The Utility and Potential Pitfalls of Fibroblast Activation Protein-Alpha (FAP) Immunohistochemistry in Diagnosing Invasive Colorectal Adenocarcinoma in Biopsies

Diagnosing invasive colorectal adenocarcinoma (CRC) in limited biopsy samples can be challenging. This study evaluates fibroblast activation protein-α (FAP)'s role in diagnosing CRC and identifies potential pitfalls. To evaluate the expression pattern of FAP in CRC, we analyzed 168 cases of CRC with established diagnosis, including 30 whole slides and 138 cases from a tissue microarray (TMA). The study group includes 33 colorectal lesion biopsies with diagnostic challenges, identified retrospectively over 9 years in 1 institution. These biopsies were initially not diagnosed as CRC and confirmed to be so upon subsequent resection. Six additional cases of adenoma and 4 cases of solitary rectal ulcers were also evaluated. FAP was expressed in the cancer stroma of 161/168 (96%) of CRC cases, including 29/30 whole slides and 132/138 TMA. The diagnosis of 33 challenging cases by H&E included adenoma (n=4), high-grade dysplasia (HGD, n=16), and concerning for CRC (n=13). Among these cases, all 20 cases of adenoma or HGD were FAP-negative. Out of 13 "concerning for CRC" cases, 9 (69%) showed FAP positivity in the stroma. Five of 6 additional adenomas were FAP-negative, while 1 adenoma showed FAP staining in scar tissue from the prior biopsy site. Of the 4 cases of solitary rectal ulcers, 1 exhibited weak FAP expression in the granulation tissue. In conclusion, FAP immunohistochemistry may assist in diagnosing CRC in challenging cases. Due to FAP positivity in benign conditions, such as scar tissue and benign ulcers, awareness of the pitfalls is important when applying FAP immunostaining in practice.