A PAX-8-Positive Female Urethral Adenocarcinoma, Intestinal-Type: A Case Report with Diagnostic Challenges and a Review of the Literature

Female urethral adenocarcinoma (FUA) is extremely rare. It is an aggressive malignancy, and clear cell and columnar/mucinous (“intestinal”) represent the two primary histologic subtypes. Diagnosis is often delayed in patients because of their vague symptomatology; hence, they present with an advanced disease and a poor prognosis. The rarity of FUA brings challenges when determining treatment and management, and treatment guidelines for various stages are lacking. We report an intestinal-type FUA that developed from inflammation-related metaplasia in urethral diverticulum with positive paired box 8 (PAX-8) staining. In addition to intestinal-type FUA being extremely rare, this particular entity exhibiting PAX-8 positivity has not been previously described, to the author's best knowledge. The present report highlights the importance of clinical and radiological assessment as well as histomorphologic and immunophenotypic features for an accurate diagnosis of this rare and aggressive malignancy.


Introduction
Primary urethral carcinoma in females is rare, and it represents 0.02% of cancers in females and <1% of cancers in the female urogenital tract [1]. The most common histologic type is squamous cell carcinoma (SCC) (representing 70% of all cases), followed by transitional cell carcinoma (TCC) (20%) and adenocarcinoma (10%) [1,2]. Female urethral adenocarcinoma (FUA) is an extremely rare and aggressive malignancy, and the two primary histologic subtypes of FUA are clear cell and columnar/mucinous ("intestinal") [3]. While the origin remains unclear, previous studies have suggested one possibility to be the periurethral Skene gland (the embryologic homologue to the male prostate gland); others have proposed urethritis glandularis with or without metaplasia [4][5][6][7]. To our knowledge, FUA exhibiting colonic adenocarcinoma with paired box 8 (PAX-8) immunoexpression has not been reported to date.
Herein, we report an intestinal-type FUA that developed from inflammation-related metaplasia in urethral diverticulum with positive PAX-8 staining. Because of this distinct immunophenotype, we emphasize the diagnostic accuracy and importance in excluding other sites, including bladder, colorectal, and gynecologic/Mullerian tumors. Additionally, search of PubMed in the English literature of primary FUA was performed, and the accompanying literature review highlights the rarity of intestinal-type FUA.

Case Presentation
The patient is a 64-year-old female with a 32-pack-year history of tobacco use who underwent a screening chest computed tomography (CT) scan which revealed multiple bilateral pulmonary nodules, measuring up to 7.8 mm (Figure 1).
Needle core biopsy of a right lower lobe nodule was performed and demonstrated moderately differentiated adenocarcinoma ( Figure 2 Because of the immunophenotype, the differential diagnosis included a colorectal primary, and PAX-8 immunoexpression raised the possibility of a gynecologic/Mullerian primary. She had markedly elevated carcinoembryonic antigen (CEA) and cancer antigen 19-9 (CA 19-9) levels. Colonoscopy and extensive imaging were recommended to determine the primary site. Imaging with abdominal and pelvic CT scan showed no evidence of colorectal or gynecologic primary tumors. CT scan and colonoscopy were otherwise normal. A follow-up CT of the abdomen/pelvis revealed multiple new and enlarging solid bilateral pulmonary nodules accompanied by scattered ground glass opacities (Figure 3(a)) and new sclerotic lesions in the right ilium ( Figure 3(b)) and vertebral body, concerning for osseous metastasis.
The patient then began having hematuria with intermittent urinary retention. Cystoscopy was performed and showed an irregular/thickened bladder wall. A pelvic exam revealed a firm, fixed urethral mass measuring approximately 4 × 3 cm and invading into the anterior vaginal wall. Cystoscopy revealed that the mass circumferentially involved the urethra and extended up to the bladder neck.
There was no disease within the bladder. Urethral and vaginal tumor biopsies were performed. Morphologic examination of the urethral biopsy demonstrated intestinal metaplasia of squamous mucosa with transition from a mature to dysplastic phenotype where the adenocarcinoma originated from (Figure 4(a)).
The vaginal wall biopsy showed the same morphology. The urethral and vaginal wall biopsies showed a similar immunophenotype as the pulmonary nodule biopsy: posi-tive for CK7, CK20 (Figure 4 Per the request of the primary oncology team, the case was sent for second pathologic opinion. The consulting service concurred with our diagnosis. This very rare and aggressive subtype warranted further discussion with the tumor board, and a management plan was formulated in consultation with a multidisciplinary team including oncology, surgery, radiology, and pathology. The patient started on the following modified FOLFOX regimen and received it every two weeks: oxaliplatin 85 mg/m 2 intravenous (IV) day 1, leucocorin 400 mg/m 2 IV day 1, fluorouracil (FU) 400 mg/m 2 IV bolus day 1, and FU 2400 mg/m 2 IV day 1, administered over 46 hours. Cycles 4-6 of oxaliplatin were held due to concern for ocular toxicity and ultimately discontinued upon cycle 13. The patient also received palliative radiotherapy to the lumbar spine and sacral metastases. She suffered from additional complications and side effects secondary to therapy, and the tumor burden progressed. CT and brain magnetic resonance imaging (MRI) demonstrated interval disease progression, and a palliative care consultation was requested.

Discussion
Female urethral carcinoma is a rare malignancy, and it represents approximately 0.02% of all female cancers and 0.003% of female urogenital malignancies [1,2]. The reported annual incidence is 1.5 cases per million and increases with age [1]. Among SCC, TCC, and FUA, FUA is exceedingly rare, accounting for approximately 10% of malignant neoplasms involving the female urethra [1].
The female urethral mucosa is lined, from proximal to distal, by transitional, pseudostratified columnar, and squamous epithelia. The composition of the lining epithelium changes from more squamous in young women to predominantly columnar epithelium in the older population. Additionally, paraurethral glands are present in the urethral mucosa and are formed by columnar epithelium or intestinal metaplasia. Although the exact etiology of FUA is unknown, the histogenesis may help explain the pathogenesis. Some have proposed a possible origin from periurethral Skene's glands, while others suggest the epithelium of urethritis glandularis with or without intestinal/glandular metaplasia as a potential source [4][5][6][7].
Clinically, signs and symptoms of FUA differ and are often nonspecific. Risk factors are also numerous. The clinical presentation and risk factors are discussed in detail elsewhere [1]. The vague symptomatology can lead to a delay in diagnosis, and patients often present with advanced-stage disease. Even in early-stage disease, this aggressive malignancy is associated with poor outcomes. The rarity of this disease entity creates added challenges when determining  Case Reports in Pathology treatment options and management, and unfortunately, a general agreement regarding the treatment modality of choice for various stages is lacking [1,8].
For early-stage disease, partial resection with preservation of the urethra, including local and wide local excisions, is the most common surgical approach, and lymphadenectomies do not affect survival [1]. For advanced-stage disease, treatment has included radical urethrectomy with resection of the paraurethral tissues and anterior vaginal wall. In advanced pathologic stage cancers, there is increased local recurrence following surgery as the only treatment and disease-specific survival is poor [1]. Furthermore, even with extensive and aggressive surgical resections or pelvic exenterations, with or without lymph node dissections, the reported local recurrence rate is 63%. The data for radiother-apy alone is further limited, and when previously used with the intent of cure, local control was not attained [1]. A multimodal treatment approach with surgery, radiation, and systemic therapy (often with multiple chemotherapeutic agents) for patients with advanced disease is commonly practiced. Adjuvant polychemotherapy with gemcitabine, cisplatin, and ifosfamide, or cisplatin, 5-FU, and gemcitabine has been recommended by some [1].
The two primary histologic subtypes of FUA are clear cell type and columnar/mucinous ("intestinal"), with the morphologic features of the former described in detail elsewhere [1]. Columnar/mucinous adenocarcinoma is morphologically similar to well-differentiated colonic or endocervical adenocarcinoma, and the architectural pattern consists of tubular to irregular glands and can also show focal papillary and   Case Reports in Pathology villous areas. The cytologic features include predominantly atypical columnar cells with eosinophilic to amphophilic cytoplasm, round to oval nuclei with regular contours, and generally small, inconspicuous nucleoli. Mucinous cells are often readily identified and can resemble goblet cells [1,9]. Rarely, mild to moderate pleomorphism is appreciated and can be associated with large mucin pools and rare malignant glands within [1]. Because of its paucity, immunohistochemical analysis for FUA has not been adequately reported, and characteristic immunophenotypic profiles are lacking. There are histologic mimickers, and the pathologic differential diagnosis includes primary tumors from adjacent anatomic sites such as the colorectum, bladder, and gynecologic/Mullerian structures, all of which may extend to and involve the urethra. Hence, IHC is of upmost importance in establishing a diagnosis to guide clinical management, particularly in the cases with initial presentation of metastatic disease.
Immunohistochemistry on the prior reported cases has been reviewed in the English literature in PubMed (Table 1), and no PAX-8 immunoexpression has been documented. The largest, contemporary clinicopathologic study  5 Case Reports in Pathology to date of primary mucinous adenocarcinoma of the female urethra was published in 2016 by Harari et al. [10] where they searched through two major academic institutions for cases and found five cases of confirmed primary mucinous adenocarcinoma arising from the female urethra. The mean age of the patients is 67 years, and all presented with at least a pT2 pathologic stage. IHC was performed for GATA3, p63, CK7, CK20, CDX2, ER, PAX-8, and β-catenin and showed the following results: positivity for CDX2 in 4/5 (80%) cases, focally positive for CK20 in 4/5 (80%) cases, and focally positive for CK7 in 4/5 cases (80%); p63, GATA3, ER, PAX-8, and β-catenin were negative in all cases.
The diagnosis in the current case is particularly challenging because of the immunophenotype, including PAX-8 immunohistochemical positivity. Also, because the tumor showed concurrent CK7 and CK20 immunoexpression, the differential diagnosis included metastatic BRAF-mutated MSI (microsatellite instability) colorectal carcinoma [20]. Extensive imaging, colonoscopy, and pelvic exams were performed to evaluate for a primary malignancy of the bladder, colorectum, and female reproductive system, all of which were ruled out. The patient was ultimately diagnosed with intestinal-type FUA that developed from inflammationrelated metaplasia in urethral diverticulum with positive PAX-8 staining. To our knowledge and review of the English literature, this is the first report on PAX-8 immunoexpression. The patient presented with locally advanced disease and pulmonary metastasis, exemplifying the aggressive nature of this particular entity. The patient received chemotherapy and palliative radiation for bone metastasis. Unfortunately, her disease progressed and she was ultimately referred to palliative care. She succumbed to the disease one year after diagnosis.

Conclusion
FUA is a rare, aggressive tumor, often presenting with metastatic disease. The nonspecific symptomatology often leads to a delay in diagnosis in most patients, and the prognosis is poor in advanced-stage disease. The clinical and pathologic diagnosis can be challenging due to its rarity. Furthermore, delay in presentation has made standardization of treatment difficult to determine, and treatment guidelines currently do not exist. The present report increases the awareness of PAX-8 immunoreactivity in FUA and emphasizes the consideration of this entity in the differential diagnoses in a metastatic tumor, in addition to the importance of clinical and radiological assessment.

Data Availability
The data used to support the findings in this case report are included within the article.

Consent
The patient is unavailable to provide consent. There is no patient-identifying information in the materials provided.