Immunohistochemistry has become an essential ancillary examination for the identification and classification of carcinomas of unknown primary site (CUPs). Over the last decade, the diagnostic accuracy of organ- or tumour-specific immunomarkers and the clinical validation of effective immunohistochemical panels has improved significantly. When dealing with small sample sizes, diagnostic accuracy is crucial, particularly in the current era of targeted molecular and immune-based therapies. Effective systematic use of appropriate immunohistochemical panels enables accurate classification of most of the undifferentiated carcinomas as well as careful preservation of tissues for potential molecular or other ancillary tests. This review discusses the algorithmic approach to the diagnosis of CUPs using CK7 and CK20 staining patterns. It outlines the most frequently used tissue-specific antibodies, provides some pitfalls essential in avoiding potential diagnostic errors and discusses the complementary tools, such as molecular tumour profiling and mutation-specific antibodies, for the improvement of diagnosis and prediction of the treatment response.

In 2018, cancer was the most frequent cause of premature death in 48 countries in the world.¹ Breast cancer was the most frequently diagnosed cancer among females in 154 countries.¹ When early diagnosis is made, conservative surgery is preferred to partially preserve organ function and maintain the patient’s body image and quality of life.^2,3 Nonetheless, these surgeries could be associated with higher risk of recurrence, risk of re-excision, and potential delay to the initiation of other therapies.^3,4 The difficulty partly lies in the efficient evaluation of tumor margins directly during surgery. Techniques such as intraoperative touch-preparation cytology or frozen-section analysis can be performed during the surgery and have shown promising potential in reducing the re-excision rate after primary breast conserving surgery by a factor 3 compared with permanent section histopathology.⁵ Nonetheless, these techniques still require about 30 min during the surgery, are resource intensive, are associated with various artifacts (sampling, freezing artifacts), and offer a less precise diagnosis compared with standard histopathological assessment.^{6 – 8} A recent review compared frozen sections and touch-preparation cytology to regular histology on 4300 and 1900 breast cancer cases and reported a sensitivity/specificity of 0.86/0.96 for frozen sections and 0.91/0.95 for cytology.⁸ This review also pointed out that these intraoperative margin assessment techniques are not commonly routinely used in hospitals. Thus, improving intraoperative diagnosis toward a real-time, easily applicable process combining large-scale assessment and cell resolution measurement to accurately evaluate cancer margins is an important step toward reducing the re-excision rate, the risk of recurrence, as well as patient stress and use of medical resources.

Fine-needle aspiration (FNA) cytology has been widely used for the diagnosis of breast cancer lesions with the objective of differentiating benign from malignant masses. However, the occurrence of unsatisfactory samples and false-negative rates remains a matter of concern. Major improvements have been made thanks to the implementation of rapid on-site evaluation (ROSE) in multidisciplinary and integrated medical settings such as one-stop clinics (OSCs). In these settings, clinical and radiological examinations are combined with a morphological study performed by interventional pathologists. The aim of our study was to assess the diagnostic accuracy of the on-site cytopathology advance report (OSCAR) procedure on breast FNA cytologic samples in our breast OSC during the first three years (April 2004 till March 2007) of its implementation. To this goal, we retrospectively analyzed a series of 1820 breast masses (1740 patients) radiologically classified according to the American College of Radiology (ACR) BI-RADS lexicon (67.6% being either BI-RADS 4 or 5), sampled by FNA and immediately diagnosed by cytomorphology. The clinicoradiological, cytomorphological, and histological characteristics of all consecutive patients were retrieved from the hospital computerized medical records prospectively registered in the central information system. Histopathological analysis and ultrasound (US) follow-up (FU) were the reference diagnostic tests of the study design. In brief, we carried out either a histopathological verification or an 18-month US evaluation when a benign cytology was concordant with the components of the triple test. Overall, histology was available for 1138 masses, whereas 491 masses were analyzed at the 18-month US-FU. FNA specimens were morphologically nondiagnostic in 3.1%, false negatives were observed in 1.5%, and there was only one false positive (0.06%). The breast cancer prevalence was 62%. Diagnostic accuracy measures of the OSCAR procedure with their 95% confidence intervals (95% CI) were the following: sensitivity (Se) = 97.4% (96.19–98.31); specificity (Sp) = 94.98% (92.94–96.56); positive predictive value (PPV) = 96.80% (95.48–97.81); negative predictive value (NPV) = 95.91% (94.02–97.33); positive likelihood ratio (LR+) = 19.39 (13.75–27.32); negative predictive ratio (LR−) = 0.03 (0.02–0.04), and; accuracy = 96.45% (95.42–97.31). The respective positive likelihood ratio (LR+) for each of the four categories of cytopathological diagnoses (with their 95% CI) which are malignant, suspicious, benign, and nondiagnostic were 540 (76–3827); 2.69 (1.8–3.96); 0.03 (0.02–0.04); and 0.37 (0.2–0.66), respectively. In conclusion, our study demonstrates that the OSCAR procedure is a highly reliable diagnostic approach and a perfect test to select patients requiring core-needle biopsy (CNB) when performed by interventional cytopathologists in a multidisciplinary and integrated OSC setting. Besides drastically limiting the rate of nondiagnostic specimens and diagnostic turn-around time, OSCAR is an efficient and powerful first-line diagnostic approach for patient-centered care.