of patients. Advancements in prostate cancer Positron Emission Tomography (PET) molecular imaging and recent United States Food and Drug Administration (FDA) approvals have led to several new options for evaluating prostate cancer. This manuscript will review the commonly used molecular imaging agents, with an emphasis on Fluorine-18 fluciclovine (Axumin) and PSMA-ligand agents, including their protocols, imaging interpretation, and pitfalls.

An overview of radiolabeled amino acid tracers in oncologic imaging

Molecular imaging has witnessed a great progress in the field of oncology over the past few decades. Radiolabeled amino acid (AA) tracers are particularly helpful in the areas where the utility of 18F-Fluorodeoxyglucose (18F-FDG) positron emission tomography with computed tomography imaging has been limited such as in evaluating brain tumors, neuroendocrine tumors (NETs), and prostate cancer. Radiolabeled AA tracers such as 6-[18F]-L-fluoro-L-3, 4-dihydroxyphenylalanine (18F-FDOPA), 18F-fluoro-ethyl-tyrosine (18F-FET), and 11C-methionine have found wide applications in brain tumors, which, unlike 18F-FDG, concentrate in the tumor tissue to a greater extent than that in normal brain tissue by providing accurate information about tumor volume and boundaries. 18F-FDOPA is also useful in evaluating NETs. Tracers such as 18F-FACBC (Fluciclovine) and anti-1-amino-2-[18F]fluorocyclopentyl-1-carboxylic acid (18F-FACPC) are used in imaging of prostate cancer and provide valuable information of locoregional, recurrent, and metastatic disease. This review highlights AA tracers and their major applications in imaging, viz., in evaluating brain tumors, NETs, and prostate cancer.

Potential Pitfall of Fluciclovine PET/CT in Castrate-Resistant Prostate Cancer With Adrenal Metastasis

ABSTRACT

An 83-year-old man with castrate-resistant prostate cancer underwent an 18 F-fluciclovine PET/CT scan, which was negative for local disease recurrence or locoregional lymphadenopathy, but there were multiple fluciclovine-avid bone metastases. In addition, mildly avid bilateral adrenal nodules were thought to be benign. However, on follow-up PET/CT 10 months later, while on additional therapy with enzalutamide, the bilateral nodules became mass lesions with interval decreased fluciclovine avidity. Adrenal metastases were suspected given their rapid growth, with subsequent CT-guided biopsy revealing metastatic prostate cancer without tumor necrosis. This false-negative case highlights the diagnostic challenge of fluciclovine PET in characterizing adrenal lesions.

F-18 Fluciclovine PET-CT Findings and Pseudoprogression on Immunotherapy

Immunotherapy plays a vital role in the treatment of several types of malignancies. While the molecular targets of immunotherapy differ, the desired goal is to increase the host immune response against neoplastic tissue. This upregulated immune state results in infiltration of the tumor with activated immune cells and may be misinterpreted as disease progression in anatomical and metabolic imaging studies, known as pseudoprogression. We present a case of pseudoprogression demonstrated on a fluoride-18 (F-18) fluciclovine positron emission tomography-computed tomography (PET-CT) scan of an 85-year-old male with metastatic castrate-resistant prostate cancer who underwent treatment with sipuleucel-T. An understanding of the pseudoprogression phenomenon and its manifestations is critical for both treating physicians and imaging specialists to facilitate decision-making regarding treatment.

Confirmation of Ectopic Pancreatic Tissue: A Novel Use for 18 F-Fluciclovine PET

ABSTRACT

This case features a 21-year-old woman with a history of ulcerative colitis who was incidentally found to have a para-jejunal mass when she presented with an ulcerative colitis flare. The mass was further characterized on MR enterography as most likely representing ectopic pancreatic tissue. Given the normal intense pancreatic uptake of 18 F-fluciclovine, PET/CT was subsequently used to confirm that the mass represented ectopic pancreatic tissue.

18F-Fluciclovine Uptake in Intramuscular Injecting Site of Antiandrogen

ABSTRACT

18F-fluciclovine, a radiolabeled amino acid analog, has been approved by US Food and Drug Administration for detecting lesions of biochemical recurrence of prostate adenocarcinoma with PET/CT. However, it is not specific for prostate cancer and has been found to be present in variety of malignant and benign etiologies. We herein present an interesting case of the incidental finding of increasing uptake of 18F-fluciclovine related to intramuscular injection of antiandrogen.

18F-Fluciclovine versus PSMA PET Imaging in Primary Tumor Detection during Initial Staging of High-Risk Prostate Cancer: A Systematic Review and Meta-Analysis

Purpose Fluorine 18 (18F)-fluciclovine and prostate-specific membrane antigen (PSMA) tracers are commonly used for localizing biochemical recurrence of prostate cancer, but their accuracy in primary tumor detection in the initial staging of high-risk prostate cancer has not been established. Materials and Methods A systematic review was performed of the electronic databases for original studies published between 2012 and 2020. Included studies were those in which 18F-fluciclovine or PSMA PET was used for initial staging of patients with high-risk prostate cancer. The diagnostic performance data were collected for primary tumor with histopathologic results as reference standard. The Quality Assessment of Diagnostic Accuracy Studies-2 tool was used for quality appraisal. A random-effects model was used to summarize the effect sizes and to evaluate the difference between two groups. Results Overall, 28 studies met the eligibility criteria, and 17 were included in the meta-analysis (18F-fluciclovine = 4, PSMA = 13). Of these 17 studies, 12 (70%) were judged to have high risk of bias in one of the evaluated domains, and nine studies were deemed to have applicability concerns. The pooled sensitivity, specificity, and diagnostic odds ratio for 18F-fluciclovine versus PSMA were 85% (95% CI: 73%, 92%) versus 84% (95% CI: 77%, 89%) (P = .78), 77% (95% CI: 60%, 88%) versus 83% (95% CI: 76%, 89%) (P = .40), and 18.88 (95% CI: 5.01, 71.20) versus 29.37 (95% CI: 13.35, 64.60) (P = .57), respectively, with no significant difference in diagnostic test accuracy. Conclusion 18F-fluciclovine and PSMA PET demonstrated no statistically significant difference in diagnostic accuracy in primary tumor detection during initial staging of high-risk prostate cancer. Keywords: PET, Prostate, Molecular Imaging-Cancer, Staging Supplemental material is available for this article. © RSNA, 2022.

Mismatched Lesions on 18F-FDG PET and 18F-Fluciclovine PET Images in a Patient With Metastatic Prostate Small Cell Carcinoma

ABSTRACT

A 65-year-old man with fluciclovine-avid metastatic prostate small cell carcinoma with prostate-specific antigen (PSA) 19.4 ng/mL at diagnosis underwent system therapy and subsequent surgery and achieved hormonal response with PSA <0.1 ng/mL. An 18F-fluciclovine PET/CT scan 3 months after surgery was negative for disease. Although PSA remained <0.1 ng/mL, the rising carcinoembryonic antigen prompted an 18F-FDG PET/CT 6 weeks later. It showed multiple hypermetabolic lesions in the prostatectomy bed, liver, and right iliac bone, suggestive of malignant disease. The FDG-avid prostatectomy lesions were further confirmed on MRI. This case demonstrates that FDG PET/CT has a role in suspected metastatic prostate small cell carcinoma with negative fluciclovine PET examination.

Going Nuclear with Amino Acids and Proteins - Basic Biochemistry and Molecular Biology Primer for the Technologist

In recent years, there has been an influx of new tracers into the field of nuclear medicine and molecular imaging. Most of these tracers that have been FDA approved for clinical imaging exploit various mechanisms of protein biochemistry and molecular biology to bring about their actions, such as amino acid metabolism, protein folding, receptor-ligand interactions, and surface transport mechanisms. In this review, we attempt to paint a clear picture of the basic biochemistry and molecular biology of protein structure, translation, transcription, post-translational modifications, and protein targeting, in the context of the various radiopharmaceuticals currently used clinically, all in an easy-to-understand language for entry level technologists in the field. Tracer characteristics, including indications, dosage, injection-to-imaging time, and the logic behind the normal and pathophysiologic biodistribution of these newer molecular tracers, are also discussed.

Initial evaluation of [18F]-FACBC for PET imaging of multiple myeloma

RATIONALE

Multiple myeloma (MM) cells synthesize large amounts of paraproteins, making radiolabeled amino acids promising candidates for PET imaging of MM patients.

METHODS

We compare tumor uptake of the two amino acid analogs [18F]-fluoroethyltyrosine and [18F]-FACBC in a MM xenograft model and show the feasibility of PET imaging with [18F]-FACBC in a MM patient.

RESULTS

Preclinically [18F]-FACBC showed superior performance, mainly due to the uptake via the ASC-system. In a subsequent proof-of-concept investigation [18F]-FACBC PET was performed in a MM patient. It allowed identification of both lesions with and without CT correlate (SUVmean 8.0 or 7.9) based on higher uptake compared to normal bone marrow (SUVmean 5.7). Bone signal was elevated compared to non-MM patients, and, thus [18F]-FACBC potentially allows the assessment of bone marrow infiltration.

CONCLUSION

The FDA/EMA approved PET agent [18F]-FACBC is promising for imaging MM and should be further evaluated in prospective clinical studies.

Identification of the Optimal Cut-Off Value of PSA for Assessing Severity of Disease in [68Ga]Ga-PSMA-11 PET/CT Study in Prostate Cancer Patients after Radical Prostatectomy

Objective: The objective of this study was to identify the optimal cut-off value of prostate specific antigen (PSA) to assess the extent of the disease in [68Ga]Ga-PSMA-11 PET/CT study in patients after radical prostatectomy. Materials and Methods: Retrospective analysis was performed on a group of 215 patients who underwent a [68Ga]Ga-PSMA-11 PET/CT examination because of suspected recurrence after radical prostatectomy. Patients were divided into four groups: 1, no active lesions suggesting recurrence (n = 92); 2, suspected isolated local recurrence (n = 19); 3, oligometastatic disease (n = 82); and 4, polymetastatic disease (n = 22). Results: In group 1, the mean PSA level was 0.962 ng/mL (median: 0.376; min: 0.004; max: 25 ng/mL); in group 2, it was 4.970 ng/mL (median 1.320; min: 0.003; max: 40.350 ng/mL); in group 3, it was 2.802 ng/mL (median: 1.270; min: 0.020; max: 59.670 ng/mL); and in group 4, it was 4.997 ng/mL (median: 3.795; min: 0.007; max 21.110 ng/mL). Statistically significant differences were shown in PSA levels when comparing groups 1 and 2 (p = 0.0025) and groups 3 and 4 (p = 0.0474). The PSA cut-off point for discriminating groups 1 and 2 was 0.831 (sensitivity: 0.684; specificity: 0.772; area under the curve (AUC): 0.775), and for groups 3 and 4, it was 2.51 (sensitivity: 0.682; specificity: 0.780; AUC: 0.720). Conclusions: Our preliminary data suggested that the PSA level has an essential influence on determining the extent of disease in a [68Ga]Ga-PSMA-11 PET/CT study in patients after radical prostatectomy. Identification of the optimal cut-off values for the oligo- and polymetastatic diseases might be helpful in stratifying these patients.

PSMA-Targeting Imaging and Theranostic Agents-Current Status and Future Perspective

In the past two decades, extensive efforts have been made to develop agents targeting prostate-specific membrane antigen (PSMA) for prostate cancer imaging and therapy. To date, represented by two recent approvals of [⁶⁸Ga]Ga-PSMA-11 and [¹⁸F]F-DCFPyL by the United States Food and Drug Administration (US-FDA) for positron emission tomography (PET) imaging to identify suspected metastases or recurrence in patients with prostate cancer, PSMA-targeting imaging and theranostic agents derived from small molecule PSMA inhibitors have advanced to clinical practice and trials of prostate cancer. The focus of current development of new PSMA-targeting agents has thus shifted to the improvement of in vivo pharmacokinetics and higher specific binding affinity with the aims to further increase the detection sensitivity and specificity and minimize the toxicity to non-target tissues, particularly the kidneys. The main strategies involve systematic chemical modifications of the linkage between the targeting moiety and imaging/therapy payloads. In addition to a summary of the development history of PSMA-targeting agents, this review provides an overview of current advances and future promise of PSMA-targeted imaging and theranostics with focuses on the structural determinants of the chemical modification towards the next generation of PSMA-targeting agents.

Biological Utility of Fluorinated Compounds: from Materials Design to Molecular Imaging, Therapeutics and Environmental Remediation

The applications of fluorinated molecules in bioengineering and nanotechnology are expanding rapidly with the controlled introduction of fluorine being broadly studied due to the unique properties of C-F bonds. This review will focus on the design and utility of C-F containing materials in imaging, therapeutics, and environmental applications with a central theme being the importance of controlling fluorine-fluorine interactions and understanding how such interactions impact biological behavior. Low natural abundance of fluorine is shown to provide sensitivity and background advantages for imaging and detection of a variety of diseases with ¹⁹F magnetic resonance imaging, ¹⁸F positron emission tomography and ultrasound discussed as illustrative examples. The presence of C-F bonds can also be used to tailor membrane permeability and pharmacokinetic properties of drugs and delivery agents for enhanced cell uptake and therapeutics. A key message of this review is that while the promise of C-F containing materials is significant, a subset of highly fluorinated compounds such as per- and polyfluoroalkyl substances (PFAS), have been identified as posing a potential risk to human health. The unique properties of the C-F bond and the significant potential for fluorine-fluorine interactions in PFAS structures necessitate the development of new strategies for facile and efficient environmental removal and remediation. Recent progress in the development of fluorine-containing compounds as molecular imaging and therapeutic agents will be reviewed and their design features contrasted with environmental and health risks for PFAS systems. Finally, present challenges and future directions in the exploitation of the biological aspects of fluorinated systems will be described.

PSA and PSA Kinetics as Predictors for 18F-Fluciclovine PET/CT Positivity in Biochemically Recurrent Prostate Cancer

INTRODUCTION

18F-Fluciclovine PET/CT is one of the imaging techniques currently employed to restage prostate cancer (PCa). Due to the conflicting results reported in the literature, it is not yet known at what PSA threshold 18F-fluciclovine PET/CT could reliably demonstrate the presence of recurring disease. We explored the association between 18F-fluciclovine PET/CT positivity and prescan PSA, PSA doubling time, and PSA velocity in patients with biochemical recurrence (BCR) of PCa after curative-intent treatment.

METHODS

Data from 59 patients who underwent 18F-fluciclovine PET/CT for BCR after radical prostatectomy or radiotherapy were retrieved from a single institution database. Patients already undergone salvage treatments at the time of PET/CT, with newly diagnosed PCa or with initial diagnosis of metastatic PCa were excluded. A 2-sided independent samples Bayesian t test and Bayesian Mann-Whitney U test were used to assess the association between PET/CT and prescan PSA, PSA doubling time, and PSA velocity.

RESULTS

Evidence for no difference between PET/CT-positive and -negative patients for log-transformed PSA was found (BF01 3.61, % error: 0.01). Robustness check and sequential analysis showed stability across a wide range of prior distribution specifications. The hypothesis of no difference in terms of PSA-dt and for PSA-vel between groups was found to be more likely compared to the alternative hypothesis (BF01 of 3.44 and 3.48, respectively).

CONCLUSION

PSA and PSA kinetics are unlikely to be associated with 18F-fluciclovine PET/CT positivity in patients with BCR, and none of these serum biomarkers might be used as single predictors of PET/CT detection. Larger studies might be needed to evaluate the role of different predictors.

18F-Fluciclovine Avid Axillary Lymph Nodes After COVID-19 Vaccination on PET/CT for Suspected Recurrence of Prostate Cancer

Abnormally increased ¹⁸F-FDG avidity of axillary lymph nodes has become a frequent diagnostic dilemma on PET/CT in the current climate of global vaccinations directed against severe acute respiratory syndrome coronavirus 2. This avidity is due to the inflammatory response evoked by vaccines and the nonspecific nature of ¹⁸F-FDG uptake, which is increased in both malignant and inflammatory processes. Similarly, ¹⁸F-fluciclovine, an amino acid analog indicated for the assessment of biochemical recurrence of prostate cancer, may also demonstrate nonspecific inflammatory uptake. We report a case of ¹⁸F-fluciclovine PET/CT obtained for concern about prostate cancer. In this case, isolated avid lymph nodes were seen in the left axilla. A screening questionnaire revealed that the patient had recently received the second dose of the Pfizer-BioNTech coronavirus disease 2019 vaccine in his left shoulder, and hence, the uptake was determined to be reactive.

Incidental Serous Cystadenoma Presenting as a Photopenic Pancreatic Defect on 18F-Fluciclovine PET/CT

ABSTRACT

18F-fluciclovine (Axumin) PET/CT has been widely used for the evaluation of biochemically recurrent prostate cancer following prior treatment. While lymph node and visceral organ metastases typically show increased radiotracer uptake, altered patterns of normal physiologic activity may also provide insight into other disease processes. We present a case of an incidental pancreatic head mass presenting as a photopenic defect on a staging 18F-fluciclovine PET/CT, which was subsequently confirmed to be a benign serous cystadenoma using multisequence MRI.

The lymphatic system and sentinel lymph nodes: conduit for cancer metastasis

The lymphatic system is a complicated system consisting of the lymphatic vessels and lymph nodes draining the extracellular fluid containing cellular debris, excess water and toxins to the circulatory system. The lymph nodes serve as a filter, thus, when the lymph fluid returns to the heart, it is completely sterile. In addition, the lymphatic system includes the mucosa-associated lymphoid tissue, such as tonsils, adenoids, Peyers patches in the small bowel and even the appendix. Taking advantage of the drainage system of the lymphatics, cancer cells enter the lymphatic vessels and then the lymph nodes. In general, the lymph nodes may serve as a gateway in the majority of cases in early cancer. Occasionally, the cancer cells may enter the blood vessels. This review article emphasizes the structural integrity of the lymphatic system through which cancer cells may spread. Using melanoma and breast cancer sentinel lymph node model systems, the spread of early cancer through the lymphatic system is progressive in a majority of cases. The lymphatic systems of the internal organs are much more complicated and difficult to study. Knowledge from melanoma and breast cancer spread to the sentinel lymph node may establish the basic principles of cancer metastasis. The goal of this review article is to emphasize the complexity of the lymphatic system. To date, the molecular mechanisms of cancer spread from the cancer microenvironment to the sentinel lymph node and distant sites are still poorly understood and their elucidation should take major priority in cancer metastasis research.

PET/CT-Based Salvage Radiotherapy for Recurrent Prostate Cancer After Radical Prostatectomy: Impact on Treatment Management and Future Directions

Biochemical recurrence is a clinical situation experienced by 20 to 40% of prostate cancer patients treated with radical prostatectomy (RP). Prostate bed (PB) radiation therapy (RT) remains the mainstay salvage treatment, although it remains non-curative for up to 30% of patients developing further recurrence. Positron emission tomography with computed tomography (PET/CT) using prostate cancer-targeting radiotracers has emerged in the last decade as a new-generation imaging technique characterized by a better restaging accuracy compared to conventional imaging. By adapting targeting of recurrence sites and modulating treatment management, implementation in clinical practice of restaging PET/CT is challenging the established therapeutic standards born from randomized controlled trials. This article reviews the potential impact of restaging PET/CT on changes in the management of recurrent prostate cancer after RP. Based on PET/CT findings, it addresses potential adaptation of RT target volumes and doses, as well as use of androgen-deprivation therapy (ADT). However, the impact of such management changes on the oncological outcomes of PET/CT-based salvage RT strategies is as yet unknown.

18F-fluciclovine PET/CT detection of biochemical recurrent prostate cancer in patients with PSA levels <2.00 ng/mL

OBJECTIVE

To establish the detection rate of prostate cancer recurrence following definitive therapy by 18F-fluciclovine PET/computed tomography (CT) in patients with biochemical recurrence (BCR) and prostate-specific antigen (PSA) levels less than 2.00 ng/mL.

METHODS

In this retrospective study, 78 patients with a PSA level of less than 2.00 ng/mL were selected from the 211 patients who underwent at least one 18F-fluciclovine PET/CT scan at our institution for the detection of biochemical recurrent prostate cancer between April 2017 and December 2018. Inherent differences in the characteristics of patients with and without a positive scan were investigated for possible associations using multivariable analysis.

RESULTS

One or more positive sites of recurrence were identified in 44 out of 78 patients (56.4%). Patients with a Gleason score between 8 and 10 were more likely to have a positive scan compared to patients with Gleason scores of 6-7 [adjusted odds ratio: 3.53, 95% confidence interval (1.13-10.99), P = 0.03]. No other significant association was found between PSA, T classification, and detection rate.

CONCLUSION

18F-fluciclovine PET/CT demonstrated a detection rate of 56.4% among patients with a PSA below 2.0 ng/mL. The results of this study support the use of 18F-fluciclovine PET/CT for the detection of recurrent prostate cancer at lower PSA levels, even at PSA levels less than 0.5 ng/mL.

Pictorial essay: incidental findings on 18F-Fluciclovine PET/CT scan

Introduction

Fluorine-18 (18F) Fluciclovine (anti-1-amino-3-18F-fluorocyclobutane- 1-carboxylic acid [FACBC]) is a synthetic amino acid labeled with 18F, currently used as PET radiopharmaceutical to investigating prostate cancer, namely in the recurrent setting.

Fluciclovine is transported to cell membranes by amino acid transporters, such as LAT1 and ASCT2. The upregulation of LAT-1 and ASCT2 activities is typical of prostate cancer but is also present in other pathological conditions such as non-prostatic neoplasms (e.g., lung cancer) and in benign inflammatory process (e.g., benign prostatic hyperplasia, chronic prostatitis, high-grade prostatic hyperplasia intraepithelial).

Methods

In this short essay we present a retrospective FACBC PET/CT analysis consisting of a selection of the five most relevant cases of patients referred in our centre to FACBC PET/CT for prostate cancer, with concomitant FACBC uptake in sites atyipical for prostate cancer.

Results

These five selected cases demonstrate FACBC uptake at the level of the pancreatic head, adrenal incidentalomas, pulmonary nodules, mediastinal lymph nodes and neoformative tissue of the rectal wall.

Discussion

Clinical cases selected in this pictorial essay have demonstrated that Fluciclovine is not an exclusive and specific radiotracer for prostate cancer and, therefore, can induce misdiagnosis. In fact, incidental benign and malignant uptake might occur and should be further evaluated with clinical correlation or other imaging.

18F-fluciclovine (Axumin) PET/CT detecting occult bone metastasis

Background

18F-fluciclovine (Axumin) positron emission tomography (PET) is a novel agent for detection of biochemical recurrence of the prostate cancer. In this article, we present an interesting case of prostate cancer recurrence with bone metastasis not visualized with conventional CT, but detected with Axumin PET/CT. This case report will help reader to understand shortcomings of conventional modalities (CT/MRI/bone scan) in evaluation of biochemical recurrence of prostate cancer.

Case presentation

We report a case of 73-year-old male previously diagnosed with adenocarcinoma of prostate, status post-radiation treatment with excellent response to therapy. He presented with progressively increasing right groin pain and rising PSA level indicating biochemical recurrence. A bony metastasis to right inferior pubic ramus was diagnosed with Axumin PET/CT, not detected with conventional CT. He underwent local radiation treatment with excellent response to therapy.

Conclusion

18F-fluciclovine (Axumin) PET/CT is the most sensitive tool in detection of biochemical recurrence of prostate cancer. CT and MRI may not detect early cases of tumor recurrence which affects disease prognosis and patient management.

Disparities in PET Imaging for Prostate Cancer at a Tertiary Academic Medical Center

The purpose of this study was to evaluate differences between patients receiving ¹⁸F-fluciclovine and ⁶⁸Ga-prostate-specific membrane antigen (⁶⁸Ga-PSMA-11) for biochemically recurrent prostate cancer at a tertiary medical center. Methods: All ¹⁸F-fluciclovine and ⁶⁸Ga-PSMA-11 PET studies performed at the University of California San Francisco from October 2015 to January 2020 were reviewed. Age, race/ethnicity, primary language, body mass index, insurance type, and home address were obtained through the electronic medical record. A logistic regression model was used to evaluate the predictor variables. Results: In total, 1,502 patients received ⁶⁸Ga-PSMA-11 and 254 patients received ¹⁸F-fluciclovine. Black patients had increased odds of receiving imaging with ¹⁸F-fluciclovine versus ⁶⁸Ga-PSMA-11 compared with non-Hispanic White patients (odds ratio, 3.88; 95% CI, 1.90-7.91). There were no other statistically significant differences. Conclusion: In patients receiving molecular imaging for prostate cancer at a single U.S. tertiary medical center, access to ⁶⁸Ga-PSMA-11 for Black patients was limited, compared with non-Hispanic White patients, by a factor of nearly 4.

Prostate-specific Membrane Antigen PET: Therapy Response Assessment in Metastatic Prostate Cancer

Therapy response assessment is a critical step in cancer management, leading clinicians to optimize the use of therapeutic options during the course of the disease. Imaging is a pivotal biomarker for therapy response evaluation in oncology and has gained wider use through the development of reproducible data-based guidelines, of which the Response Evaluation Criteria in Solid Tumors is the most successful example. Disease-specific criteria have also been proposed, and the Prostate Cancer Working Group 3 criteria are the mainstay for prostate cancer (PC). However, conventional imaging evaluation in metastatic PC has several limitations, including (a) the inability to detect small-volume disease, (b) the high prevalence of bone (nonmeasurable) lesions at imaging, and (c) the established role of serum prostate-specific antigen (PSA) levels as the biomarker of choice for response assessment and disease progression. In addition, there are an increasing number of newer treatment options with various effects on imaging features. Prostate-specific membrane antigen (PSMA) PET has improved patient selection for newer treatments, such as metastasis-directed therapy (MDT) or radionuclide therapy. The role of PSMA PET in response assessment for many metastatic PC therapeutic options (MDT, androgen deprivation therapy, chemotherapy, radionuclide therapy, and immunotherapy) is an evolving issue, with emerging data showing good correlation with PSA levels and clinical outcome. However, there are specific implications of each therapy (especially androgen deprivation therapy and immunotherapy) on PSMA expression by PC cells, leading to potential pitfalls and inaccuracies that must be known by radiologists. Despite some limitations, PSMA PET is addressing gaps left by conventional imaging methods (eg, CT and bone scanning) and nonimaging biomarkers (PSA levels) in metastatic PC therapy response assessment, a role that can be improved with advances like refinement of interpretation criteria and whole-body tumor burden quantification.^© RSNA, 2020See discussion on this article by Barwick and Castellucci.

Deep learning detection of prostate cancer recurrence with 18F-FACBC (fluciclovine, Axumin®) positron emission tomography

PURPOSE

To evaluate the performance of deep learning (DL) classifiers in discriminating normal and abnormal ¹⁸F-FACBC (fluciclovine, Axumin®) PET scans based on the presence of tumor recurrence and/or metastases in patients with prostate cancer (PC) and biochemical recurrence (BCR).

METHODS

A total of 251 consecutive ¹⁸F-fluciclovine PET scans were acquired between September 2017 and June 2019 in 233 PC patients with BCR (18 patients had 2 scans). PET images were labeled as normal or abnormal using clinical reports as the ground truth. Convolutional neural network (CNN) models were trained using two different architectures, a 2D-CNN (ResNet-50) using single slices (slice-based approach) and the same 2D-CNN and a 3D-CNN (ResNet-14) using a hundred slices per PET image (case-based approach). Models' performances were evaluated on independent test datasets.

RESULTS

For the 2D-CNN slice-based approach, 6800 and 536 slices were used for training and test datasets, respectively. The sensitivity and specificity of this model were 90.7% and 95.1%, and the area under the curve (AUC) of receiver operating characteristic curve was 0.971 (p < 0.001). For the case-based approaches using both 2D-CNN and 3D-CNN architectures, a training dataset of 100 images and a test dataset of 28 images were randomly allocated. The sensitivity, specificity, and AUC to discriminate abnormal images by the 2D-CNN and 3D-CNN case-based approaches were 85.7%, 71.4%, and 0.750 (p = 0.013) and 71.4%, 71.4%, and 0.699 (p = 0.053), respectively.

CONCLUSION

DL accurately classifies abnormal ¹⁸F-fluciclovine PET images of the pelvis in patients with BCR of PC. A DL classifier using single slice prediction had superior performance over case-based prediction.

Isolated Hepatic Metastasis of Prostate Cancer with Variable 18F-fluociclovine Uptake by PET/CT Imaging

A 74-year-old man presented with rapid rising prostate-specific antigen (PSA) 2 years after treatment of prostate cancer with prostatectomy and salvage radiation therapy. PSA increased from 923 to 4349 ng/mL within 2 months. No osseous metastatic lesions of prostate cancer were detected by ¹⁸F-sodium fluoride PET/CT imaging at an outside facility. ¹⁸F-fluciclovine PET/CT imaging was performed to evaluate local recurrence of prostate cancer at surgical bed of prostatectomy and distant metastasis. One small focus of low-level ¹⁸F-fluciclovine radiotracer uptake was noted in the surgical bed of prostatectomy without corresponding soft tissue mass on CT. No fluciclovine-avid lymph nodes or osseous metastatic lesions were detected, but multiple hypodense lesions of variable ¹⁸F-fluciclovine radiotracer uptake were noted in the liver, concerning for isolated liver metastasis of prostate cancer. The patient underwent docetaxel chemotherapy for treatment of prostate cancer liver metastasis and showed a favorable response to treatment by significant decreased size of the hypodense lesions in the liver on post treatment abdominal CT, along with dramatic reduction of PSA level and improvement of liver function. The findings from this case highlight the importance of checking hypoattenuating lesions in the liver for the presence of prostate cancer metastatic lesions that might appear similar to other benign hypoattenuating lesions of low fluciclovine uptake relative to physiological ¹⁸F-fluciclovine uptake in the normal liver tissues, a potential pitfall at interpretation of ¹⁸F-fluociclovine PET/CT imaging.

Axumin Positron Emission Tomography: Novel Agent for Prostate Cancer Biochemical Recurrence

Prostate cancer remains one of the top common cancers in terms of incidence and cancer-related deaths. Approximately 1/3rd cases develop biochemical recurrence during surveillance post-definite therapy. Multiple imaging modalities, including computed tomography (CT), magnetic resonance imaging (MRI) (including multiparametric prostate MRI), bone scan, and positron emission tomography (PET) using different tracers are being used for the characterization of the prostate cancer recurrence. CT and MRI do not provide physiological information, thus have lower sensitivity in detecting the metastasis. A bone scan has low sensitivity (depending on the prostate-specific antigen level) with low specificity as well. Among different PET tracers, Axumin PET appears to be the most promising tool. Axumin PET is Food and Drug Administration approved for the evaluation of prostate cancer biochemical recurrence. Several studies have shown that Axumin PET findings played a key role in treatment modification by finding otherwise undetected lesions. We briefly discuss the salient characteristics, imaging protocol and image interpretation criteria for Axumin PET in the workup of prostate cancer biochemical recurrence.