The present study demonstrated that PET/CT underestimates the extent of PC when compared to the intraoperative findings, which is in line with the results reported in the literature [
11,
16‐
18]. An earlier study evaluating metastatic patterns of colorectal carcinoma showed that mucinous carcinoma and SRCC more frequently metastasize to the peritoneum compared to adenocarcinoma (48%, 51%, and 20%, respectively) [
19]. Especially in mucinous carcinoma or SRCC an underestimation might occur due to the fact that these histological subtypes are generally less [
18F]F-FDG avid, probably due to the relative hypocellularity of these tumors [
20]. Therefore, it seems more difficult to assess the extent of the PC of these histological tumor types, which unfortunately are common subtypes in patients scheduled for CRS-HIPEC [
21]. The PCI score was underestimated by PET/CT regardless of the histological typing. Small peritoneal metastases are easily missed by PET/CT due to its limited image resolution. Secondly, it is difficult to distinguish peritoneal metastases from increased physiological FDG-uptake in the intestines. Another reason for underestimation might be the median time between the PET/CT scan and surgery (33 days). Surgical assessment has also been shown to overestimate PCI scoring, possibly adding to the gap between scoring methods [
16]. Further limitations of the current study are the retrospective character and relatively small number of cases. Three studies showed similar results compared to the present study [
11,
20,
22]. Dromain et al. [
11] compared surgical PCI scores with PET/CT scores and found an ICC of 0.12. No difference in the value of [
18F]F-FDG PET/CT between mucinous and non-mucinous PC (ICC of 0.11 vs. 0.14) was found. On a contrary, De Vos et al. [
20] included 55 patients with PC from colorectal origin. PC was detected by PET/CT in 96% of the patients with non-mucinous histology and in 60% of the patients with mucinous histology. When comparing the surgical results with the PET/CT results for the extent of PC, an adequate correlation (
ρ = 0.623) for patients with non-mucinous tumors and a poor correlation for patients with mucinous tumors (
ρ = − 0180) was found. The difference between patients with non-mucinous histology and mucinous histology is in contrast with results of the present study. De Vos et al. quantified the extent of PC by using the modified 7 region count (7RC), whereas the present study used the modified PCI score. The PCI is the most accepted method in scoring PC, furthermore it has been identified as a separate prognostic factor for patients with PC [
23‐
25]. This might explain the difference in results between both studies. In a retrospective study of 58 patients, Pasqual et al. [
26] found that PET/CT did not detect PC in 17% of the patients and underestimated the extent of PC in 43% of the cases. Four studies showed more favorable results for PET/CT in the detection of PC [
16‐
18,
27].These studies included patients with various primary tumors (colorectal, gastric, ovarian, pseudomyxoma peritonei, mesothelioma), [
16‐
18] while our study focused on colorectal PC. Colorectal PC has a different biology and prognosis compared to other origins of disease. The inclusion of different disease origins might cause heterogeneity in the study population when investigating limits of detection methods like PET/CT. Pfannenberg et al. [
17] compared both PET and CT and fused PET/CT for the extent of PC, using a contrast-enhanced PET/CT scan where abnormalities not related to FDG-uptake were also interpreted as positive. In the present study, however, patients underwent a PET/CT scan with a low-dose CT. Using a contrast-enhanced PET/CT (especially with mucinous tumors) might improve the accuracy of rPCI. Different outcomes might also be explained by the fact that Pfannenberg et al. had a mean sPCI of 18.6 ± 11.6, whereas in the current study the mean sPCI was 11.8 ± 6.8 (
p < 0.001). It might be more difficult to correctly score patients based on imaging with less extensive peritoneal disease, probably consisting of smaller lesions. CT tends to have lower sensitivity in smaller lesions and is deemed not reliable in lesions smaller than 5 cm [
28,
29]. Also, the evaluation of CT is dependent of the radiologist, possibly leading to outcome bias [
10]. Klumpp et al. [
18] compared the diagnostic accuracy of contrast-enhanced PET/CT and MRI in relation to the surgical results. A diagnostic accuracy of 94% for PET/CT and 88% for MRI was found. Dubreuil et al. [
30] assessed the prognostic impact of contrast-enhanced PET/CT in peritoneal mesothelioma. A diagnostic accuracy of 87% was found with a sensitivity and specificity of 86% and 89%, respectively. Liberale et al. [
27] focused on colorectal PC. Their study found a correlation of 77% between sPCI and rPCI, but specific PCI scores were not described. Berthelot et al. [
16] showed a good correlation between the PET/CT and the surgical assessment of the extent of PC (
ρ 0.615;
p = 0.0005). They also found a good correlation between rPCI and histological data. Compared to the histological PCI (hPCI), the sPCI was overestimated (hPCI 9.5 ± 5.6 vs. sPCI 12.3 ± 8.0,
p < 0.05). Reasons for surgical overestimation might be due fibrous non-malignant tissue or adhesions mimicking visual and tactile characteristics of PC during surgery. Therefore, the rPCI might correlate better with hPCI than with sPCI. This might explain the difference in PCI scoring in the current study.