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Matsuoka et al. J Cancer Metastasis Treat 2018;4:6 I http://dx.doi.org/10.20517/2394-4722.2017.85 Page 3 of 13

[9]
20% false negative for peritoneal dissemination . Thus, there is still a need for more sensitive methods of
PLC with lower false-positive and false-negative rates.

GENETIC DETECTION OF PLC
The greater sensitivity of RT-PCR analysis has made it possible to detect micrometastasis in the basis of
cancer-tissue-specific messenger RNA (mRNA) expression in peripheral veins, lymph nodes, bone marrow,
[23]
and the peritoneal cavity . Molecular diagnosis using RT-PCR is generally reflected to be a more sensitive
and quantitative method than conventional cytology for the detection of micrometastasis in PLC [3,10] . Thus,
RT-PCR analysis of PLF should have clinical significance in the diagnostic evaluation of suspected peritoneal
metastasis and the development of therapeutic strategies. Based on a range of studies, there is a robust
correlation between the results of RT-PCR analysis of PLF and prognosis after curative operation in patients
with advanced gastric cancer [Table 1]. In this section, we will discuss selected molecular markers of PLC
including those based on genetic approaches.

Carcinoembryonic antigen
Studies conducted over the last couple of decades have demonstrated the usefulness of measuring CEA
mRNA to detect micrometastasis in the peritoneal cavity. Nakanishi et al. were the first to describe the
[24]
high sensitivity for detecting free cancer cells through RT-PCR amplification of CEA mRNA. Positive rate
of analysis through RT-PCR was elevated to 20% than that of cytology alone . Subsequent to their study,
[24]
many further studies examining CEA mRNA in PLF as the target molecular marker in gastric cancer
were published [11,15,25-32] . For instance, one study using RT-PCR for CEA mRNA showed a detection rate
[33]
of free cancer cells of 28%, with a 14% higher detection rate than for PLC . A recent prospective study of
quantitative CEA mRNA detection in PLF using the most desirable cutoff value of CEA mRNA of 0.1 by ROC
curve analyses found that the positive rates for CEA mRNA were 45.7% and 50.0% in T3 and T4 patients,
respectively. Among the CEA mRNA-positive patients, 55.0% induced peritoneal metastasis. In contrast,
only 3.0% of patients who were negative for CEA mRNA had peritoneal relapse, 84.6% of the positive rate of
CEA mRNA in PLF from patients with peritoneal dissemination for the period of postoperative surveillance.
CEA mRNA was shown to be only an independent prognostic factor in multivariate analysis with peritoneal
recurrence-free survival .
[30]
Nevertheless, the sensitivity of the CEA RT-PCR assay for detecting peritoneal micrometastasis is still
insufficient. Moreover, false positive results, caused by expression of CEA in no malignant cells such as
mesothelial cells and lymphocytes, remains a key problem of this technique . To overcome these problems,
[34]
a recent study showed that a novel and rapid molecular method of diagnosis using the technique of
transcription-reverse transcriptase concerted reaction (TRC) has been developed [35,36] . A prospective study at
multiple institutions to examine the clinical benefit of TRC diagnosis with PLF from gastric cancer patients
was carried out. Accordingly, TRC can be a prognostic factor for the prediction of patient outcome and
peritoneal metastasis of gastric cancer with serosa-infiltrating tumors. On the other hand, another paper
showed that CEA mRNA index (CmRI) (CEA mRNA/porphobilinogen deaminase mRNA × 10,000) values
in PLF may be a useful tool for reflecting the response of peritoneal relapse to induction chemotherapy and
that the advantage of conversion gastric surgery could be predicted by CmRI values .
[37]
Cytokeratin
Keratins are intermediate filament proteins which are closely related with the structural integrity of epithelial
cells. Recently, several studies have identified cytokeratin-20 (CK-20) as one of the potential cancer-related
biomarkers for the detection of peritoneal free cancer cells for the patients with gastric cancer [38,39] . CK-20 has
been used as a factor with CEA in a multiple-marker analysis for the detection of peritoneal micrometastasis .
[40]
In a recent study, real-time quantitative RT-PCR analysis of the CEA and/or CK20 transcripts in PLF was