To the best of our knowledge, this is the first study to examine the positional relationship between the second HV and the PR using CTC and MRI.
The positions of the second HV and the PR were nearly concordant, with a displacement of up to 3 cm on the cephalocaudal side (Fig. 2C). Two earlier studies reported the distance from the anal verge to the peritoneal reflection. One measured the distance from the anal verge to the PR intraoperatively using an endoscope with a median of 13.2 cm and a range of 8.5–21.0 cm [6], and the other used an endoscope with a median of 8.8 cm and a range of 6.4–11.2 cm [9]. The distance from the anal verge to the second HV was also measured in 400 patients using a rigid endoscope, with a mean of 9.4 cm and a range of 7–12 cm [10]. Thus, from the anal verge, the measured distances between the valve and the PR were generally similar, with variations within a certain range, but there were large individual differences. We found no report that evaluated the positional relationship between the second Houston valve and the peritoneal reflection in the same patient. In this study, it was observed that the second HV tended to be located more cranially than the PR.
We examined the correlation between the position of the second HV and the PR, and patient variables. Univariate analysis revealed that the second Houston valve was caudal to the peritoneal reflection, and the peritoneal reflection was significantly shallower in males with high body weight and BMI (Fig. 3), whereas no correlation was observed with height or age. In the multivariate analysis, males were found to have a significantly shallower peritoneal reflection (Table 2). Previous studies examined the correlation between the position of the peritoneal reflection and patient variables. Wasserman et al. reported that the distance from the anal verge to the peritoneal reflection was greater in cases of high body weight and high BMI [6], and that the average distance tended to be greater in males than in females, as also reported by Memon et al. [11] and Najarian et al. [12]. These observations are in agreement with the results of the present study. We examined whether the shallower peritoneal reflection of the second Houston valve in males with a high BMI was due to a shallower PR or a deeper second HV. Using the reference line as the standard, we examined whether the positioning of the valve and the PR differed depending on patient variables. The PR was significantly more cranial than the reference line for heavier patients in the multivariate analysis. However, the position of the second HV was not affected by patient variables in the univariate analysis (Table 2). This suggests that the reason for the elevation of the second Houston valve in males with high body weight is due to a change in the height of the peritoneal reflection without a change in the position of the valve. The second HV is a structure in the intestinal tract with its position not dependent on patient variables. Higher peritoneal reflection in men with heavy body weight can possibly be attributed to a higher amount of retroperitoneal fat. This may result in a greater amount of fat below the peritoneal reflection, resulting in a higher position. In men, the seminal vesicles and prostate gland are located caudal to the PR, and thus the position of the PR is relatively higher in men than that in women.
Table 2
– Logistic regression analysis for the anatomical landmarks of the rectum
| | Univariate analysis | Multivariate analysis |
| | P-value | Odds Ratio | 95% CI | P-value |
Depth of the second Huston valve in CTC |
Sex | Male vs. female | 0.4558 | | | |
Age (years) | < 63 vs. ≥63 | 0.3954 | | | |
Height (m) | < 1.63 vs. ≥1.63 | 0.2951 | | | |
Weight (kg) | continuous variable | 0.5398 | | | |
Depth of peritoneal reflection in MRI |
Sex | Male vs. female | 0.1311 | | | |
Age (years) | < 63 vs. ≥63 | 0.6809 | | | |
Height (m) | < 1.63 vs. ≥1.63 | 0.2491 | | | |
Weight (kg) | continuous variable | 0.0968 | | | |
Positional relationship between the second Huston valve and peritoneal reflection |
Sex | Male vs. female | 0.0047 | 2.75 | 1.15–6.56 | 0.023 |
Age (years) | < 63 vs. ≥63 | 0.3084 | | | |
Height (m) | < 1.63 vs. ≥1.63 | 0.1633 | | | |
Weight (kg) | continuous variable | 0.0913 | 1.64 | 0.26–10.3 | 0.5973 |
* Stratified by median value |
In general, it is important to determine preoperatively whether a rectal cancer tumor is located cranial or caudal to the PR because cancers on the caudal side are associated with a higher local recurrence rate [13–15]. Preoperative CRT is recommended to reduce local recurrence for these cases [16–18]. The location of the PR is generally evaluated by MRI [19, 20], although a study reported that the PR was visible by MRI in only 74.4% cases [7]. Another study reported that the peritoneal reflection could be recognized with certainty in only 68.2% of cases, and could not be evaluated in approximately 30% of cases. The same study also revealed that when preoperative MRI was used to determine whether the lesion was located cranial or caudal to PR and results were compared with intraoperative findings, the assessment was incorrect in 10% of cases [16]. Therefore, MRI is not always reliable. The positional relationship between the second HV and the lesion can be easily observed during preoperative examination using endoscopy. Endoscopy of the rectums of 400 adults revealed that the second HV was visible in 92% of cases, while in 6.5% only one HV was visible, and in 1.5% no Houston valve was present [10]. The present study has revealed that second HV can be confirmed by endoscopy when it is difficult to evaluate the PR by MRI, and conversely, the second HV can be evaluated by MRI when it is difficult to evaluate endoscopically. It is important to make a comprehensive judgment based on multiple modalities, rather than a single modality, because the locations of the second HV and the PR may differ depending on patient background.
In Japan, upper and lower rectal cancers are customarily classified based on the location of the lesion in relation to the second HV, and the indication for CRT is determined. It is not clear whether dividing the upper and lower rectum based on the second HV or on the information obtained from MRIs reflect better prognosis. It was difficult to evaluate this in the present study because of the small number of cases and the single-center nature of the study. In future, it is necessary to examine which method reflects better prognosis, the method based on the second HV or the one based on MRI, and whether it should be included in CRT.
This study had several limitations. First, it was conducted at a single institution and only included Asians. Since the anatomical location of the second HV may differ based on race, further investigations are warranted in multi-center, multi-racial studies. Second, all patients had rectal cancer, which may have affected the original position of the second HV and the PR due to changes associated with tumor growth, such as invasion of the intestinal wall and intestinal dilatation. It would be desirable to conduct a similar study in patients without lesions. Third, CTC may cause the second HV to be pushed up cranially due to airflow, while with MRI, the position of the PR is stable due to the lack of air inflation. Thus, the second HV tends to be cranial to the PR. Intraoperative endoscopy or cadaveric studies might be used to examine the positional relationship between the second HV and the PR in real time and to evaluate the degree of displacement caused by air inflation.
Our study shows that the position of the second HV is concordant with that of the PR, although its position may differ depending on patient variables. We found that in males with a high BMI, the PR is located more cranially than the second HV. In conclusion, the location of the second HV is useful as a surrogate marker for estimating the positional relationship of the lesion and the PR at the time of preoperative endoscopic examination.