Primary Mucinous Borderline Tumor of the Testis: A Case Report

ABSTRACT

Introduction

Testicular mucinous borderline tumor is a rare primary testicular neoplasm with a generally favorable prognosis. Herein, we report a case diagnosed after high orchiectomy, with a brief literature review.

Case Presentation

A 77‐year‐old man presented with discomfort in the left scrotum. Computed tomography revealed enlargement and calcification of the left testis and indicated a testicular tumor. High orchiectomy was performed, and pathological examination revealed a mucinous borderline tumor. Postoperative upper and lower gastrointestinal endoscopy showed no abnormal findings, leading to the diagnosis of primary testicular mucinous borderline tumor. Twelve months postoperatively, no evidence of postoperative recurrence or metastasis was observed.

Conclusion

We presented a rare case of testicular mucinous borderline tumor. Considering the existence of a case with metastasis, long‐term imaging follow‐up is advised.

Keynote Message

Testicular mucinous borderline tumor is extremely rare, and the initial treatment is high orchiectomy. In the diagnostic process, unlike typical germ cell tumors, such as seminoma, gastrointestinal endoscopy must be performed in addition to chest and abdominal computed tomography to exclude metastatic testicular tumors. Although many reported cases indicate a favorable prognosis, a case with metastasis has been reported. Therefore, postoperative imaging is necessary to rule out recurrence or metastasis.

Abbreviation

CT

computed tomography

1. Introduction

Mucinous borderline tumors are frequently reported among ovarian tumors; however, primary mucinous borderline tumors of the testis are extremely rare [1]. Testicular tumors of ovarian epithelial type are similar to ovarian tumors, and may consist of various histological subtypes [1]. While germ cell tumors account for the majority of testicular tumors, mucinous borderline tumors are classified as non‐germ cell tumors. Primary mucinous borderline tumor of the testis is a rare neoplasm with a generally favorable prognosis. To date, including the present case, only 16 cases of primary mucinous borderline tumor of the testis have been reported worldwide [1, 2, 3]. Herein, we report a case diagnosed following high orchiectomy, along with a brief literature review.

2. Case Presentation

2.1. Patient

A 77‐year‐old man presented with left scrotal discomfort. Computed tomography (CT) revealed a tumor in his left testis.

2.2. Physical Findings

Height 170 cm, weight 73.3 kg, palpable enlargement and induration of the left testis.

2.3. Laboratory Findings

No remarkable abnormalities were observed in the testicular tumor markers, lactate dehydrogenase, alpha‐fetoprotein, or human chorionic gonadotropin. However, the soluble interleukin‐2 receptor concentration was elevated at 552.3 U/mL (122–496 U/mL).

2.4. Testicular Ultrasonography

Partial calcification (red dashed line sections) and heterogeneous internal echogenicity (blue dashed line sections) were observed (Figure 1A).

FIGURE 1.

(A) Testicular ultrasonography of the tumor. Partial calcification (red dashed line sections) and heterogeneous internal echogenicity (blue dashed line sections) were observed. (B) Photograph of the gross testicular surgical specimen showing a yellowish, gelatinous mass occupying the entire testicular parenchyma.

2.5. Chest and Abdominal CT

No obvious evidence of metastasis to the lungs or lymph nodes was observed. The left testis was enlarged and calcified.

On the basis of these findings, a malignant testicular tumor was suspected, and left high orchiectomy was performed.

2.6. Resected Specimen

The resected left testis was yellowish in color and gelatinous, and this tissue occupied the entire testicular parenchyma (Figure 1B).

2.7. Pathological Findings

A unilocular cystic lesion with marked cytological atypia was observed histologically. The boundary between the normal testis and tumor was clear, with no evidence of invasion (Figure 2A,B). Glandular and papillary structures were present (Figure 2C). No findings suggestive of teratoma were seen histologically. Immunohistochemistry showed the following: caudal type homeobox 2 gene (CDX2): positive (Figure 3A), cytokeratin (CK)7: negative (Figure 3B), CK20: positive (Figure 3C), Mucin‐2 (MUC‐2) protein: positive (Figure 3D), Mucin 5AC (MUC5AC): positive (Figure 3E), and MUC6: negative (Figure 3F). Findings were negative for both estrogen receptor (Figure 4A) and progesterone receptor (Figure 4B). On the basis of the results of hematoxylin and eosin staining and immunohistochemistry, the diagnosis was intestinal‐type mucinous borderline tumor of the testis.

FIGURE 2.

Hematoxylin and eosin‐stained pathological specimens of the tumor. (A) Histopathological image showing a unilocular cystic lesion with marked architectural atypia. There is a distinct boundary between the lesion tissue and normal tissue, and no evidence of invasion was observed. (B, C) Histopathological image showing glandular and papillary structures.

FIGURE 3.

Immunohistochemical‐stained histopathological images. (A) CDX2‐positive, (B) CK7‐negative, (C) CK20‐positive, (D) MUC‐2 protein‐positive, (E) MUC5AC‐positive, (F) MUC6‐negative.

FIGURE 4.

Immunohistochemical‐stained histopathological images. (A) Estrogen receptor (ER): Negative, (B) Progesterone receptor (PgR): Negative.

2.8. Clinical Course

To rule out testicular metastasis from a gastrointestinal malignancy, chest and abdominal CT as well as upper and lower gastrointestinal endoscopy were performed. No neoplastic lesions were found, and the diagnosis of mucinous borderline tumor of the testis was confirmed. Chest and abdominal CT at 6 and 12 months after surgery were planned for the evaluation of metastasis and recurrence. At the time of writing, 12 months postoperatively, there is no evidence of recurrence or metastasis, and the patient is under follow‐up with imaging studies.

3. Discussion

Mucinous borderline tumors are frequently reported among ovarian tumors; however, primary mucinous borderline tumors of the testis are extremely rare [1]. Testicular tumors of ovarian epithelial type are similar to ovarian tumors, and have been described in all histological subtypes, including serous, mucinous, endometrioid, clear cell, transitional cell, and squamous cell types [1]. Among these, the serous type is most common, followed by the mucinous type [4]. To date, 16 cases of primary mucinous borderline tumor of the testis have been reported worldwide, including the present case [1, 2, 3].

In previously reported cases and in the present case, cystic lesions lined by columnar epithelial cells were observed, which are histopathological features similar to those of ovarian mucinous tumors. Testicular mucinous tumors are classified as cystadenoma, borderline tumor, or carcinoma, on the basis of the degree of cellular atypia and stromal invasion. In the present case, marked cellular atypia was observed without invasion into the normal testicular parenchyma, leading to a diagnosis of mucinous borderline tumor.

Similar to ovarian tumors, most primary testicular mucinous tumors exhibit an intestinal‐type histology, although cases with Müllerian‐type histology have also been reported [5]. In the present case, immunohistochemical staining for estrogen receptor and progesterone receptor was negative, making Müllerian‐type histology unlikely. According to Ulbright et al., it is important to exclude germ cell tumors, such as teratomas with prominent mucinous components [5]. In the present case, no histological features suggestive of teratoma were identified in any of the pathological slides, and a teratomatous origin was excluded.

Immunohistochemically, primary testicular mucinous tumors often show CK7 and CK20 co‐positivity, or CK7 negativity with CK20 positivity [4]. CDX2 expression has been reported in both primary and metastatic colorectal carcinomas [6, 7]. In the present case, the tumor was positive for CDX2 and CK20, and negative for CK7, supporting the diagnosis of intestinal‐type mucinous borderline tumor. To differentiate between primary and metastatic lesions, upper and lower gastrointestinal endoscopy and radiographic examinations are recommended [4]. In the present case, no evidence of a primary lesion or metastasis was found, confirming the diagnosis of primary mucinous borderline tumor of the testis.

The histogenesis of testicular mucinous tumors remains unclear, with several proposed hypotheses [7], namely, origin from mesothelial metaplasia of the tunica vaginalis; mucinous metaplasia following mesothelial cell migration into the testis due to inflammation; development from Müllerian duct remnants located outside the testis, such as in the appendix or scrotum; and differentiation from teratomatous cells.

In nearly all reported cases, high orchiectomy was performed, leading to a definitive diagnosis. The prognosis of mucinous borderline tumors of the testis is generally favorable; however, because a case with metastasis has been reported, careful follow‐up is warranted [2]. Regarding the follow‐up protocol, there are no clear guidelines established, and a report suggests that annual chest and abdominal CT scans, gastrointestinal endoscopy every 5 years, and tumor marker evaluation should be performed [1]. Since a case of recurrence at 6 months postoperatively was reported [2], we established a follow‐up protocol consisting of chest and abdominal CT imaging at 6 months after surgery, followed by annual examinations. Although no definitive follow‐up duration has been established, follow‐up for a minimum of 5 years is advisable. Although there is no specific tumor marker, multiple reports have documented cases of elevated CEA and CA19‐9 in patients with mucinous tumor of the testis [3, 8]. Hao et al. reported that increased CA19‐9 and CEA levels might provide an early indication of a mucinous tumor of the testis, especially mucinous adenocarcinoma or mucinous borderline tumor [9]. Unfortunately, CEA and CA19‐9 were not measured and could not be evaluated in this case; however, no signs of recurrence or metastasis were observed on chest and abdominal CT scans half and 1 year postoperatively.

4. Conclusion

Primary mucinous borderline tumors of the testis are exceptionally rare, with only 16 cases reported worldwide to date, including the present case. Accurate diagnosis requires exclusion of metastatic disease and germ cell tumor origins, particularly teratomas with mucinous components. While the prognosis is generally favorable following high orchiectomy, the occurrence of at least one metastatic case underscores the need for vigilant long‐term follow‐up.

Ethics Statement

The protocol for this research project has been approved by a suitably constituted Ethics Committee of the institution and it conforms to the provisions of the Declaration of Helsinki. The Ethics Committee of the Japan Community Health Care Organization, Isahaya General Hospital, Approval No. 154. All informed consent was obtained from the subject(s) and/or guardian(s).

Consent

Written informed consent was obtained from the patient.

Conflicts of Interest

The authors declare no conflicts of interest.

Shirota T., Nakanishi H., Hakariya T., Nishimura N., Sato S., and Nakashima M., “Primary Mucinous Borderline Tumor of the Testis: A Case Report,” IJU Case Reports 9, no. 1 (2026): e70139, 10.1002/iju5.70139.

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.