LAMN is extremely rare, with no specific clinical manifestations. Histologically, LAMN has low-grade cytological characteristics. However, it is easy to develop into PMP with recurrence and metastasis, so regular follow-up is required. Attention should be paid to observing the position of the tumour and the distance between the tumour and the margin. The pathologist needs to examine the serosal surface of visible mucus and solid areas. It is difficult to distinguish appendiceal mucinous tumours from ovarian mucinous tumours because of the close location and the nonspecific serum tumour markers, increasing the challenge of diagnosing LAMN .
Early and accurate diagnosis has a great significance for the prognosis of patients. Therefore, pathologists especially need to make a distinction between LAMN and the following types of similar tumours. Histologically, prominent features of LAMN are atrophy of submucosa and lamina propria at the lesion, but the mucosal muscular of serrated polyps/adenomas is intact. Furthermore, LAMN produces much mucus, but serrated polyps/adenomas have not . HAMN has papillary and sieve structure, and the tumour cells showed considerable atypia, polymorphonuclear and pathological mitosis. LAMN often has low-grade cell atypia . A villous adenoma is similar to LAMN and can be arranged in villous shape, but LAMN tumour cells are close to the sclerotic fibrous stroma rather than lamina propria. Mucinous adenocarcinoma often grows infiltratively, with single cells, small cell clusters, or irregular glands, often accompanied by the interstitial reaction. Mucinous adenocarcinoma cell atypia and pleomorphism are obvious. In immunohistochemistry, LAMN usually expresses CK (pan), CDX2, and SATB2, which suggested the origin of the lower gastrointestinal tract . Without PAX8 expression, LAMN can be distinguished from ovarian mucinous tumours. The microsatellite is stable in LAMN . There are usually KRAS / GNAS co-mutations in LAMN and ovarian mucinous tumors are usually accompanied by KRAS and CDKN2A mutations . The presence of a collision between LAMN and ANET is exceedingly rare. In ANET, large and small nests were composed of polygonal cells with salt-and-pepper chromatin and cytoplasmic brightly eosinophilic basally-located granules. ANET cells were positive for SSTR2, CgA, and Syn. In this case, the lesion at the pulmonary pleura was confirmed as a LAMN metastasis via immunohistochemical staining and morphology. The elastic fiber staining showed that the pleura was intact, confirming that LAMN metastasized to the pulmonary pleura via the esophageal fissure.
There is no consensus regarding surgical treatment for LAMN. For patients without extra-appendiceal disease, appendectomy and follow-up are recommended. Right colectomy should not be performed, since it gives no advantage for patients with LAMN . Some experts suggest a simple cecectomy in the case of a mucinous neoplasm with positive margins and negative appendiceal lymph nodes. Peritoneal mucus should be cleared and hyperthermic intraperitoneal perfusion chemotherapy (HIPEC) should be performed on LAMN patients with PMP . In this case, the patient underwent right hemicolectomy, omentum resection, cauterization of peritoneal lesions (sub-diaphragm, rectal fossa of the uterus, retroflexed peritoneum of the bladder, parametrium), total hysterectomy, bilateral adnexal resection, and HIPEC. Currently, follow-up is recommended to start yearly, including an abdominal computed tomography (CT) scan and determination of serum tumour markers. Furthermore, CA125, CA19-9, and CEA are significant prognostic elements to predict patients’ prognosis . Our case is rare, and its description and report will help in the diagnosis of the pulmonary pleura metastasis from LAMN with ANET.