Endometrial endometrioid adenocarcinoma with a malignant squamous component: is the unusual metastatic pattern unique of these tumors?

The FIGO scheme is currently applied for tumor grading of endometrioid adenocarcinoma. The current report presents a series of ten cases of endometrioid carcinomas that when applying the FIGO grading does not fully convey the true biological nature of the disease. The squamous component of these tumors is malignant; it constitutes the predominant invasive component, and it often metastasizes to unconventional sites. Half of the cohort developed distant disease recurrence within 2 years, even those with early-stage disease. Somatic mutations were analyzed, targeting 101 genes in all ten cases, and mutations in PTEN, MMR, PIK3CA, ATM, RB1, and TP53 genes were detected, often multiple mutations in the same case. None of the cases revealed unique molecular signatures or previously unreported gene mutations. Immunohistochemical staining for beta-catenin showed aberrant nuclear staining in eight of ten cases and remaining two showed cytoplasmic and membranous staining. Aggressive behavior and unusual sites of metastases are observed in this series even in low-grade tumor. The FIGO grading on smaller samples may be deceptive for these cases. Even if FIGO is applied, the pathology report should emphasize the malignant squamous component and its potential significance so that the gynecologic oncology team can formulate appropriate adjuvant treatment upfront. This case series argues that this histology should be regarded as a high-grade endometrioid carcinoma and can show unusual metastatic patterns. Further research is needed with more cases within this histologic subtype to guide recommendations on adjuvant therapies for this aggressive tumor type.