Primary nuclear protein in testis (NUT) carcinoma is a rare, aggressive, and malignant epithelial cell tumor characterized by an acquired rearrangement of the gene encoding NUT on chromosome 15q14 (1). NUT carcinoma is rare in clinical practice and arises preferentially in children and adolescents. It is most commonly seen in midline anatomical structures such as the mediastinum, head and neck, previously known as NUT carcinoma, but it can also involve non-midline sites such as the lung, kidney, bladder, and iliac bone (2–4). Due to the lack of characteristic histopathological features, it is frequently missed and misdiagnosed. In recent years, the application of anti-NUT immunohistochemical staining has improved the rate of accurate diagnosis. Primary pulmonary NUT carcinoma is very rare, and clinical diagnosis and differential diagnosis are still challenging for physicians (5). In this paper, we analyzed the clinical and imaging features of pulmonary NUT carcinoma and summarizes the novel treatment options to improve the understanding and awareness of it.

Nucleoprotein of the testis carcinoma is a highly aggressive malignant epithelial cell tumor that accounts for approximately 0.22% of primary lung tumors (7, 8). It harbors unique molecular genetic changes, that is, rearrangements or fusions of the NUT gene on chromosome 15q14 with bromodomain-containing 4 (BRD4), BRD3, nuclear receptor binding SET domain protein 3 (NSD3) or other genes, among which BRD4-NUT fusion is the most common. In 2015, WHO listed it as an independent solid tumor of the chest and classified it as undifferentiated carcinoma (9). NUT carcinoma has no gender preference and can occur at any age, while more prevalent in children and young adults (10), with the median age of lung NUT patients being 30 years (21–68 years) (7, 11). None of the patients in this study had a history of smoking, and the mean age was 49 years, which was higher than that in previous reports. The clinical manifestations of pulmonary NUT carcinoma are closely related to the size and location of the tumor, and the presence of complications or metastasis. And common symptoms include cough, chest pain, hemoptysis, wheezing, dyspnea, fever, weight loss, etc. (7, 12). As for the laboratory tests, it lacks specific tumor markers.

It is still unclear about the specific etiology, pathogenesis, and risk factors of NUT carcinoma (13). The occurrence of it may be related to MYC, P63, and MED24 as well as Wnt, MAPK, and PI3K signaling pathways, but it has no connection with environmental factors and viral infection, such as Epstein-Barr virus and human papillomavirus (7, 14). The tissue origin of NUT carcinoma is likewise unrevealed. Some studies speculate that it may originate from cells derived from primitive neural grooves (15), while others believe that it may arise from pluripotent stem cells (16).

The morphology of lung NUT carcinoma is difficult to distinguish from other poorly differentiated tumors such as Ewing’s sarcoma and small cell lung cancer, and the diagnosis is challenging due to the possibility of misdiagnosis. However, it shows unique alterations in the NUT gene. As for the histological features, small to medium-sized completely undifferentiated or poorly differentiated tumor cells are diffusely arranged in sheets, with sparse, eosinophilic or double eosinophilic cytoplasm, and some foci are accompanied by obvious interstitial connective tissue proliferation, with irregular nuclear contours, most of which are naked nuclei, prominent nucleoli, and frequent nuclear fission (3, 7). What is more, local squamous differentiation can be observed in 33–40% of cases (4, 10), and extensive necrosis sometimes exists. Lung cancer with adenocarcinoma morphology should be excluded from the differential diagnosis of NUT carcinoma because NUT carcinoma is considered a non-glandular epithelioid tumor (13). NUT carcinoma can simultaneously express markers of squamous cell carcinoma and neuroendocrine carcinoma (1, 3), and immunohistochemical detection demonstrates that tumor cells are positive for NUT staining, with a specificity of 100% and a sensitivity of 87% (17). And staining of epithelial markers such as AE1/AE3, CK5/6, P63, P40, and EMA is usually positive (18). The diagnosis relies on molecular tests such as immunohistochemical staining, FISH, and RT-PCR (5), but these methods have not been routinely used in clinical practice, resulting in misdiagnosis or delayed diagnosis of NUT carcinoma.

Chest CT is often used to evaluate the characteristics of lung lesions and abnormalities of lymph nodes, pleura, and adjacent bone. Combined with the results of our study and related literature review, the imaging manifestations of NUT carcinoma are summarized as follows: (1) It was reported that pulmonary NUT carcinoma is mainly located in the right lower lobe (19, 20), while in our group it is mostly located in the left upper lobe, which is inconsistent with previous reports. The tumor is large, mostly central-located, with aggressive and infiltrating growth, irregular shape, and mostly lobulated margins. It can also be manifested as massive pleural effusion, lobar collapse, obstructive atelectasis, and mediastinal widening or displacement (21, 22). (2) The tumor showed low-density soft tissue mass with the low-density necrotic area and occasional calcification in it on plain scan (19), and heterogeneous enhancement on enhanced images. (3) NUT carcinoma can invade the bronchial wall or blood vessels (7, 23), and is frequently associated with ipsilateral mediastinal lymph node metastasis and pleural involvement in the form of pleural thickening or nodules (13, 21), while some cases presenting with supra clavicular, contralateral mediastinal and subcarinal lymph node metastasis. The location of extrathoracic metastasis is mainly bone, with osteolytic destruction manifestation on CT, and liver, adrenal gland, and subcutaneous soft tissue involvement has also been reported (7). However, brain metastasis is rare, and we speculate that it is due to the rapid progression of tumors and the short survival time of patients, which is not enough to develop brain metastasis. Mediastinal and left supraclavicular lymph node metastasis, bone metastasis, and liver metastasis occurred in our patients, and brain metastasis occurred in one patient during disease progression. Therefore, there is no specific metastatic route for lung NUT cancer.

¹⁸F-FDG PET has been widely used for staging, restaging, and response evaluation of lung cancer. Considering lung NUT cancer is highly aggressive and usually accompanied by symptoms and signs caused by extrathoracic distant metastasis, it is recommended that patients undergo PET/CT examination to comprehensively evaluate whether they have developed other invaded sites, which is useful for the staging, early detection of metastasis, and evaluation of the metabolic and anatomical characteristics of the tumor. We searched the literature in the PubMed and Google Scholar databases using the keywords containing lung, NUT and PET. The PET/CT manifestations of total 18 cases from 2014 to 2022 are summarized in Table 3 (12, 22, 24–29). The most frequently involved lobe is right upper lobe of the lung among these cases (8 in 17) and the mean max diameter of the lung lesions is 5.0 cm (1.4–12.0 cm). The feature of lung NUT on PET/CT is the increased uptake of ¹⁸F-FDG in the lesion and the metastatic areas. In the reported cases we summarized in Table 3, the SUV_max of the tumor could range from 5 to 40 and the mean value is 12.8, and the number of patients with a SUV_max of more than 10.0 was up to 10 in these 14 available cases. And the most common metastatic sites are lymph nodes and bones, with FDG-avid presentation on PET/CT as well. We additionally searched for the extrapulmonary NUT for more extensive analysis using the keywords including NUT and PET and screened out the primary lung NUT. Totally 22 cases containing descriptions of PET/CT are included and summarized in Table 4 (30–50). Extrapulmonary sites involved include commonly the mediastinum and nasal sinuses and sometimes the neck and abdomen. The presentation of extrapulmonary lesions on PET/CT is similar to that of pulmonary lesions with a range of 4.5–64.1 and a mean of 13.8. Interestingly, the age of onset of extrapulmonary NUT is significantly younger than that of pulmonary NUT (23.9 ± 4.2 of extrapulmonary lesions vs. 42.7 ± 3.8 of pulmonary lesions, P = 0.002).

Through the analysis of our cases and the literature review, we concluded that PET/CT plays a vital role in the diagnosis, staging and treatment monitoring of NUT. The abnormally elevated FDG uptake on PET/CT is beneficial to distinguish between benign and malignant diseases. Rutt et al. (31) reported an 8-year-old girl with a neck mass mimicking tonsillitis. The presentation on CT indicated a tonsillar abscess while the SUV of 13 on PET scans was suggestive of a malignant disease, and the results of histopathological examination confirmed the diagnosis of NUT. Additionally, with the advantages of the whole body scan and available information of tissue function and biological metabolism, PET/CT is a superior imaging technique of choice for detection and staging of NUT, especially for distant metastases (30, 49). PET scans can accurately reveal the most common bony metastases with significant hypermetabolic activity while conventional imaging modalities such as bone scintigraphy will substantially underestimate the degree of bone involvement (7). Rosenbaum et al. (32) reported a patient developing extensive lytic bone metastases at the later stage of treatment. They found that whole-body bone scintigraphy neglected some lesions revealed by PET scans. Moreover, some studies suggested that PET/CT should be used to monitor treatment response and guide the therapy regimen (34, 36). A patient reported by Niederkohr et al. (51) received ¹⁸F-FDG PET/CT examination for four times after chemotherapy and the PET results clearly demonstrated the process of remission after initial chemotherapy and aggravated disease burden several months later. PET/CT can reveal both the transient chemotherapeutic response prior to disease progression and residual lesion after operation. Furthermore, the decreased uptake in PET at the center of the mass indicates necrosis within the lesion (19, 32, 52), therefore, PET/CT is an excellent imaging modality of choice for directing percutaneous tissue biopsy (22).

It is difficult to distinguish NUT from other pulmonary tumors based solely on PET/CT findings. Previous literature reported that the SUV_max of pulmonary squamous cell carcinoma ranged from 3.1 to 30.2 (6), the SUV_max of lung adenocarcinoma ranged from 2.2 to 18.1 (53), the SUV_max of pulmonary primary lymphoma ranged from 2.58 to 22.6 (54), the SUV_max of small cell lung cancer ranged from 3.3 to 29.9 (55). And there is no obvious difference compared with the data for lung NUT we summarized above. Although lack of pathognomonic imaging findings, NUT should be taken into consideration for differential diagnosis with aggressive clinical presentations, exponential interval growth of the tumor and extensive metastasis (22, 33).

To date, there is no standard treatment for primary pulmonary NUT carcinoma and the prognosis is poor, associated with its rapid progression, susceptibility to recurrence, and unsatisfactory treatment outcome, with a median overall survival of 6.7 months (1, 8). If patients can be diagnosed early, surgery remains the treatment of choice for NUT carcinoma and will help improve the prognosis of patients (5). Due to the aggressive growth of the tumor, most patients already have metastases at the time of initial diagnosis and the outcome of the surgery will be unfavorable or the opportunity for surgery has been missed (56). One study reported a case of primary pulmonary NUT in which the progression of the tumor was inhibited by radiation therapy combined with anlotinib (57). It was suggested that approximately 40% of patients initially responded to chemotherapy and radiotherapy, but always relapsed rapidly and did not respond to subsequent therapeutic interventions (5, 13). Chemotherapeutic agents commonly used include vincristine, cyclophosphamide, etoposide, and adriamycin (21). The use of PD-1 inhibitors as second-line therapy has been reported in the literature to further improve survival (7). The overall outcome of primary lung NUT is unsatisfactory, and molecular targeted therapy is being investigated as a promising therapy, and two molecular targeted agents targeting the underlying pathogenic mechanism have emerged: bromodomain and extra-terminal protein inhibitors (BETi) and histone deacetylase inhibitors (HDACi) are applied in clinical trials and have proved efficacy in some patients, both of which can induce NUT carcinoma cell differentiation and inhibit their growth (4). BETis inhibit the function of BRD4-NUT protein, allowing cell differentiation to proceed. One study showed that two patients administered with a novel BET inhibitor (OTX015/MK-8628) had a significantly higher overall survival (19 and 18 months, respectively) than the previously reported median survival of 6.7 months (58). However, variant NUT may be insensitive to the treatment, and therefore identification of the molecular type is necessary for predicting responsiveness to Beti treatment (59). BRD4-NUT oncoprotein binds and activates histone acetyltransferases, resulting in chromatin acetylation and a feed-forward mechanism, and finally leading to tumorigenesis. HDACis can reverse the function of BRD4-NUT and restore normal cellular processes and is expected to be a new therapeutic target for lung NUT (4), but toxicity and side effects are still issues that need to be addressed (14).

In summary, the possibility of NUT should be considered in patients with central lung masses and rapidly progressive disease, and anti-NUT immunohistochemical staining should be performed as soon as possible, combined with genetic testing if necessary. CT and PET/CT imaging are essential for staging, management, treatment response assessment, and monitoring of pulmonary NUT cancer.

The original contributions presented in this study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

This study was reviewed and approved by the Medical Ethics Committee of the First Affiliated Hospital of Zhengzhou University and Peking University First Hospital. The patients/participants provided their written informed consent to participate in this study. Written informed consent was obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

WH: draft the manuscript. YZ: acquisition and analysis of the work and imaging data collection and analysis. QY and GG: imaging data collection and analysis and resources. YQ and LL: formal analysis and resources. LK: supervision and writing—review and editing. All authors met the requirements for authorship for the submitted version and agreed to its submission.