Case 110

A 45-year-old male with a ill-defined appendix mass.

1. What's the morphologic diagnosis?

   A: Adenocarcinoma

   B: Well differentiated neuroendocrine tumor

   C: Goblet cell adenocarcinoma

   D: Benign entrapped glands

Answer

The correct answer is B. This is a case of well-differentiated neuroendocrine tumor. Immunostains performed show that the tumor cells are positive for pancytokeratin, synaptophysin, and INSM1 with a Ki-67 proliferation index of less than 3%.

• EC-cell NETs (most common type):

  • Uniform polygonal tumor cells.

  • Frequently arranged in large nests with peripheral palisading and glandular formations.

  • Similar to ileal EC-cell NETs.

  • Mitoses are rare or absent; necrosis is uncommon.

  • Often associated with fibrotic stromal response.

  • Tumor bulk typically located in the deep muscular wall and subserosa.

  • In muscular wall, nests may appear as small tumor cell ribbons.

• Mesoappendix involvement:

  • ~1/3 of appendiceal NETs infiltrate the mesoappendix.

  • 50–82% of those involve <3 mm depth.

• L-cell NETs:

  • Composed of different cell type from EC-cell NETs.

  • Exhibit trabecular or glandular growth pattern.

  • Cells produce GLP-1 and other proglucagon-derived peptides.

  • Similar to rectal L-cell NETs.

• Tubular NETs (rare subtype):

  • Formerly termed "tubular carcinoid."

  • Important to distinguish from adenocarcinoma and goblet cell adenocarcinoma.

  • Stromal retraction artifact should not be mistaken for vascular invasion.

• Grading:

  • Vast majority are G1 (86–91%) or G2 (9–14%).

• Reporting:

  • Specifying L-cell vs. EC-cell NET is not required in pathology reports.

  • No prognostic or therapeutic implications.

Case credit: UCSD Pathology

Author: Wangpan Jackson Shi, MD