Trends of low-grade serous ovarian carcinoma in the United States

To the Editor, In the United States, ovarian carcinoma remains the most deadly gynecologic malignancy, and approximately 14,000 deaths are projected from the disease in 2017 [1].Ovarian carcinoma comprises various histologic subtypes based on the cell of origin, with serous carcinoma being the most common subtype.Serous ovarian carcinomas are commonly grouped into 2 types based on histologic characteristics (high-grade and low-grade) [2,3].Mounting evidence has shown that low-grade serous ovarian carcinoma (LGSOC) has distinct clinical and molecular differences from high-grade serous ovarian carcinoma (HGSOC) [4,5].Unlike HGSOC, LGSOC is considered a rare tumor and has been understudied due to its low incidence.To date, there have been no population-based reports of LGSOC statistical trends.The objective of our study was to examine the temporal changes in the proportion of LGSOC among women with epithelial ovarian cancer.This retrospective observational study examined data from the National Cancer Institute, Surveillance, Epidemiology, and End Results (SEER) program between 1973 and 2013.The SEER database is the largest tumor registry in the United States covering approximately 28% of the US population.The SEER data are publicly available and de-identified.The University of Southern California Institutional Review Board exempted this study.SEER*Stat 8.3.2 (IMS Inc., Calverton, MD, USA) was used to extract the SEER 18 cases, generating the dataset from "Ovary" limited to malignancy and female sex.Primary invasive epithelial ovarian cancer was eligible for analysis, and the International Classification of Diseases for Oncology, Third Edition (ICD-O-3) site/histology validation and World Health Organization histological classification codes were used for identifying serous ovarian cancer cases as previously described [6].In our study, tumor grade was combined with histology types, and serous ovarian carcinomas that were designated as well-differentiated tumors (grade 1) were considered LGSOC whereas moderately-differentiated (grade 2) and poorly-differentiated (grade 3) tumors were considered HGSOC.This classification is based on the rationale that historical grade 1 tumors correlate with LGSOC and grade 3 tumors correlate with HGSOC in the 2-tier system [3].In addition, moderately-differentiated (grade 2) tumors were grouped as HGSOC in this study as the majority of this group are generally high-grade tumors.