Adult patients suspected of Squamous Cell Carcinoma of Unknown Primary (SCCUP) will be prospectively enrolled at a tertiary head & neck cancer center at Copenhagen University Hospital - Rigshospitalet, Copenhagen, Denmark. All patients will undergo Magnetic Resonance Imaging (MRI) and Positron Emission Tomography-Computerized Tomography (PET-CT) prior to examination in general anesthesia. During general anesthesia, Intraoperative Transoral Ultrasound (ITUS) will be performed prior to panendoscopy. Detected tumors will be registered with specified oropharyngeal sub-locations. Blinded to ITUS, a consultant head & neck surgeon will perform panendoscopy. After examination, the surgeon is unblinded to ultrasound results. Final histopathology results from location-specified biopsies performed will be used as reference standard. The overall detection rate will be compared between ITUS, panendoscopy, PET-CT and MRI with sensitivity and specificity analysis. Oropharynx sub-location specific detection rate of ITUS vs. panendoscopy will be compared with logistic regression analysis.

A prospective diagnostic study examining patients with suspected SCCUP will be performed at the Department of Otorhinolaryngology & Head & Neck Surgery, Copenhagen University Hospital - Rigshospitalet, Denmark. This specialized cancer center is responsible for the surgical work-up for approximately half of suspected SCCUP patients in the Danish population of 5.8 million.

Standard work-up of patients with suspected SCCUP in Denmark consists of same-day clinical examination combined with high resolution video endoscopy with NBI, surgeon-performed neck ultrasound and fine-needle aspiration cytology (FNAC) of suspected lymph nodes with same-day results. Cytology results can be further tested for Human Papillomavirus (HPV), and patients are booked for panendoscopic examination under general anesthesia with frozen section biopsy. Pre-operative cross-sectional imaging to locate the primary tumor includes MRI and 18-F Fluordeoxyglucose (FDG) PET-CT.

Intraoperative transoral ultrasound

During general anesthesia and prior to panendoscopy, ITUS will be performed while a consultant head & neck surgeon performing panendoscopy is blinded. A Fujifilm Arietta 850 or Arietta 65 (Fujifilm, Tokyo, Japan), BK5000 (BK Medical ApS, Herlev, Denmark), or Samsung RS85 (Samsung Medison, Seoul, South Korea) ultrasound machines will be used. Small-footprint, high-frequency transducers such as the Arietta L51K linear-array intraoperative robot transducer attached to a forceps, the BK5000 "flexible hockey-stick" 18XL5s, or the Samsung RS85 LA3-22AI "hockeystick" will be used. A Boyle-Davis gag or laryngoscope will be used to visualize the tonsils (Figure 1), while the transducer is placed onto each tonsil in transverse and sagittal planes. The tongue base will be scanned by pulling the tongue anteriorly by an assistant and placing the transducer directly onto the tongue base. Alternatively, the tongue can be retracted with a video-laryngoscope while revealing the tongue base. The tongue base will be scanned from side to side in the sagittal plane. The tongue can then be pulled to either the left or right side to angle the probe obliquely for transverse imaging. If a tumor is suspected, the tumor location is registered in the following categories, including indication of right or left side:

• Tonsil cranial.
• Tonsil caudal.
• Glossotonsillar sulcus.
• Anterolateral tongue base.
• Anteromedial tongue base.
• Posterolateral tongue base.
• Posteromedial tongue base.
• "Other" oropharynx sub-locations, including anterior & posterior pharyngeal arches, soft palate, uvula, posterior oropharynx wall, and vallecula.

B-mode video clips of both tonsils and the tongue base will be recorded. Power doppler for suspected tumors and the contralateral side will be performed. If a well-defined lesion is seen, the greatest tumor diameter and if possible, three-dimensional size is measured. The grade of tumor suspicion will be rated on a 5-point Likert scale ranging from 0 (very low) to 5 (very high).

Panendoscopy Following ITUS, the surgeon will perform standard panendoscopy of the pharynx including palpation of the oral cavity and oropharynx, and high-resolution video endoscopy with NBI. If the surgeon suspects a tumor, the location is registered with the same categories as above. Tumor suspicion will be rated on a 5-point Likert scale ranging from 0 (very low) to 5 (very high). The surgeon will categorize visual findings such as ulcers, exophytic tumors, asymmetric tissue, vulnerable/bleeding mucosa, suspect vasculature on NBI, or no suspect visual findings. The surgeon will categorize palpatory findings as soft or firm. The clinically visualized or palpated greatest tumor diameter will be estimated. The surgeon is then unblinded to ultrasound results. After unblinding from ultrasound, the investigators will register whether further directed biopsies from ultrasound suspected location are performed.

If the primary tumor is not located during panendoscopy, patients will tonsillectomied and the tonsils will be sliced in 2mm slices by head & neck pathologists in search of the primary tumor. If the primary tumor is still unknown and the neck metastasis is HPV+, then patients are offered Transoral Robotic Surgery (TORS) with diagnostic tongue base mucosectomy. These tongue base specimens will also be sliced in 2mm slices and examined by head & neck pathologists. If the primary tumor is located in final histopathology from either tonsillectomies or tongue base mucosectomy, the location of the tumors will be compared to the initially registered suspected ultrasound locations from panendoscopy.

Medical chart data The patient's age, sex, alcohol, smoking, largest cervical lymph node size, cervical lymph node side (right/left), date of panendoscopy, date of conclusive histopathology results, and date of treatment initiation will be obtained from medical charts.

Inclusion Criteria:

• Cytology-verified cervical lymph node metastasis from squamous cell carcinoma in neck levels I-IV with HPV+ cytology.
• Booked for panendoscopic examination under general anesthesia.
• Pre-operative MRI and PET-CT available.

Exclusion Criteria:

• Unable to provide written informed consent.
• Unable to complete full surgical work-up including palatine- and lingual tonsillectomy.