Circulating tumour DNA (ctDNA) facilitates longitudinal study of the tumour genome, which, unlike tumour tissue biopsies, globally reflects intratumor and intermetastatis heterogeneity. Despite its costs, next-generation sequencing (NGS) has revolutionised the study of ctDNA, ensuring a more comprehensive and multimodal approach, increasing data collection, and introducing new variables that can be correlated with clinical outcomes. Current NGS strategies can comprise a tumour-informed set of genes or the entire genome and detect a tumour fraction as low as 10−5.
1. Introduction
Ovarian cancer (OC) is the 7th most frequent gynaecological malignancy worldwide and the main cause of gynaecological cancer death
[1][2]. OC is of epithelial origin in 90% of cases, and these can be classified into five different histological subgroups based on the World Health Organization’s (WHO) current classification: high-grade serous ovarian carcinoma (HGSOC), endometrioid carcinoma, clear-cell carcinoma, low-grade serous carcinoma, and mucinous carcinoma
[3]. Most cases are diagnosed at advanced stages with peritoneal involvement, indicating poor overall survival (OS), despite the best therapeutic efforts
[1][4]. However, different subtypes have diverse molecular and phenotypical behaviours, as well as distinct prognosis and treatment options
[1].
HEA-4 and CA-125 are the two clinically useful serum protein biomarkers for OC. Only CA-125 is approved for evaluating treatment response and disease recurrence
[5][6]. The absence of higher-sensitivity biomarkers capable of early detection and prognostication remains an area of need in the management of EOC
[1][4][5][7][8]. In numerous cancers, cell-free DNA (cfDNA) has shown promise in predicting prognosis, assessing treatment response, and recurrence detection
[4][7][9].
Table 1 compares CA-125 and ctDNA as biomarkers of OC.
Table 1. Comparison between Ca-125 and ctDNA as biomarkers of OC.
| CA-125 |
ctDNA |
| Non-invasive |
| Can be altered by other coexisting physiological and pathological conditions |
| Inexpensive and highly available |
Expensive and restricted to specialist centres |
| Simple methodology |
Complex methodology |
| Results in minutes-hours |
Results in days and weeks |
| Quantitative marker |
Quantitative and qualitative markers |
| One continuous variable |
Can generate multiple continuous and discrete variables |
| Only informative regarding the presence/absence of treatment response and recurrence |
Yields more information regarding treatment response and tumour recurrence, like resistance mechanisms and targetable genetic alterations |
| Directly interpreted by the clinician |
Requires specialised interpretation |
| Easily detected in blood and urine |
Low concentrations in biological fluids |
| The utility is limited to producing tumours (mainly restricted to HGSOC) |
Theoretically applicable to all histological subtypes |
| Established and recognised clinical utility in trials |
Clinical utility is debatable and requires confirmation in prospective trials |