Cancers of unknown primary site(CUPs) represent a heterogeneous group of metastatic tumours for which a standardised diagnostic work-up fails to identify the site of origin at the time of diagnosis. At present, there is no international consensus on the pathogenesis of CUPs: i The primary lesion is recognized and attacked by the autoimmune mechanism, causing the primary lesion to recede on its own; ii The primary lesions are subtle and cannot be detected by the current inspection instruments; iii The location of the primary lesion is relatively special and cannot be reached by current examination methods; iv The primary tumor is not manifested at first, and the primary tumor was removed during the treatment due to drugs, radiation or surgery; v The course of the disease progress rapidly, and the patient die when the primary lesion is not found.
It has been reported that about 50% of CUPs patients were well-differentiated adenocarcinoma, 30% were poorly differentiated or undifferentiated adenocarcinoma, 15% were squamous cell carcinoma, and 5% were undifferentiated malignant tumor [5]. There were approximately 3% of melanomas lack an identifiable primary, otherwise known as melanoma of unknown primary(MUP). Some reports has been published that patients with MUP site seem to present better outcomes compared to those with stage-matched melanoma of known primary (MKP), probably due to higher immunogenicity as reflected in the immunologically mediated primary site regression. As such, MUP patients on immunotherapy probably display better outcomes when compared to the MKP site subset [6]. The pathological types of this study were 25%(8/32) of moderately well-differentiated adenocarcinoma, 21.9%(7/32) of poorly differentiated adenocarcinoma, 25%(8/32) of squamous cell carcinoma, and 12.5(4/32) of undifferentiated carcinoma. In our research, the proportions of squamous cell carcinoma and undifferentiated carcinoma were higher than those reported in previous literature. There are some discrepancies with the literature, which may be related to the small number of cases. This results suggests that adenocarcinoma is more common in metastatic cancers of unknown primary focus. The onset of CUPs is relatively insidious and the clinical symptoms are atypical. The first symptoms are general weakness, loss of appetite, chest tightness, abdominal distention, weight loss and unexplained lymph node enlargement which are often ignored by patients and found accidentally during physical examination or other reasons. The most common sites of involvement are lymph, liver, lung and bone, followed by pleura, peritoneum and brain. Sixteen of the thirty-two patients in this research had enlarged cervical, supraclavicular and abdominal lymph nodes as their first symptom. There were four cases of masses and vague pain in the liver area, three cases of multiple nodules in the lung tissue found by chest CT, low back pain(one case of spinal metastasis and one case of retroperitoneal metastasis), chest tightness and shortness of breath, abdominal distention(seven cases of chest and abdomen with multiple plasma cavity effusion), the origin of which could not be identified after certain clinical and pathological detailed examination.
The treatment of patients with metastatic cancer of unknown primary origin is now considered to be inextricably linked to factors such as the location of the primary tumour, the type of pathology and the stage of the tumour. Therefore, it is important to identify the primary site of the tumour as soon as possible for the patient's prognosis. CT, ultrasound, PET-CT and endoscopy should be perfected as a first step in clinical work. All of the cases in this study, the above examinations were completed, but the primary lesion site could not be found. Several gene expression profiling assays have become commercially available, claiming to blindly and correctly identify a known primary cancer and a likely tissue of origin in patients with 80% of CUPs [7, 8]. These assays are based on mRNA or miRNA RT-PCR or oligonucleotide microarray technologies [9,10,11]. No significant differences in the tumour microRNA expression profile were evident when CUPs metastases biologically assigned to a primary tissue of origin were compared with metastases from typical solid tumours of known origin [12]. These tests may aid in the diagnosis of the putative primary tumour site in some patients. While it is important to seek the primary lesion, treatment should not be delayed by the pursuit of examination results, and the long time and costly tests can add to the financial burden of patients. We believes that efforts should be made in the clinical diagnosis of patients with CUPs and strive to to do the following: i A history of previous malignancy needs to be excluded for the diagnosis of CUPs, and metastatic cancer with a clear primary focus should be ruled out by thorough history inquiry. ii Comprehensive physical examination: including body surface, extremities for lumps, enlarged lymph nodes, superficial body cavities such as the mouth and pharynx,nasal cavity, vagina tract, anus, etc. iii For new admissions, relevant tumour markers should be completed. If prostate-specific antigen(PSA) is elevated, it is often indicative of a tumour from the prostate. Neuron-specific enolase(NSE) is a marker of neuroendocrine tumours. iv .It is the most basic for patients to complete X-ray, B-ultrasound, CT, MRI, PET-CT and other imaging examinations. v Endoscopycan observe the lesion site more directlyandbiopsies can be taken to determine the nature of the lesion. VI Histocytological biopsy is an effective means of identifying the primary focus means.
Up to now, there is no uniform and effective treatment method for CUPs. The main treatment for CUPs is chemotherapy, which varies greatly in terms of regimen, duration, dose and route of administration. Now, more and more immunotherapy is applied to CUPs. Chromosomal instability (CIN) is not a frequent phenomenon in CUP, which may favour immune checkpoint inhibitors(ICI) among patients with CUP. Conversely, these patients present individual gene alterations implicated in immune-evasion and resistance to ICI. Further clinical investigations are needed to provide more information regarding the interplay between CIN, point mutations and the immune system, allowing a better understanding of ICI use in patients with CUP and potentially improving their efficacy [13].
The indications and extent of surgical resection are still controversial and their application is mostly limited. There is no standard for the timing, mode and scope of radiotherapy, but comprehensive treatment has been recognized by scholars [14]. The treatment of unknown primary metastatic cancer should be tailored to local conditions, different sites, different pathological types, different stages and available evidence regarding potential biomarkers in CUP. It has been recently reported that 28% of patients with CUP present one or more predictive biomarkers to immune checkpoint inhibitors(ICI), such as programmed death-ligand 1 (PD-L1) expression on ≥ 5% cancer cells in 22.5%(≥ 1% in 34%) and lymphocytes in 58.7%, microsatellite instability (MSI)-high in 1.8% and tumour mutational burden (TMB) ≥ 17 mutations per megabase in 11.8%. However, these biomarkers are not yet validated. Generally, CUP patients with TMB > 10 mutations per megabase have a trend for better outcomes when treated with ICI [15]. In this study, there were 7 patients who have similar features to known primary tumors. Therefore, the protocol was consistent with the primary tumor. Three female patients were treated with an ovarian cancer chemotherapy regimen due to clinical features similar to ovarian cancer, and two achieved complete response and one achieved partial response after treatment. Two male patients with metastatic adenocarcinoma of bone with elevated serum PSA were treated as metastatic prostate cancer, two patients both achieved complete response. The response to treatment for ovarian and prostate cancer has been very good, the reason why these five patients could achieve good results may be originated from the ovary and prostate. 2 patients were adenocarcinomas with immunohistochemical features of colorectal cancer which could be treated with chemotherapy according to the indications for metastatic colorectal cancer, they got partial response.19 patients received empirical treatment. The empirical chemotherapy regimen is carboplatin combined with epirubicin, etoposide. Patients treated empirically had a poor prognosis [16]. After completion of treatment,3 patients had PR, accounting for only 16% (3/19), another 4 patients were stable and 12 patients had disease progression.Empirical treatment did not show an advantage in this study. Gene expression profiling have also been exploited to determine a diagnosis for CUPs patients. 6patients with CUPs were predicted to have probable bowel cancer based on 64 gene expression profiles. After FOLFOX6 regimen chemotherapy, 3 cases had partial response, 1 case was stable and 2 cases progressed. Compared to imaging and histopathological methods, molecular molecular diagnosis has the advantages of high sensitivity and specificity, objective interpretation of results, etc. A large prospective non-randomised phase II study of 252 patients suggested that survival may be improved by site-specific therapy determined by a gene expression profile assay of the biopsy specimen, particularly for patients with a tissue of origin diagnosis of more responsive tumour types [17]. At the end of the follow-up, of the 32 patients, 2 were in complete remission, 3 were in partial remission, 4 were stable and 29 had progressive disease. Most people with CUPs did not respond well to treatment.
About the clinical diagnosis of CUPs, emphasis should be placed on the interplay of imaging, histopathology and molecular diagnostics to guide and corroborate each other. In the era of targeted therapies, accurate histopathological and molecular classification of tumours is essential, in order to administer the best tailored therapeutic strategy, the classifications based on epigenetic alterations have served this purpose. Indeed, cancer cells are characterized by a massive overall loss of DNA methylation(20–60% overall decrease in 5-methylcytosine), and by the simultaneous acquisition of specific patterns of hypermethylation at CpG islands of certain promoters, which can reversibly or irreversibly alter gene function, thereby contributing to cancer progression [18].
With advances in imaging, histopathology and molecular, the development of CUPs clinical diagnosis will be driven by the combination and complementarity of different tests with advances in imaging, histopathology and molecular diagnostic techniques. This will facilitate the development of clinical diagnosis of CUPs and help more patients to identify the primary site, and achieve precise treatment of cancer.
At the present time, there are significant deficiencies in the available studies comparing site-specific therapy and empiric chemotherapy. These deficiencies include patient accrual problems (oversampling treatment-resistant tumor types and long recruitment), study design limitations (observational and problematic trials), heterogeneity among the CUP classifiers(epigenetic vs. Transcriptomic profiling), and incomparable therapies. The assessment of recently published CUP literature allows to recommend two comprehensive clinical trial designs, a visionary and a pragmatic approach. Both are amenable to implementing the latest diagnostics and therapeutic advances to improve the quality of CUP research and the prognosis of many patients [19].
