Prostate Cancer Recurrence Mimicking a Primary Paranasal Sinus Tumor on <sup>18</sup>F-FDG PET/CT During Treatment for Metastases After 10 Years of Remission.

Abstract

Metastatic tumors of the nasal cavity and paranasal sinuses are rare, and the prostate is a particularly uncommon primary site. Including metastatic prostate cancer in the differential diagnosis of sinonasal tumors can be clinically challenging. To our knowledge, there are no detailed case reports confirming uptake in paranasal sinus metastases from prostate cancer using 18F-fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT). The patient was a 68-year-old man who was diagnosed with prostate cancer 16 years prior to his visit. After a 10-year remission, he received radiation therapy and radium-223 for prostate cancer 6 years earlier, followed by multiple courses of radiation therapy for lymph node and bone metastases over the next 5 years. His chief complaints were a 2-month history of nasal congestion and a postnasal drip. The tumor was located mainly in the right paranasal sinus and showed signs of bone infiltration and tumor necrosis. FDG-PET/CT showed intense uptake in the mass (maximum standardized uptake value = 15.2). The differential diagnosis included primary and metastatic tumors. Biopsy revealed histological features similar to the primary prostate cancer site from 16 years prior, with prostatic acid phosphatase and NKX3.1 positive staining. The patient was diagnosed with metastatic prostate cancer, underwent palliative radiotherapy, and passed away approximately 23 months after the detection of paranasal sinus metastasis. This case highlights the importance of considering metastasis even in atypical lesions in patients with prostate cancer.