Low-grade serous ovarian carcinoma (LGSOC) and high-grade serous ovarian carcinoma (HGSOC) represent two different entities with distinct clinical, histological, immunohistochemical (IHC), and molecular features. HGSOC accounts for 60–70 % of ovarian cancers compared to LGSOC, accounting for approximately 5 % [[1], [2], [3], [4]]. LGSOC is typically diagnosed in younger women and is less sensitive to chemotherapy compared to HGSOC [[5], [6], [7]]. Both LGSOC and HGSOC tend to be diagnosed at stage III-IV, but with different prognoses [6]. Prolonged survival is observed in LGSOC when compared to HGSOC, often due to the indolent nature of LGSOC as well as patients' younger age at diagnosis and thus longer clinical course [1]. Histologically, LGSOC has uniform cells with mild to moderate atypia and typically up to 12 mitoses per 10 high-power fields (HPFs). In contrast, HGSOC is characterized by the presence of marked nuclear atypia, conspicuous mitotic activity, and more than 12 mitoses per 10 HPFs [8].

The origins of these two cancers also differ, with LGSOC typically arising in the ovarian epithelium in the setting of serous borderline tumors (SBTs) [8], and HGSOC typically arising from serous tubal intraepithelial carcinoma (STIC) [9]. This difference in pathogenesis leads to different molecular features. LGSOCs are associated with RAS/RAF pathway mutations [10,11], with greater than 50 % harboring a KRAS, NRAS, or BRAF mutation [12,13]. Alternatively, HGSOCs have almost ubiquitous mutations in TP53 [14,15].

Typically, serous ovarian carcinomas occur in a pure form, either LGSOC or HGSOC, and standard treatment options, both for primary treatment and relapse, generally differ. However, a subset of cases presents as a combination of both tumor types, as either a synchronous or metachronous phenomenon. Little is known about the best treatment practices or clinical outcomes for patients with mixed HGSOC and LGSOC. This study presents the clinical and pathologic features, including a discussion of treatment modalities, of a cohort of synchronous or metachronous mixed LGSOC and HGSOC.