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  • Review Article
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Rethinking cancer of unknown primary: from diagnostic challenge to targeted treatment

Nature Reviews Clinical Oncology volume 22, pages 781–799 (2025)Cite this article

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Abstract

Cancer of unknown primary (CUP) is a metastatic malignancy for which a primary site of origin cannot be identified despite a thorough and standardized diagnostic work-up, and accounts for 1–3% of all malignancies. An unfavourable subgroup of CUP has a poor prognosis, with a median overall survival of <1 year when treated with current standard-of-care platinum-based chemotherapy. Virtually no progress in elucidating the disease biology and improving outcomes for patients with unfavourable CUP has been made over the past several decades, including a failure of initial randomized clinical trials to demonstrate the superiority of tissue-of-origin (ToO) identification by gene-expression profiling and subsequent primary-site-directed treatment over standard chemotherapy. However, large-cohort randomized trials have now shown that molecularly guided therapy improves outcomes for patients with CUP harbouring an actionable target, both in a tissue-agnostic as well as a primary tumour site-specific context. Moreover, data from non-randomized phase II trials suggest that immunotherapy using immune-checkpoint inhibitors can be beneficial even in patients with CUP that has relapsed after, or is refractory to, standard chemotherapy. In addition, a plethora of refined and novel strategies, including DNA and RNA sequencing, DNA-methylation profiling, circulating tumour DNA analysis, and artificial intelligence-based pathology, have been leveraged to facilitate ToO identification. In light of these developments, we review current ToO methodologies and compare the evidence supporting the use of a primary tumour site-guided approach versus a histology-agnostic approach to the management of CUP. We also discuss whether CUP can be viewed as a model disease for the development of histology-agnostic precision oncology treatment strategies.

Key points

  • Several methods based on DNA or RNA sequencing, transcriptomics microarrays, DNA-methylation profiling, circulating tumour DNA analysis and artificial intelligence-based pathology have been developed to predict the tissue of origin (ToO) of cancer of unknown primary (CUP) with high accuracy; however, no benchmarking and reporting standards have been established to date, making it challenging to judge the value of different tests.

  • Next-generation sequencing can aid in the differential diagnosis, ToO identification and detection of therapeutic targets in patients with unfavourable CUP.

  • Randomized trials have revealed that molecularly guided therapy and immunotherapy improve progression-free survival compared with standard platinum-based chemotherapy in patients with newly diagnosed unfavourable CUP, in both ToO-informed and histology-agnostic settings.

  • Non-randomized trials suggest that second-line treatment with immune-checkpoint inhibitors can increase response rates and prolongs survival in patients with a high tumour mutational burden and/or PD-L1 expression.

  • Comprehensive genomic profiling should be performed at initial diagnosis in all patients with unfavourable CUP, and liquid biopsy circulating tumour DNA assays can overcome the technical difficulties and scarcity of tumour material commonly associated with tissue biopsy approaches.

  • Access to molecular testing as well as to molecularly guided treatment and immunotherapy for patients with CUP remains limited in many countries.

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Fig. 1: Major advances in the diagnosis, classification and treatment of CUP.
Fig. 2: Diagnostic algorithm for CUP.
Fig. 3: CUP subtypes and their respective treatment options.

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Acknowledgements

The work of M.P. was supported by the physician scientist programme of the Medical Faculty of the University of Heidelberg. The work of A.K. is funded by the Priority Program Translational Oncology of the Deutsche Krebshilfe (grant 70115167) and the proof-of-concept trial programme of the National Center for Tumour Diseases (NCT) Heidelberg.

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Authors and Affiliations

  1. Clinical Cooperation Unit Molecular Hematology/Oncology, German Cancer Research Center (DKFZ) and Department of Internal Medicine V, University of Heidelberg, Heidelberg, Germany

    Maria Pouyiourou, Tilmann Bochtler & Alwin Krämer

  2. Department of Internal Medicine V, University of Heidelberg, Heidelberg, Germany

    Maria Pouyiourou, Tilmann Bochtler & Alwin Krämer

  3. Department of Medical Oncology, Heidelberg University Hospital, Medical Faculty Heidelberg, Heidelberg University, Heidelberg, Germany

    Tilmann Bochtler

  4. Department of Pathology and Molecular Pathology, University and University Hospital Zürich, Zürich, Switzerland

    Chantal Pauli & Holger Moch

  5. Medical Faculty, University of Zürich, Zürich, Switzerland

    Chantal Pauli & Holger Moch

  6. Institute of Pathology, University of Heidelberg, Heidelberg, Germany

    Alexander Brobeil & Albrecht Stenzinger

  7. Institute of Tumour Biology, University Medical Center Hamburg–Eppendorf, Hamburg, Germany

    Klaus Pantel

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Contributions

M.P., C.P., K.P., A.S. and A.K. researched data for the article and wrote the manuscript. All authors contributed to discussions of content and reviewed and/or edited the manuscript before submission.

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Correspondence to Alwin Krämer.

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Competing interests

T.B. has received travel support from F. Hoffmann-La Roche and served as study oncologist for the CUPISCO trial (which was sponsored by F. Hoffmann-La Roche). C.P. has received research funding from F. Hoffmann-La Roche and served as study pathologist for the CUPISCO trial. H.M. has received research funding and honoraria for lectures from, and has served as a consultant on data safety monitoring boards or advisory boards for Amgen, Astellas, AstraZeneca, Bayer, F. Hoffmann-La Roche, Merck and Stemline Therapeutics. K.P. has received honoraria from Eppendorf, Menarini, MSD, NRICH, Roche and Sysmex. A.S. has acted as adviser for Aignostics, Amgen, Astellas, AstraZeneca, Bayer, Beigene, Bristol Myers Squibb, Eli Lilly, Illumina, Incyte, Janssen, Jazz Pharmaceuticals, MSD, Novartis, Pfizer, Qlucore, Roche, Sanofi, Servier, Takeda and Thermo Fisher, and has received research funding from Bayer, Bristol Myers Squibb, Chugai and Incyte. A.K. has received research funding from Bristol Myers Squibb, F. Hoffmann-La Roche and Molecular Health; consulting fees, travel support and/or remuneration for advisory board participation from F. Hoffmann-La Roche; and served as study oncologist for the CUPISCO trial. M.P. and A.B. declare no competing interests.

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Pouyiourou, M., Bochtler, T., Pauli, C. et al. Rethinking cancer of unknown primary: from diagnostic challenge to targeted treatment. Nat Rev Clin Oncol 22, 781–799 (2025). https://doi.org/10.1038/s41571-025-01060-8

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