Does Cancer of Unknown Primary (CUP) Truly Exist as a Distinct Cancer Entity?

Rare axillary cancer of unknown primary originating from the breast of a 64‑year‑old male patient: A case report and literature review

Cancers of unknown primary (CUPs) are a heterogeneous group of tumors characterized by a difficult diagnosis. The primitive tumor remains unknown, whereas metastases are the most common manifestation. Occult male breast cancers are very rare types of CUPs. The present study describes the case of a 64-year-old man affected by a CUP of presumed mammary origin. The aim of the article and the present review was to focus on their management. To the best of our knowledge, only thirteen cases have been reported in the literature. Because no specific guidelines are available, various approaches have been applied, influencing the treatment and the prognosis of patients with CUP.

Beyond boundaries: unraveling innovative approaches to combat bone-metastatic cancers

Evidence demonstrated that bones, liver, and lungs are the most common metastasis sites in some human malignancies, especially in prostate and breast cancers. Bone is the third most frequent target for spreading tumor cells among these organs and tissues. Patients with bone-metastatic cancers face a grim prognosis characterized by short median survival time. Current treatments have proven insufficient, as they can only inhibit metastasis or tumor progression within the bone tissues rather than providing a curative solution. Gaining a more profound comprehension of the interplay between tumor cells and the bone microenvironment (BME) is of utmost importance in tackling this issue. This knowledge will pave the way for developing innovative diagnostic and therapeutic approaches. This review summarizes the mechanisms underlying bone metastasis and discusses the clinical aspects of this pathologic condition. Additionally, it highlights emerging therapeutic interventions aimed at enhancing the quality of life for patients affected by bone-metastatic cancers. By synthesizing current research, this review seeks to shed light on the complexities of bone metastasis and offer insights for future advancements in patient care.

Pathological Fracture of the Proximal Humerus Occurred on Metastases of Probable Kidney Origin in the Absence of Primary Lesions: A Case Report

Cancer of unknown primary (CUP) origin represents a diagnostic and therapeutic challenge. These tumours spread to different parts of the body even if the site of origin has not been identified. When renal metastases are observed without an obvious primary lesion, it is important to exclude the possibility of a primary kidney tumour that may be unknown or too small to be detected. The diagnosis of CUP is established after a careful clinical evaluation and diagnostic tests, including blood chemistry and laboratory tests, instrumental exams (CT, MRI, PET, bone scan), biopsy, and molecular and cytogenetic analysis. Once the diagnosis of CUP with kidney metastases is confirmed, treatment depends on the location of the metastases, the patient's health status, and available treatment options. The latter includes surgery to remove metastases, radiation therapy, or systemic treatment such as chemotherapy or immunotherapy. It is important that patients with CUP are evaluated by a multidisciplinary team of specialists, who can contribute to planning the most appropriate treatment. In this article, we report the clinical case of a patient with a pathological fracture of the proximal humerus which occurred on metastases of probable renal origin in the absence of primary lesions.

Machine learning for genetics-based classification and treatment response prediction in cancer of unknown primary

Cancer of unknown primary (CUP) is a type of cancer that cannot be traced back to its primary site and accounts for 3-5% of all cancers. Established targeted therapies are lacking for CUP, leading to generally poor outcomes. We developed OncoNPC, a machine-learning classifier trained on targeted next-generation sequencing (NGS) data from 36,445 tumors across 22 cancer types from three institutions. Oncology NGS-based primary cancer-type classifier (OncoNPC) achieved a weighted F1 score of 0.942 for high confidence predictions ([Formula: see text]) on held-out tumor samples, which made up 65.2% of all the held-out samples. When applied to 971 CUP tumors collected at the Dana-Farber Cancer Institute, OncoNPC predicted primary cancer types with high confidence in 41.2% of the tumors. OncoNPC also identified CUP subgroups with significantly higher polygenic germline risk for the predicted cancer types and with significantly different survival outcomes. Notably, patients with CUP who received first palliative intent treatments concordant with their OncoNPC-predicted cancers had significantly better outcomes (hazard ratio (HR) = 0.348; 95% confidence interval (CI) = 0.210-0.570; P = [Formula: see text]). Furthermore, OncoNPC enabled a 2.2-fold increase in patients with CUP who could have received genomically guided therapies. OncoNPC thus provides evidence of distinct CUP subgroups and offers the potential for clinical decision support for managing patients with CUP.

Cancers of Unknown Primary: A Descriptive Study in the U.S. Military Health System

INTRODUCTION

Cancers of unknown primary (CUP) are defined as histologically confirmed metastatic cancers that do not have an identified primary site of origin despite an appropriate diagnostic workup. Although accessibility to and quality of medical care influence diagnosis of cancer including CUP, previous studies describing CUP have generally been conducted in patients with various accessibilities to care. This study aimed to describe the demographic, histologic, and temporal trend characteristics of CUP patients in the DoD Cancer Registry of the Military Health System (MHS), which provides universal health care access, reducing the potential effects of accessibility to care on research results.

MATERIALS AND METHODS

The data were obtained from the DoD's Automated Central Tumor Registry (ACTUR), which collects cancer data from beneficiaries who were diagnosed or received treatment in the MHS. We described the demographic and histologic distributions in CUP patients aged 18 years or older diagnosed from 1987 to 2013. We calculated the proportion of CUP patients among all metastatic cancers and the most common histologic categories of those tumors. We then evaluated whether the proportion of histologic types changed over time.

RESULTS

CUP comprised 13.3% of all metastatic cancers in ACTUR during the study period. The majority of CUP within ACTUR was moderately and well-differentiated adenocarcinoma (51.3%) and poorly differentiated carcinomas (23.2%) followed by squamous cell carcinomas (12.5%). The percentages of CUP among metastasized cancers of the same histologic category ranged 12%-15% for moderately and well-differentiated adenocarcinomas, squamous cell, and poorly differentiated carcinomas, and 41%-46% for malignant neuroendocrine carcinomas and undifferentiated neoplasms. However, the percentages varied by sex, race, and age for certain pathologies. The proportion of CUP patients among all metastatic cancer patients has steadily declined from 22.4% to 8.3% from 1987 to 2013.

CONCLUSION

The proportion and trends of CUP in the ACTUR were generally consistent with other descriptive CUP studies. This study provides a description of CUP in a health care system with universal access in the USA and provides a foundation for future studies on CUP.

Utilizing Electronic Health Records (EHR) and Tumor Panel Sequencing to Demystify Prognosis of Cancer of Unknown Primary (CUP) patients

Cancer of unknown primary (CUP) is a type of cancer that cannot be traced back to its original site and accounts for 3-5% of all cancers. It does not have established targeted therapies, leading to poor outcomes. We developed OncoNPC, a machine learning classifier trained on targeted next-generation sequencing data from 34,567 tumors from three institutions. OncoNPC achieved a weighted F1 score of 0.94 for high confidence predictions on known cancer types (65% of held-out samples). When applied to 971 CUP tumors from patients treated at the Dana-Farber Cancer Institute, OncoNPC identified actionable molecular alterations in 23% of the tumors. Furthermore, OncoNPC identified CUP subtypes with significantly higher polygenic germline risk for the predicted cancer type and significantly different survival outcomes, supporting its validity. Importantly, CUP patients who received first palliative intent treatments concordant with their OncoNPC-predicted cancer sites had significantly better outcomes (H.R. 0.348, 95% C.I. 0.210 - 0.570, p-value 2.32 × 10-5). OncoNPC thus provides evidence of distinct CUP subtypes and offers the potential for clinical decision support for managing patients with CUP.

Cancer-of-Unknown-Primary-Origin: A SEER-Medicare Study of Patterns of Care and Outcomes among Elderly Patients in Clinical Practice

Simple Summary

Cancer-of-unknown-primary-origin (CUP) is challenging to diagnose and treat, and little is known about its diagnostic work-up, treatment, and outcomes in routine healthcare. We examined data from elderly patients (at least 66 years old) diagnosed with CUP in real-world US clinical practice, using the Surveillance, Epidemiology, and End Results–Medicare-linked database. Only half of elderly patients with CUP received all three diagnostic work-up procedures (biopsy, immunohistochemistry, and imaging), as recommended by guidelines. Patients who received all three diagnostic work-up procedures were more likely to receive any type of anticancer treatment, and patients who did not receive full diagnostic work-up had shorter median overall survival, particularly with increasing age. Overall, these results suggest that further studies are needed to understand why many patients given a diagnosis of CUP do not receive complete diagnostic work-up or treatment. Further research into improving diagnostic work-up and treatment effectiveness in patients diagnosed with CUP is required.

Abstract

Knowledge of contemporary patterns of cancer-of-unknown-primary-origin (CUP) diagnostic work-up, treatment, and outcomes in routine healthcare is limited. Thus, we examined data from elderly patients diagnosed with CUP in real-world US clinical practice. From the Surveillance, Epidemiology, and End Results–Medicare-linked database, we included patients ≥ 66 years old with CUP diagnosed between 1 January 2013 and 31 December 2015. We analyzed baseline demographics, clinical characteristics, methods of diagnostic work-up (biopsy, immunohistochemistry, imaging), treatment-related factors, and survival. CUP diagnosis was histologically confirmed in 2813/4562 patients (61.7%). Overall, 621/4562 (13.6%) patients received anticancer pharmacotherapy; among these, 97.3% had a histologically confirmed tumor and 83.1% received all three procedures. Among those with a histologically confirmed tumor, increasing age, increasing comorbidity score, not receiving all three diagnostic measures, and having a not-further specified histologic finding of only ‘malignant neoplasm’ were all negatively associated with receipt of anticancer pharmacotherapy. Median overall survival was 1.2 months for all patients. Median time between CUP diagnosis and treatment initiation was 41 days. Limited diagnostic work-up was common and most patients did not receive anticancer pharmacotherapy. The poor outcomes highlight a substantial unmet need for further research into improving diagnostic work-up and treatment effectiveness in CUP.

Treatment of Metastasis in the Appendicular Skeleton

Bone metastases may evolve with events (pain, fractures and compression) that the orthopedic surgeon will encounter regardless of his subspecialty. Accumulated surgical knowledge is predictive for the prevention of impending fractures, as well as of pathological fractures. We will present a guide to properly evaluate and conduct a patient with bone implant for surgeons who are not specialists in this area.

Systematic review of the CUP trials characteristics and perspectives for next-generation studies

BACKGROUND

Research on therapeutic strategies for patients with unknown primary cancer (CUP) has been underwhelming. This paper summarized and evaluated the CUP therapeutic research over the previous five years. Based on this evaluation, recommendations for clinical trial designs are made to improve the impact of CUP research on patients.

METHODS

Published and ongoing research were evaluated. PubMed was searched from January 1, 2015, to November 1, 2021. The start date of 2015 was chosen to identify research published after ESMO issued new diagnostic and therapeutic guidelines. The US National Library of Medicine indexed ongoing clinical trials.

FINDINGS

Of the 244 CUP studies indexed in PubMed, 11.9% were prospective studies, and 4.9% were clinical trials. The review protocol deemed 65 publications eligible for full-text review. Eleven studies evaluating therapeutic regimens were retained. The two prospective studies and non-randomized trials showed promising outcomes for site-specific treatments. Randomized clinical trials were less promising; however, the trials had recruitment challenges resulting in biased accrual and the inability to keep pace with advancing diagnostics and therapeutics. Most of the 35 ongoing studies were phase II single-arm trials assessing immune checkpoint inhibitors (ICI) or site-specific therapies among CUP patients with suspected favorable prognoses.

CONCLUSION

Our evaluation suggests two prospective clinical trial designs that addressed recent study design and recruitment challenges. A visionary approach uses a multi-arm, multistage randomized trial to address rapid advancements in diagnosis and therapy. A pragmatic approach utilizes a single-arm trial with historical controls to overcome comparison group and recruitment challenges.

The Treatment for a Patient with Cancer of Unknown Primary: A Case Report

Background

Cancer of unknown primary (CUP) is metastatic at diagnosis with an unknown primary site, indicating a high degree of malignancy with a poor prognosis. The development and application of targeted therapy and immunotherapy are current research hotspots, which provide additional treatment options for CUP.

Case Presentation

A 36-year-old male presented with pain on the right hip in April 2018. After various examinations, he was diagnosed with CUP. This patient received chemotherapy, immunotherapy, and local radiotherapy in our department. However, the use of radiotherapy after immunotherapy resulted in severe pneumonia.

Conclusion

Compared with traditional treatments, immunotherapy is an effective treatment with fewer side effects and better patient tolerance. However, treating physicians should be still pay special attention to the occurrence of side effects when radiotherapy is combined with immunotherapy.

Local ablative treatment with surgery and/or radiotherapy in single-site and oligometastatic carcinoma of unknown primary

BACKGROUND

Single-site carcinoma of unknown primary (CUP) is recognised as a distinct favourable subtype in the European Society of Medical Oncology (ESMO) classification. There is broad consensus that these patients are candidates for local ablative treatment strategies with surgery and/or radiotherapy, but data on their outcomes are scarce.

PATIENTS AND METHODS

In this study, we have addressed the prospects of cure and prognostic factors in a retrospective cohort of 63 patients who were eligible for local treatment at our centre.

RESULTS

Median event-free (EFS) and overall survival (OS) were 15.6 months and 52.5 months, respectively. Of 61 patients who received local treatment, 20 (32.8%) remained event-free over a median follow-up of 28 months. Baseline clinical parameters including affected organ, number, volume and histology of metastases had no significant impact on prognosis, whereas deleterious TP53 mutations and DNA copy number loss emerged as independent adverse risk factors with respect to EFS. Surgical treatment was associated with improved OS as compared to radiation-based therapy.

CONCLUSION

Our study advocates to pursue localised treatment with surgery and/or radiotherapy whenever feasible and implies that genetic parameters might additionally determine the clinical course of single-site CUP patients.

Cancer of unknown primary in a mare: case report and comparative pathology review

A 25-y-old Percheron mare was admitted to the teaching hospital because of lethargy and intractable dyspnea. Thoracoabdominal ultrasound examination identified severe peritoneal effusion, mild bilateral pleural effusion, and a diffuse pulmonary nodular pattern. Cytology of peritoneal fluid revealed a hypercellular sample with clusters of neoplastic polygonal cells and admixed macrophages. Euthanasia was followed by postmortem examination; marked bi-cavitary effusion was present, and innumerable up to 4-cm diameter, round-to-floriform nodules were diffusely evident throughout serosal surfaces as well as the pulmonary and hepatic parenchyma. Disseminated adenocarcinoma, predominantly affecting lung and liver with widespread serosal implantation, was confirmed on light microscopy. Neoplastic cells had strong immunolabeling for pancytokeratin and lacked immunoreactivity to vimentin, napsin A, and Pax8. Cytokeratin 7 and thyroid transcription factor-1 were non-contributory given absent and inconsistent internal control reactivity, respectively. Such results, combined with the lack of a major mass that would indicate a primary site, were supportive of carcinoma of unknown primary site, which remains a conundrum in human oncology, and is poorly explored in veterinary medicine, mainly as a result of clinical and diagnostic limitations.

Expression of Glycolysis-Related Proteins in Cancer of Unknown Primary Origin

Introduction

Cancer of unknown primary origin (CUP) is defined as metastatic cancer without identification of the primary site. Considering that only 15–20% of patients with CUP show a favorable outcome, identifying biomarkers may help improve the clinical management of patients who do not respond well to conventional therapies. In this context, the study of the metabolic profile of CUP may pave the way to establish new biomarkers and/or therapeutic targets; therefore, this study aimed to characterize the expression of metabolism-related proteins in CUP.

Materials and Methods

The expression of monocarboxylate transporters MCT1, MCT2 and MCT4, their chaperone CD147, the glucose transporter GLUT1 and the pH regulator CAIX was evaluated by immunohistochemistry in a series of 118 CUP patients, and the results were associated with the available clinicopathological information.

Results

The metabolism-related proteins MCT1, MCT4, CD147, GLUT1 and CAIX were expressed in a critical portion of the CUP (approximately 20 to 70%). MCT1 and CD147 were both more frequently expressed in cases with lymph nodes as metastasis dominant sites (p = 0.001) as well as in samples from lymph nodes (p <0.001 and p = 0.002, respectively), while MCT1 expression was more frequently expressed in squamous cell carcinomas (p = 0.045). A higher overall survival was observed in patients with tumors positive for GLUT1 and CAIX expression (p = 0.011 and p = 0.041, respectively), but none of the proteins was an independent prognostic factor for overall survival in multivariable analysis.

Conclusion

The results suggest that a portion of CUPs present a hyperglycolytic phenotype, which is associated with higher overall survival.

A Challenging Task: Identifying Patients with Cancer of Unknown Primary (CUP) According to ESMO Guidelines: The CUPISCO Trial Experience

BACKGROUND

CUPISCO is an ongoing randomized phase II trial (NCT03498521) comparing molecularly guided therapy versus platinum-based chemotherapy in patients newly diagnosed with "unfavorable" cancer of unknown primary (CUP).

MATERIALS AND METHODS

Patients with an unfavorable CUP diagnosis, as defined by the European Society of Medical Oncology (ESMO), and available cancer tissue for molecular sequencing are generally eligible. Potential patients with CUP entering screening undergo a review involving reference histopathology and clinical work-up by a central eligibility review team (ERT). Patients with "favorable" CUP, a strongly suspected primary site of origin, lack of tissue, or unmet inclusion criteria are excluded.

RESULTS

As of April 30, 2020, 628 patients had entered screening and 346 (55.1%) were screen failed. Screen fails were due to technical reasons (n = 89), failure to meet inclusion and exclusion criteria not directly related to CUP diagnosis (n = 89), and other reasons (n = 33). A total of 124 (35.8%) patients were excluded because unfavorable adeno- or poorly differentiated CUP could not be confirmed by the ERT. These cases were classified into three groups ineligible because of (a) histologic subtype, such as squamous and neuroendocrine, or favorable CUP; (b) evidence of a possible primary tumor; or (c) noncarcinoma histology.

CONCLUSION

Experience with CUPISCO has highlighted challenges with standardized screening in an international clinical trial and the difficulties in diagnosing unfavorable CUP. Reconfirmation of unfavorable CUP by an ERT in a clinical trial can result in many reasons for screen failures. By sharing this experience, we aim to foster understanding of diagnostic challenges and improve diagnostic pathology and clinical CUP algorithms.

IMPLICATIONS FOR PRACTICE

A high unmet need exists for improved treatment of cancer of unknown primary (CUP); however, study in a trial setting is faced with the significant challenge of definitively distinguishing CUP from other cancer types. This article reports the authors' experience of this challenge so far in the ongoing CUPISCO trial, which compares treatments guided by patients' unique genetic signatures versus standard chemotherapy. The data presented will aid future decision-making regarding diagnosing true CUP cases; this will have far-reaching implications in the design, execution, and interpretation of not only CUPISCO but also future clinical studies aiming to find much-needed treatment strategies.

Redefining cancer of unknown primary: Is precision medicine really shifting the paradigm?

The concept of Cancer of Unknown Primary (CUP) has evolved with the advent of medical oncology. CUP can be difficult to diagnose and represents 2 to 5% of new cancers, therefore not exceptionally rare. Within CUPs can be identified a subset of favourable prognosis tumours, however the vast majority of CUP patients belongs to a poor prognosis group. CUP features significant oncological challenges, such as unravelling biological and transversal issues, and most importantly, improving patient's outcomes. In that regard, CUP patients' outcomes regrettably showed minimal improvement for decades and CUP remains a cancer group of very poor prognosis. The biology of CUP has two main hypotheses. One is that CUP is a subgroup of a given primary cancer, where the primary is present but cannot be seen due to its small size. The other, the "true" CUP hypothesis, states that CUP share features that make them a specific entity, whatever their tissue of origin. A true biological signature has not yet been described, but chromosomal instability is a hallmark of poor prognosis CUP group. Precision oncology, despite achieving identifying the putative origin of the CUP, so far failed to globally improve outcomes of patients. Targeting molecular pathways based on molecular analysis in CUP management is under investigation. Immunotherapy has not shown ground-breaking results, to date. Accrual is also a crucial issue in CUP trials. Herein we review CUP history, biological features and remaining questions in CUP biology, the two main approaches of molecular oncology in CUP management, in order to draw perspectives in the enormous challenge of improving CUP patient outcomes.

[Cervical CUP syndrome]

The term CUP syndrome (cancer of unknown primary) refers to a multifaceted clinical picture in which the affected patients have lymph node and/or distant metastases without the location of the primary tumor being known. The criteria of CUP syndrome are fulfilled if a histologically or cytologically confirmed malignancy is present without a detectable primary tumor after completion of the primary diagnosis. Due to the usually poor prognosis and the manifold appearance, which often does not allow a standardized therapy, cervical CUP syndrome makes specific demands on clinical and imaging diagnostics. Using modern techniques such as positron emission tomography-computed tomography (PET-CT), imaging plays a key role in the detection of a possible occult primary tumor as well as in staging and prognosis assessment.

The role of molecular profiling in the diagnosis and management of metastatic undifferentiated cancer of unknown primary✰: Molecular profiling of metastatic cancer of unknown primary

Cancer of unknown primary (CUP) refers to metastatic tumors for which the primary tumor of origin cannot be determined at the time of diagnosis, despite extensive clinicopathologic investigations. Molecular profiling is increasingly able to predict a probable primary tumor type for CUP when clinicopathologic workup is inconclusive. Numerous studies have explored the use of various molecular profiling techniques for identification of site/tissue of origin of CUP. These techniques include gene expression profiling utilizing microarray, reverse transcriptase polymerase chain reaction, RNA-sequencing, somatic gene mutation profiling with next-generation DNA sequencing, and epigenomics including DNA methylation profiling. Despite the generally poor prognosis of CUP, a minority of patients can expect to benefit from targeted therapy despite being agnostic to the tissue of origin. Studies have explored the use of various molecular profiling techniques to predict prognostic and therapeutic biomarkers, with the goal of improving outcome for patients with CUP. However, discordant results between non-randomized and randomized clinical trials in evaluating tumor-type specific therapies raise uncertainties of the benefits of molecularly-predicted tissue of origin-based treatment in routine clinical use. Nevertheless, the current overall trend is in favor of using molecular tools to refine the diagnosis and clinical management of patients with CUP. More large-cohort, randomized prospective studies are needed to assess and validate the utility and feasibility of molecular profiling to uncover potentially targetable genetic alterations. These efforts will also yield further biological insights into the biology and pathogenesis of CUP (Graphical Abstract).

Synergism between anti-angiogenic and immune checkpoint inhibitor drugs: A hypothesis

Hepatocellular cancer (HCC) and renal cell cancer (RCC) are singularly resistant to conventional chemotherapy drugs but therapies targeting the supporting stroma have significantly altered their management. Two recent trials combining anti-angiogenic (AA) agents with immune checkpoint inhibitors (ICIs)- the IMbrave150 and IMmotion151 - have reported impressive progress over targeted agents. It has been suggested that bevacizumab, by improving tissue perfusion, changes the immune suppressive tumour microenvironment to an immune stimulatory one where the ICIs can be more effective. This hypothesis proposes an alternative explanation: That bevacizumab, by increasing tissue hypoxia, amplifies the mutational burden of the tumour by stress-induced mutagenesis, creating a hypermutator profile, which is more vulnerable to the ICI drug, atezolizumab. Additionally, ICIs are known to cause hyperprogression in some tumours, and bevacizumab could provide further benefit by starving these rapidly proliferative tumours of blood supply and nutrients.

Brain Metastasis from Unknown Primary Tumour: Moving from Old Retrospective Studies to Clinical Trials on Targeted Agents

Simple Summary

Brain metastases (BMs) are the most common intracranial tumours in adults and occur up to 3–10 times more frequently than primary brain tumours. In up to 15% of patients with BM, the primary tumour cannot be identified. These cases are known as BM of cancer of unknown primary (CUP) (BM-CUP). The understanding of BM-CUP, despite its relative frequency and unfavourable outcome, is still incomplete and clear indications on management are missing. The aim of this review is to summarize current evidence on the diagnosis and treatment of BM-CUP.

Abstract

Brain metastases (BMs) are the most common intracranial tumours in adults and occur up to 3–10 times more frequently than primary brain tumours. BMs may be the cause of the neurological presenting symptoms in patients with otherwise previously undiagnosed cancer. In up to 15% of patients with BMs, the primary tumour cannot be identified. These cases are known as BM of cancer of unknown primary (CUP) (BM-CUP). CUP has an early and aggressive metastatic spread, poor response to chemotherapy, and poor prognosis. The pathogenesis of CUP seems to be characterized by a specific underlying pro-metastatic signature. The understanding of BM-CUP, despite its relative frequency and unfavourable outcome, is still incomplete and clear indications on management are missing. Advances in diagnostic tools, molecular characterization, and target therapy have shifted the paradigm in the approach to metastasis from CUP: while earlier studies stressed the importance of finding the primary tumour and deciding on treatment based on the primary diagnosis, most recent studies focus on the importance of identifying targetable molecular markers in the metastasis itself. The aim of this review is to summarize current evidence on BM-CUP, from the diagnosis and pathogenesis to the treatment, with a focus on available studies and ongoing clinical trials.

The diagnostic challenges of patients with carcinoma of unknown primary

INTRODUCTION

Cancer of unknown primary (CUP) is a disease entity encompassing heterogeneous malignancies without a clinically-detectable anatomical primary. It is usually a poor prognosis malignancy with dismal prognosis where molecular and genetic testing were expected to be a major breakthrough.

AREAS COVERED

In this review, we provide an overview of the advances in the understanding of the carcinogenesis, biology, diagnosis and treatment of patients with CUP. This review focuses on the advantages and inconveniences of immunohistochemistry and CUP classifiers in assessing the progress in the management of CUP.

EXPERT OPINION

CUP classifiers were expected to gradually replace the classical multistep approach in identifying the culprit tumors to guide site-specific therapy. Immunohistochemistry staining led to the prediction of a single tissue of origin in 10.8-51%. CUP classifiers identified the primary site in 61-89% of these cases and were concordant with immunohistochemistry in 57.1-100%. Immunohistochemistry is cheap, fast and broadly available whereas CUP classifiers are less widely available and have not been validated in randomized control trials. The diagnostic recommendations consist of a standard pathology evaluation based on morphology and algorithmic immunohistochemistry assessment. Physicians should weigh in the input of the CUP classifier to the clinical picture and pathology investigations before performing additional investigations.

Sentinel node theory helps tracking of primary lesions of cancers of unknown primary

Background

Sentinel lymph node is the first stop of lymphatic spreading of cancer with known primary. The lymph node metastasis pattern of cancer of unknown primary (CUP) is unclear and has been presumed to follow the same pathway. To test this hypothesis, data of all 716 patients clinically diagnosed as CUP in our center were collected.

Methods

Diagnoses of lymph node metastasis were established by ¹⁸F-FDG PET-CT and/or biopsy pathology. Three hundred and forty-seven cases meeting the criteria were divided into three groups: pathology-confirmed primary with invasive biopsy or surgery of the suspicious lesion (group A, n = 64), primary still unknown even with invasive biopsy or surgery of the suspicious lesion (group B, n = 204), and others with no suspicious lesion or lesions who had not been sampled due to medical or other reasons (group C, n = 79). We assessed the clinicopathological features between these groups, and the relationship between lymph node metastasis pattern and confirmed primary site.

Results

In group A, the primary sites of 61 cases were compatible with sentinel node theory, resulting in a positive predictive value of 95%. No significant differences in age, sex, bone metastasis, or visceral metastasis observed between group A and group B, except that group A had a higher ratio of differentiated carcinoma (94% vs. 77%, P = 0.003).

Conclusion

To our knowledge, this is the first evidence indicating that the majority of clinical CUP cases follow the sentinel node theory to spread in lymph nodes, which helps tracking the primary, especially for differentiated carcinoma.

[CUP in the liver]

Hepatic involvement is one of the most common manifestations in cancer of unknown primary (CUP) syndrome. The most frequent secondary neoplasms of the liver are carcinomas and malignant melanomas. Most common carcinoma metastases are adenocarcinomas originating from the digestive system or metastases of breast and lung carcinomas. Therefore, hepatic CUP syndrome is an exclusion diagnosis. Immunohistochemistry and molecular examinations are an important part of histopathological diagnosis. They do not only serve to identify the tissue of histologically origin or possible primary tumor, but also contribute to the selection of a personalized targeted therapy by detecting so-called druggable targets in the interdisciplinary management.

Progress in refining the clinical management of cancer of unknown primary in the molecular era

Cancer of unknown primary (CUP) is an enigmatic disease entity encompassing heterogeneous malignancies without a detectable primary tumour, despite a thorough diagnostic workup. A minority of patients with CUP (15-20%) can be assigned a putative primary tissue of origin according to clinical and histopathological findings and typically have a more favourable prognosis with the use of corresponding tumour type-specific therapies. Thus, the majority of patients with CUP have disease that cannot be assigned to a culprit primary tumour, are treated with empirical chemotherapy and have a poor prognosis. In the molecular era, the use of (epi)genomic or transcriptomic CUP classifiers and DNA or RNA sequencing offers two, sometimes overlapping, therapeutic strategies: tumour type-specific therapy and biomarker-guided therapy. Published data reveal that the accuracy of site-of-origin predictions made using CUP classifiers ranges between 54% and 98% when compared with the assignment made according to the recommended clinicopathological criteria. These advances have led to promising results in non-randomized prospective studies evaluating the efficacy of tumour type-specific therapy; however, the favourable outcomes were not confirmed in randomized controlled studies comparing this approach with standard empirical chemotherapy. Currently, the evidence supporting the use of biomarker-guided therapies is limited to case reports and small case series. In this Review, we discuss the clinical management of CUP in the era of precision medicine. We focus on the advances in understanding the biology of CUP, the implications for the diagnosis and classification of CUP according to the tissue of origin and the shift away from empirical therapy towards tailored therapy.

Exploring the biological hallmarks of cancer of unknown primary: where do we stand today?

Cancer of unknown primary (CUP) affects a small percentage of the general population. Nonetheless, a substantial number of these patients have a poor prognosis and consequently succumb to their illness within a year of diagnosis. The natural history of CUP is characterised by early metastasis from the unknown primary site, aggressive course and resistance to conventional chemotherapy. Unfortunately, the processes by which this orphan disease originates and progresses have not been fully elucidated and its biology remain unclear. Despite the conceptual progress in genetic and molecular profiling made over the past decade, recognition of the genetic and molecular abnormalities involved in CUP, as well as the identification of the tissue of origin remain unresolved issues. This review will outline the biology of CUP by exploring the hallmarks of cancer in order to rationalise the complexities of this enigmatic syndrome. This approach will help the reader to understand where research efforts currently stand and the pitfalls of this quest.

"Metastatic Cancer of Unknown Primary" or "Primary Metastatic Cancer"?

Cancer of unknown primary (CUP) is an umbrella term used to classify a heterogeneous group of metastatic cancers based on the absence of an identifiable primary tumor. Clinically, CUPs are characterized by a set of distinct features comprising early metastatic dissemination in an atypical pattern, an aggressive clinical course, poor response to empiric chemotherapy and, consequently, a short life expectancy. Two opposing strategies to change the dismal prognosis for the better are pursued. On the one hand, following the traditional tissue-gnostic approach, more and more sophisticated tissue-of-origin (TOO) classifier assays are employed to push identification of the putative primary to its limits with the clear intent of allowing tumor-site specific treatment. However, robust evidence supporting its routine clinical use is still lacking, notably with two recent randomized clinical trials failing to show a patient benefit of TOO-prediction based site-specific treatment over empiric chemotherapy in CUP. On the other hand, with regards to a tissue-agnostic strategy, precision medicine approaches targeting actionable genomic alterations have already transformed the treatment for many known tumor types. Yet, an unmet need remains for well-designed clinical trials to scrutinize its potential role in CUP beyond anecdotal case reports. In the absence of practice changing results, we believe that the emphasis on finding the presumed unknown primary tumor at all costs, implicit in the term CUP, has biased recent research in the field. Focusing on the distinct clinical features shared by all CUPs, we advocate adopting the term primary metastatic cancer (PMC) to denominate a distinct cancer entity instead. In our view, PMC should be considered the archetype of metastatic disease and as such, despite accounting for a mere 2–3% of malignancies, unraveling the mechanisms at play goes beyond improving the prognosis of patients with PMC and promises to greatly enhance our understanding of the metastatic process and carcinogenesis across all cancer types.