The usefulness of [18F]FDG-PET/CT in detecting and managing cancers with unknown primary site depends on histological subtype

We assessed the role of [18F]FDG-PET/CT in identifying and managing cancer of unknown primary site (CUP syndrome). We reviewed [18F]FDG-PET/CT scans of individuals with CUP syndrome recorded in clinical referral letters from 2012 to 2019. We evaluated the identification of primary tumor (PT) by [18F]FDG-PET/CT, according to histological subtype, and the impact on clinical management. The median age was 65 years, 36/64 males (56%). PTs were detected in 28/64 (44%) patients. Detection was significantly lower in patients with squamous cell carcinoma (SCC) than with other histologies combined, p = 0.034. Mean age, mean SUVmax (10.6 ± 6.0) and organ involvement were similar between patients with and without discovered PTs; and between patients with SCC and with other histologies combined. However, those with SCC were less likely than the others to present with multi-lesion involvement, p < 0.001. [18F]FDG-PET/CT interpretations apparently affected treatment of 8/28 (29%) patients with PT detected, and in none of the 35 whose PT was not discovered, p < 0.001. [18F]FDG-PET/CT appeared helpful in detecting PT in almost half the patients with CUP syndrome; the lowest rate was for patients with SCC pathology. PET/CT showed limited overall value in guiding clinical management, however benefited those with discovered PT.


Results
[18F]FDG-PET/CT scans performed during the study period, 64 [18F]FDG-PET/CT included the term "Unknown Primary". Demographic and clinical characteristics of the population are presented in Table 1. The study cohort comprised 36 males and 28 females; the median age was 65 years (range: . The patients had a total number of 145 [18F]FDG-avid lesion sites, with a mean SUVmax of 10.6 (range: 2. 2-27.8). The most common sites of [18F]FDG uptake were the lymph nodes: 39/145 (27%), bones: 24 (17%), liver: 17 (12%), lungs: 17 (12%), regions of the head and neck: 7(5%) and brain: 6 (4%). The remaining 35 sites (23%) included the urogynecological system, esophagus, peritoneum, skin, colon, thyroid and muscles. The median number of sites/ Table 1. Demographic, clinical and radiographic characteristics of the study cohort. a The total number of sites exceeds 100% due to the involvement of more than one site in some patients. b 'Other' includes the urogynecological system, esophagus, peritoneum, skin, colon, thyroid and muscles. c Number of patients with management data is 63, one patient with squamous cell carcinoma and an undetected primary site was lost to follow-up.

Primary tumors (PTs) discovered by [18F]FDG-PET/CT.
[18F]FDG-PET/CT discovered the PT in 28 patients (44%); while in the remaining 36 (56%), the PTs were not located. Table 2 presents the characteristics of the two groups. Differences in mean patient age) 61.2 vs. 61.9 years, p = 0.72), mean SUVmax of the lesions (10.3 vs 10.8, p = 0.08) and the mean number of organs/sites involved by [18F]FDG-avid lesions (2.4 vs. 1.8, p = 0.47) were not statistically different between patients with and without an identified PT. Variations in anatomic organs and sites involved by [18F]FDG-avid metastatic lesions were also similar between patients with and without discovered PTs. The rates of identification of PTs relative to the histologic groups were: 18/30 (60%) of patients with tumors for which the origin was pathologically and immunohistochemically suggested, 4/9 (44%) of adenocarcinomas, 4/14 (29%) of poorly differentiated carcinomas, 1/10 (10%) of SCC and the sole (100%) neuroendocrine carcinoma.
[18F]FDG-PET/CT detection of PTs was significantly worse for the patients with SCC than for the 54 patients with other pathologies, considered as a combined group: 1/10 (10%) vs. 27/54 (50%), p = 0.03. The only patient in whom PT was detected was one of 7 (14.3%) patients with SCC with head and neck findings. Therefore, we decided to examine in more depth the subgroup of patients with SCC pathology and to compare their characteristics to the combined subgroup of patients with pathologies other than SCC (Table 3). Statistically significant differences were not observed between these two subgroups of patients, in mean age (61.7 vs 61.6 years, p = 0.85), mean SUVmax of [18F]FDG -avid lesions (11.3 vs 10.4, p = 0.38) and the mean number of involved organs/sites (1.5 vs 2.2, p = 0.09). Variations in anatomic organs and locations involved by [18F]FDGavid metastatic lesions were similar between patients with SCC and those with the other histologies combined. However, among the patients with SCC, the proportion with multi-lesion spread was substantially lower than for the rest of the cohort: 2/10 (25%) vs. 45/54 (83%), p < 0.001, see Figs. 1, 2 and 3.
The [18F]FDG-PET/CT findings did not appear to affect the clinical management of 55 (87%) of the 63 patients with available data. Treatment was apparently affected in 8/28 (29%) patients with a PT detected by [18F]FDG-PET/CT: seven received chemotherapy that was specific to the diagnosis, and one patient received palliative radiotherapy.
Treatment was apparently not affected by the [18F]FDG-PET/CT scan in any of the 35 for whom the PT was not detected (p < 0.001, Table 2). Therefore, considering the entire cohort, [18F]FDG-PET/CT findings seem to have changed clinical management in 8/63 (13%) patients. Despite the much lower detection rates among patients with SCC, the effect of [18F]FDG-PET/CT findings on clinical management did not appear to differ between these patients and those with other pathologies combined: 1/9 (11%) vs. 7/54 (13%) (p = 1.0, Table 3).
Since tumors for which the origin was pathologically and immunohistochemically suggested comprised the largest subgroup of our cohort, 30 (47%), we compared changes in management between this subgroup of patients and all the other patients combined without a pathologically and immunohistochemically suggested

Discussion
More than one decade ago, a multidisciplinary expert panel of oncologists, radiologists and nuclear physicians recommended the use of [18F]FDG PET in the diagnosis of patients with CUP syndrome 13 . Despite the common use of this imaging technique in this context, data are sparse regarding the characteristics of CUP syndrome for which [18F]FDG-PET/CT is most and least effective. Interestingly, in the current study of patients with CUP syndrome and negative conventional imaging, [18F]FDG-PET/CT detected the PT in only 1 (10%) of the patients with SCC compared to 50% of all those with other pathologies. Nonetheless, the apparent effects of the [18F]FDG-PET/CT findings on clinical management were similar between these two groups: 11% vs. 13%. Thus, surprisingly, the greater detection of PTs in pathologies other than SCC compared to SCC did not have clinical implications.
Our overall rate of tumor detection was 44%, which is within the range of 10%-75% reported in other studies 3,11,[14][15][16][17][18] . While CUP syndrome is a relatively common clinical entity, presentations and histologies are diverse 9,19,20 . Notably, consensus has not been reached as to whether CUP syndrome is simply a group of metastatic tumors with an undetected source, or a distinct entity with its own characteristics and behavior 17,21,22 . Most researchers currently believe that CUP syndrome is a heterogeneous collection of metastatic tumors 23 . Accordingly, treatment strategies have shifted from empiric cytotoxic therapies to identifying the PT and targeting therapy at the tumor type 24 . Importantly, detecting PT sites and additional metastases improves disease staging; this helps define prognosis and can better guide surgical intervention with curative intent 15 . Indeed, several studies have shown longer survival times in patients with CUP syndrome in whom a PT was detected 25,26 .
Sixteen percent of the patients in the current cohort were with SCC. This is higher than the 5% rate of CUP syndrome that was reported in a number of publications 19,27 , but substantially lower than the 57% rate that was reported in another study 11 . Among the reasons for the wide discrepancy in rates are the lack of a standardized definition of CUP syndrome, including the clinical workup and imaging tests required for the diagnosis 3,28 , and the resultant heterogeneity in selection criteria. Of our 10 patients with SCC, 7 (70%) had [18F]FDG-PET/CT uptake in the head and neck. Similarly, head and neck cancers were reported to represent 75% of patients with CUP syndrome and SCC histology 6 . In our series, the PT was detected in one of 7 (14.3%) of our patients with SCC who had head and neck findings. Similarly, Majchrzak et al. reported detection of PT in 17% (7/41) of patients with CUP syndrome of cervical lymph nodes with SCC metastases 29 . This compares with the detection by [18F]FDG-PET/CT of PTs that were not detected by other modalities in 25% of the patients with headand neck metastases in another cohort 30 . Notably, despite our relatively high proportion of patients with SCC, the age and sex distributions were comparable to those reported in other studies of [18F]FDG-PET/CT in CUP syndrome 3 . Further, patients' age, SUVmax of the lesions, and site distribution of [18F]FDG-avid lesions were similar between patients whose PT was and was not detected by [18F]FDG-PET/CT; and between patients with SCC and those with all other pathologies combined. Thus, the distributions of age, involved organ/site and [18F]FDG avidity do not explain the low detection of PTs among our patients with SCC compared to those with other pathologies. Interestingly, our patients with SCC tumors were significantly more likely to present with limited-lesion metastatic spread disease involvement than were patients with the other pathologies combined. We speculate that this finding is due to lower metastatic rates in SCC or to poor [18F]FDG-PET/CT uptake in small SCC metastases, or to a combination of the two. SCC was shown to have a lower ratio of metastases per PT than adenocarcinoma 31 , while [18F]FDG-PET/CT uptake in SCC was shown to be directly correlated to tumor size, and lower in metastatic tumors than in PTs 32 .
Changes in treatment were attributed to [18F]FDG-PET/CT detection of primary sites in 29% of our patients with a newly detected PT. However, considering the entire cohort, including patients for whom the PT was not detected, [18F]FDG-PET/CT apparently affected clinical management in only 13%. This is on the lower end of www.nature.com/scientificreports/ the range of 10-58% (mean 35%) that was reported in a review of 10 studies 15 . That review found that patients with a planned curative treatment for cancers such as breast, ovary and prostate most benefited from the [18F] FDG-PET/CT scan; thus, differences between studies in the types of cancers may explain the large variability in detection rates 15 . While its impact on clinical management may be limited to a subgroup of patients with discovered PT, [18F]FDG-PET/CT may have additional benefits for patients with CUP syndrome. This may explain disparities between studies in the interpretation of the usefulness of [18F]FDG-PET/CT for clinical decisions. Notably, Reinert et al. 11 reported a PT detection rate in only 23% of patients with CUP syndrome, but changes in treatment management in twice the number of patients 11 .
[18F]FDG-PET/CT has been recommended for accurate staging, monitoring of treatment response and follow-up in patients with CUP syndrome who undergo active therapy; and as an alternative to contrast CT in patients with severe iodine dye allergy 12 . Moreover, the use of [18F]FDG-PEFT/CT in place of conventional imaging may lead to earlier diagnosis of the PT and thus facilitate earlier targeted therapy 15 .
We acknowledge several limitations to this retrospective study. Our database search relied on proper documentation of the disease in the [18F]FDG-PET/CT reports and could therefore present an incomplete sample of patients from our institution. We did not have data regarding the workups that patients underwent according to their clinical presentations. Larger studies of patients with various pathologies and tumor sites are needed to better define the role of [18F]FDG-PET/CT in CUP syndrome and to identify the CUP syndrome subtypes whose management is most influenced by [18F]FDG-PET/CT results.

Conclusion
[18F]FDG-PET/CT appeared helpful in detecting PT in almost half the patients with CUP syndrome; the lowest rate was for patients with SCC pathology.
[18F]FDG-PET/CT showed limited overall value in guiding clinical management, however benefited those with discovered PT.

Materials and methods
Study design. We searched the Sheba Medical Center computerized database for [18F]FDG-PET/CT studies that included the term "Unknown Primary" in reports (in the graph of "indication" for the referral) recorded from April 2012 through February 2019. Medical history and tumor histopathology analysis were included in the clinical data. Imaging data were provided from the picture archive and communication system (PACS, Carestream Health 11.0, Rochester, NY), and clinical data from the computerized medical records at Sheba Medical Center.
The study inclusion criterion was an unknown PT according to the clinical referral letter at the performance of the [18F]FDG-PET/CT scan, with or without known tumor histology. For all the patients, the following were performed before [18F]FDG-PET/CT examination: a whole diagnostic workup including a physical examination, CT/MRI or US, and rhino-laryngoscopy in patients with cervical CUP syndrome. Pathological evaluation included immunohistochemical staining for tumor origin.
For this research, tumors were categorized into broad groups based on their histology type: adenocarcinoma, squamous cell carcinoma (SCC), poorly/undifferentiated carcinoma, neuroendocrine carcinoma and tumors in which the origin was pathologically and immunohistochemically suggested. The latter group was relevant when histology results from one of the metastatic lesions indicated a certain tumor origin (including: melanoma, thyroid, peritoneal mesothelioma, sarcoma, thyroid carcinoma, thymoma, ovarian carcinoma, gastrointestinal tumor, pancreatic and biliary carcinoma, carcinoma of breast and lung); however, prior conventional workup did not reveal the primary tumor site. Patients without any available histological data were not included in the study.
Metastatic spread was characterized by variations in anatomic organs or sites involved by [18F]FDG-avid metastatic lesions. Additionally, metastatic spread was evaluated according to the number of [18F]FDG-avid metastatic lesions and classified as a limited-lesion (up to two lesions) or as multi-lesion spread (more than two lesions). Image assessment. An experienced physician with two specializations (nuclear medicine and radiology) reviewed all the scans of all the patients included in the study. The intensity of [18F]FDG uptake in the lesions was calculated by standardized uptake values max (SUVmax), by manually generating a region of interest over the pathological lesion. The protocol of the [18F]FDG-PET/CT scans was similar to those described in previously reported studies 33,34 .
We assessed the impact on clinical management, of PT detection by PET/CT, by examining treatment decisions that were made by a referring physician or by a tumor board, and that were influenced by the identification or non-identification of PT. The performance of additional diagnostic procedures after a [18F]FDG-PET/CT study was not considered a change in management.
Statistical analysis. Data are demonstrated as medians with ranges, or as means with standard deviations for continuous variables; and as percentages for categorical parameters. Correlations between subgroups were analyzed using the T-test for continuous variables, and the chi-square test and Fisher's exact test for categorical variables. The SPSS version 25.0 (SPSS, IBM, USA) was used. A P value of less than 0.05 was considered statistically significant.
Ethics. The institutional review board of Sheba Medical Center approved our single-institution study, and informed consent was waived due to the retrospective design. All the methods were performed in accordance with the relevant guidelines and regulations of Sheba Medical Center.