Clinical value of computed tomography-based three-dimensional reconstruction of the pelvis and mesorectum in middle to low rectal carcinoma

Abstract

BACKGROUND

Laparoscopic anus-preserving resection for middle to low rectal carcinoma is technically challenging because of restricted pelvic space and bulky mesorectum. Although computed tomography (CT) imaging-based three-dimensional (3D) reconstruction enables accurate pelvic and mesorectal measurements, few studies have systematically quantified sex-related differences or explored their impact on surgical outcomes.

AIM

To evaluate sex-related pelvic and mesorectal anatomical differences using CT-based 3D reconstruction and assess their clinical relevance in middle to low rectal carcinoma.

METHODS

We retrospectively analyzed 103 patients with primary middle to low rectal carcinoma who underwent laparoscopic low or ultra-low anterior resection between January 2018 and January 2024. Pelvic and mesorectal parameters were obtained using CT-based 3D reconstructions. Differences between sexes were compared, and their associations with operative difficulty and short-term outcomes were evaluated.

RESULTS

Significant sex-related differences were observed in pelvic diameters, pelvic angles, pelvic volume, and mesorectal fat volume (all P < 0.05). Male patients had smaller pelvic volumes (P = 0.007) but larger mesorectal fat volumes (P = 0.047) compared with female patients. Despite a higher incidence of previous abdominal surgery among female patients (P < 0.05), intraoperative blood loss was significantly lower in females than in males (P < 0.05).

CONCLUSION

CT-based 3D reconstruction reveals clinically relevant sex-related pelvic and mesorectal anatomical differences that may aid in preoperative assessment of middle to low rectal carcinoma.

Key Words: Rectal carcinoma; Pelvic anatomy; Three-dimensional reconstruction; Pelvic volume; Mesorectal fat; Laparoscopic surgery

Core Tip: The primary innovation of this study lies in the precise measurement of pelvic volume, rectal mesenteric volume, and rectal mesenteric fat volume using three-dimensional (3D) reconstruction. The 3D computed tomography-based reconstruction and measurement of the pelvis and mesorectum reveal clear, clinically relevant sex-related differences and provide precise parameters for defining a difficult pelvis. This method offers significant potential to enhance preoperative planning, guide personalized surgical strategies, and optimize outcomes in laparoscopic anus-preserving rectal cancer surgery.

INTRODUCTION

Since Heald introduced total mesorectal excision (TME) in 1982, it has become the gold standard for radical resection of low and intermediate rectal carcinoma[1,2]. However, rectal cancer surgery, particularly for middle to low rectal tumors, remains technically challenging because of the confined pelvic space and complex pelvic anatomy, which includes vital digestive, urological, and gynecological organs, as well as delicate pelvic nerves and blood vessels.

Several studies have investigated predictors of technical difficulty during TME for rectal carcinoma[3-6]. Although increased mesorectal fat and pelvic anatomical constraints have been implicated, most previous studies did not quantitatively assess pelvic volume or mesorectal soft-tissue volume using three-dimensional (3D) approaches.

Therefore, this retrospective study aimed to evaluate sex-related differences in pelvic and mesorectal soft-tissue anatomy and to explore their clinical implications for laparoscopic low anterior resection (L-LAR) and laparoscopic ultra-low anterior resection (L-ULAR). Using computed tomography (CT)-based 3D reconstruction, pelvic volume and mesorectal soft-tissue volume were quantified to assess their associations with operative difficulty and short-term surgical outcomes.

MATERIALS AND METHODS

Case collection

Clinicopathological and imaging data were retrospectively collected from 103 patients with middle to low rectal carcinoma who underwent L-LAR at a single tertiary medical center between January 2018 and January 2024. The cohort comprised 71 males and 32 females, with ages ranging from 32 years to 89 years and body mass index (BMI) values ranging from 15.94 kg/m² to 33.98 kg/m².

CT-based 3D reconstruction and pelvic measurements

Thin-slice scans were obtained using a 64-slice multidetector spiral CT system (SIEMENS SOMATOM Definition AS+). The CT DICOM datasets were imported into E3D digital medical 3D modeling and design software (Master Edition V19.15, Nanjing Huiqing Information Technology Co., Ltd.).

Bone tissue was defined using the threshold segmentation tool, with reconstruction thresholds set between 100 HU and 1200 HU. After generating a mask through threshold segmentation, irrelevant structures were removed using tools such as simple seed points and clump separation. Finally, a 3D pelvic model was reconstructed by selecting the “Reconstruct from smooth mask” option in solid modeling.

3D reconstruction and measurement of pelvic bony parameters: A 3D pelvic model, including portions of the lumbar vertebrae and proximal femur, was reconstructed using the methods described above. Anatomical landmarks identified in the transverse, coronal, and sagittal planes of the digital model were used to define reference points, determine measurement locations, and calculate corresponding distances and angles.

CT-based 3D reconstruction of the pelvis provided the following measurements: Anterior-posterior diameter of the pelvic inlet (ab), transverse diameter of the pelvic inlet (ef), anterior-posterior diameter of the pelvic outlet (cd), interischial spine diameter (gh), interischial tuberosity diameter (ij), superior-inferior diameter of the pubic symphysis (bc), sacrococcygeal distance (ad), and sacrococcygeal-pubic angle (δ; Figure 1).

Figure 1 Pelvic measurements for three-dimensional reconstruction. A: Anterior-posterior diameter of pelvic inlet (ab); anterior-posterior diameter of pelvic outlet (cd); superior-inferior diameter of the pubic symphysis (bc); sacrococcygeal distance (ad); and sacrococcygeal-pubic angle (δ); B: Transverse diameter of pelvic inlet (ef) and interischial spine diameter (gh); C: Interischial spine diameter (gh) and interischial tuberosity diameter (ij).

Pelvic volume was measured by selecting the “inlet pelvic plane (IPP)”[7], defined as the level at which the pelvic cavity is completely enclosed by bone, from the reconstructed 3D pelvic model (Figure 2). In the two-dimensional (2D) editing menu of the 3D reconstruction, the “Enable difference function, outline key layer mask” option was selected, and the medial pelvic boundary was manually outlined layer by layer. After completion of the outlining process, the “Merge difference” function was applied to automatically generate an initial pelvic volume mask. This mask was then converted into a solid model using the “Reconstruct from smooth mask” function in the masks panel, yielding a 3D image of the pelvic volume. The volume of the reconstructed model was subsequently calculated automatically in the models panel (Figure 3).

Figure 2 Three-dimensional reconstruction of the pelvis: Pelvic volume and inlet pelvic plane. A: Anterolateral view: Pelvic volume (σ) and inlet pelvic plane (IPP); B: Lateral view: Pelvic volume (σ) and IPP (semi-translucent pelvis). IPP: Inlet pelvic plane.

Figure 3 Three-dimensional reconstruction of pelvic volume. A: Lateral view: Pelvic volume (σ; semi-translucent pelvis); B: Pelvic volume on the transverse plane; C: Pelvic volume on the sagittal plane; D: Pelvic volume on the coronal plane.

3D reconstruction and measurement of pelvic soft tissue parameters: CT-based pelvic soft tissue parameters included rectal mesenteric fat volume at the superior border of the third sacral vertebra (α), rectal volume (β), rectal mesenteric volume (γ = α + β), rectal mesenteric fat volume at the level of the sciatic spine (θ), and anorectal angle (μ; Figures 4, 5, and 6).

Figure 4 Three-dimensional reconstruction of rectal mesenteric-related volumes in different orientations. A: Anterolateral view: Rectal mesenteric fat at the level of the superior border of the third sacral vertebra (α); B: Lateral view: Rectal volume at the level of the superior border of the third sacral vertebra (β; semi-translucent rectal mesenteric fat); C: Anterolateral view: Rectal mesenteric volume at the level of the superior border of the third sacral vertebra ((γ = α + β) and pelvis; D: Lateral view: Rectal mesenteric volume at the level of the superior border of the third sacral vertebra (γ) and pelvis.

Figure 5 Three-dimensional reconstruction of rectal mesenteric fat at the level of the sciatic spine. A: Front view: Rectal mesenteric fat at the level of the sciatic spine (θ); B: Behind view: Rectal mesenteric fat at the level of the sciatic spine (θ) and the pelvis.

Figure 6 Anorectal angle on the median sagittal plane. Anorectal angle (μ).

To measure these parameters, the horizontal plane at the superior border of the third sacral vertebra was first identified on the 2D CT cross-sectional image. In the 2D editing menu of the 3D reconstruction, the “Enable difference function, outline key layer mask” option was selected, and the rectal mesentery was manually outlined layer by layer. After completion of the outlining process, the “Merge difference” function was applied to automatically generate an initial rectal mesentery mask. This mask was then used for solid modeling by applying the “Reconstruct from smooth mask” function, yielding a 3D image of the rectal mesentery. The volume of the reconstructed model was automatically calculated in the “Models” panel.

Similarly, a 3D image of the rectum was reconstructed starting from the superior border of the third sacral vertebra. The rectal mesenteric fat model at this level was obtained using the “Boolean subtract” function in the “Boolean” menu by subtracting the rectal model from the rectal mesentery model, with the corresponding volume generated automatically. The rectal mesenteric fat volume at the level of the sciatic spine was measured using the same procedure, with the cross-sectional plane selected at the level of the sciatic spine.

Finally, the anorectal angle was measured on the 2D CT image by identifying the midsagittal plane, selecting the “Measurement tools-angle” option, and determining the angle at the anorectal junction between the central axes of the rectum and anal canal.

Statistical analysis

Data management and statistical analysis were performed using SPSS software, version 16.1.1 (SPSS Inc., Chicago, IL, United States). The normality of continuous variables was assessed using the Shapiro-Wilk test. Normally distributed data are expressed as mean ± SD, while non-normally distributed data are presented as median and interquartile range (IQR; Q1, Q3). To evaluate intra-observer reliability, measurements obtained by the same observer at two different time points were compared using the paired-samples t-test for normally distributed variables and the Wilcoxon signed-rank test for non-normally distributed variables. Inter-observer reliability was assessed by comparing measurements obtained by different observers using the same statistical approach. Correlations between repeated measurements were assessed using Pearson’s correlation coefficient (r) for normally distributed data and Spearman’s rank correlation coefficient (ρ) for non-normally distributed data. For comparisons between male and female cohorts, continuous variables were analyzed using the independent-samples t-test for normally distributed data with homogeneous variance and the Mann-Whitney U test for non-normally distributed data. Categorical variables were analyzed using the Pearson χ² test, continuity-corrected χ² test, or Fisher’s exact test, as appropriate. All statistical tests were two-sided, and a P value < 0.05 was considered statistically significant.

RESULTS

Assessment of data distribution

Tests for normality of each variable are summarized in Table 1. We assessed the normality of continuous variables using the Shapiro-Wilk test. Based on these results, normally distributed data are presented as mean ± SD, while non-normally distributed data are presented as median (IQR).

Table 1 Normality test analysis for each variable.

	W value	P value
Age	0.984	0.243
BMI	0.977	0.078
Surgical time	0.776	0.000a
Intraoperative blood loss	0.648	0.000a
Duration of postoperative hospital stay	0.715	0.000a
Postoperative anal evacuation time	0.765	0.000a
Postoperative resumption of semi-liquid food time	0.941	0.000a
Tumor height	0.923	0.000a
Tumor maximum diameter	0.947	0.000a
Anterior-posterior diameter of pelvic inlet	0.980	0.122
Transverse diameter of pelvic inlet	0.991	0.729
Anterior-posterior diameter of pelvic outlet	0.977	0.070
Interischial spine diameter	0.964	0.008a
Interischial tuberosity diameter	0.981	0.163
Superior-inferior diameter of the pubic symphysis	0.082	0.000a
Sacrococcygeal distance	0.989	0.579
sacrococcygeal-pubic angle	0.131	0.000a
Anterior-posterior diameter of pelvic inlet/sacrococcygeal distance	0.937	0.000a
Rectal mesenteric fat volume at the level of the superior border of the 3rd sacral vertebra	0.948	0.001a
Rectal volume at the level of the superior border of the 3rd sacral vertebra	0.877	0.000a
Rectal mesenteric fat volume at the level of the sciatic spine	0.932	0.000a
Pelvic volume	0.986	0.390
Anorectal angle	0.993	0.853
Rectal mesenteric volume at the level of superior border of the 3rd sacral vertebra	0.944	0.000a
Rectal mesenteric fat volume at the superior border of the 3rd sacral vertebra/pelvic volume	0.981	0.195

^aP < 0.05 indicates significant deviation from normal distribution.

BMI: Body mass index.

Baseline characteristics

A total of 103 patients were included in the study. Baseline characteristics are presented in Table 2. The mean age of the cohort was 64.81 years, and the mean BMI was 22.63 kg/m². The most frequent comorbidities were hypertension (37.9%), diabetes mellitus (13.6%), and cardiac disease (7.8%). Seventy-three patients (70.9%) were anemic at the time of hospitalization.

Table 2 Clinical and pathological characteristics data of 103 patients in the laparoscopic low anterior resection for rectal cancer.

	Range	mean ± SD/median (Q1, Q3)	Number of cases, n (%)
Age (year)	32-89	64.81 ± 11.832
Gender
Male			71 (68.9)
Female			32 (31.1)
BMI (kg/m2)	15.94-33.980	22.63 ± 2.910
Underlying comorbidities			48 (46.6)
Diabetes			14 (13.6)
Hypertension			39 (37.9)
Heart disease			8 (7.8)
Pulmonary disease			3 (2.9)
Stroke			4 (3.9)
Other			4 (3.9)
History of previous abdominal surgery			16 (15.5)
Neoadjuvant therapy			4 (3.9)
Hypoalbuminemia			17 (16.5)
Anemia			73 (70.9)
Blood transfusion			2 (1.9)
Tumor height (cm)	4-10	7 (6, 8)
Tumor maximum diameter (cm)	0.9-10	3.9 (3, 5)
Intraoperative preventive ostomy			36 (35.0)
pT/ypT staging
T0			1 (1.0)
Tis			4 (3.9)
T1			12 (11.7)
T2			13 (12.6)
T3			70 (68.0)
T4a			3 (2.9)
pN/ypN staging
N0			61 (59.2)
N1			26 (25.2)
N2			16 (15.5)
pTNM/ypTNM staging
0			5 (4.9)
I			20 (19.4)
II			36 (35)
III			42 (40.8)
Tumor differentiation grade
N/A			2 (1.9)
Well-differentiated			6 (5.8)
Moderately differentiated to well-differentiated			11 (10.7)
Moderately differentiated			70 (68)
Moderately differentiated to poorly differentiated			11 (10.7)
Poorly differentiated			3 (2.9)
Surgical time (minute)	78-892	222.83 (173, 260)
Intraoperative blood loss (mL)	10-300	50 (20, 50)
Postoperative complications (CD grade)
No complications			77 (74.8)
I			5 (4.9)
II			16 (15.5)
III			4 (3.9)
IV			1 (1.0)
Duration of postoperative hospital stay (days)	6-56	14 (12, 16)
Postoperative anal evacuation time (days)	1-7	2 (2, 3)
Postoperative resumption of semi-liquid food time (days)	3-17	7 (5, 9)

If the data follows a normal distribution: mean ± SD will be used to represent numerical data. The n (%) will be used to represent count and ordinal data. If the data does not follow a normal distribution: Median (Q1, Q3) will be used to represent numerical data. The n (%) will be used to represent count and ordinal data. N/A: Not applicable, BMI: Body mass index; CD: Clavien-Dindo.

Surgical and postoperative outcomes

The median operative time was 222.83 minutes, with a median intraoperative blood loss of 50 mL. The median postoperative hospital stay was 14 days. At 30 days after surgery, 77 patients (74.8%) had no complications. Postoperative complications were classified as grade I in 5 patients (4.9%), grade II in 16 patients (15.5%), grade III in 4 patients (3.9%), and grade IV in 1 patient.

Intra- and inter-observer reliability

Pelvic measurements were performed by the same senior colorectal surgeon. To assess intra-observer variability, bony and soft tissue parameters were re-measured in 20 patients after a 4-week interval, with the initial results concealed. Inter-observer variability was evaluated independently by two senior colorectal surgeons, who measured selected pelvic parameters from 20 patients blinded to each other’s results. As shown in Tables 3 and 4, the mean ± SD or median (Q1, Q3), correlation coefficients, and mean or median differences for each parameter demonstrated strong correlations (P < 0.05), indicating high reliability and accuracy.

Table 3 Within-group comparison of pelvic measurements and soft tissue measurements in 20 patients.

	mean ± SD/median (Q1, Q3)	mean ± SD/median (Q1, Q3)	r value (correlation coefficient)	P value (correlation coefficient)	P value (mean or median difference)
Anterior-posterior diameter of pelvic inlet (mm)	114.07 ± 12.58	113.74 ± 12.72	0.998	0.000a	0.936
Transverse diameter of pelvic inlet (mm)	120.91 ± 8.22	120.60 ± 8.16	0.998	0.000a	0.905
Anterior-posterior diameter of pelvic outlet (mm)	87.98 ± 12.07	88.12 ± 11.68	0.998	0.000a	0.971
Interischial spine diameter (mm)	91.56 (85.19, 106.16)	91.02 (84.84, 106.15)	0.992	0.000a	0.892
Interischial tuberosity diameter (mm)	97.128 ± 14.21	96.53 ± 14.35	0.998	0.000a	0.896
Superior-inferior diameter of the pubic symphysis (mm)	36.94 (33.34, 40.26)	36.08 (32.99, 39.37)	0.838	0.000a	0.589
Sacrococcygeal distance (mm)	127.09 ± 12.19	126.82 ± 12.35	0.998	0.000a	0.944
sacrococcygeal-pubic angle (°)	55.49 (46.10, 60.55)	54.59 (46.58, 60.59)	0.989	0.000a	0.978
Anterior-posterior diameter of pelvic inlet/sacrococcygeal distance	0.90 (0.82, 0.98)	0.90 (0.81, 0.98)	0.994	0.000a	0.957
Rectal mesenteric fat volume at the level of the superior border of the 3rd sacral vertebra (cm3)	178.91 (137.08, 198.96)	168.68 (135.51, 189.72)	0.92	0.000a	0.534
Rectal mesenteric fat volume at the level of the sciatic spine (cm3)	39.15 (33.12, 56.53)	35.56 (30.62, 53.16)	0.974	0.000a	0.626
Pelvic volume (cm3)	849.37 ± 112.75	836.76 ± 109.53	0.978	0.000a	0.722
Anorectal angle (°)	119.99 ± 14.88	119.67 ± 14.77	0.995	0.000a	0.946
Rectal mesenteric volume at the level of superior border of the 3rd sacral vertebra (cm3)	247.14 (219.43, 301.38)	241.78 (211.90, 297.89)	0.892	0.000a	0.516
Rectal volume at the level of the superior border of the 3rd sacral vertebra (cm3)	78.49 (55.94, 119.31)	68.76 (53.53, 123.11)	0.991	0.000a	0.589
Rectal mesenteric fat volume at the superior border of the 3rd sacral vertebra/pelvic volume	0.32 ± 0.08	0.31 ± 0.07	0.964	0.000a	0.619

^aP < 0.001.

If the data are normally distributed, they are expressed as mean ± SD; if not, they are expressed as median (Q1, Q3).

Table 4 Between-group comparison of pelvic measurements and soft tissue measurements in 20 patients.

	mean ± SD/median (Q1, Q3)	mean ± SD/median (Q1, Q3)	r value (correlation coefficient)	P value (correlation coefficient)	P value (mean or median difference)
Transverse diameter of pelvic inlet (mm)	124.44 ± 6.87	123.84 ± 6.93	0.988	0.000a	0.782
Anterior-posterior diameter of pelvic outlet (mm)	87.74 ± 8.45	87.61 ± 8.27	0.983	0.000a	0.962
Interischial spine diameter (mm)	93.02 (86.26, 100.70)	91.98 (86.63, 100.32)	0.991	0.000a	0.925
Interischial tuberosity diameter (mm)	96.90 ± 13.52	96.74 ± 13.33	0.997	0.000a	0.969
Superior-inferior diameter of the pubic symphysis (mm)	38.24 (35.63, 41.85)	37.68 (33.50, 42.26)	0.896	0.000a	0.589
Sacrococcygeal distance (mm)	122.33 ± 9.10	122.12 ± 9.05	0.992	0.000a	0.940
sacrococcygeal-pubic angle (°)	50.56 (45.18, 55.68)	51.35 (44.52, 54.76)	0.956	0.000a	0.620
Anterior-posterior diameter of pelvic inlet/sacrococcygeal distance	0.91 (0.83, 1.02)	0.91 (0.84, 1.01)	0.992	0.000a	0.989
Rectal mesenteric fat volume at the level of the superior border of the 3rd sacral vertebra (cm3)	170.50 (135.04, 213.31)	169.18 (111.89, 235.48)	0.814	0.000a	0.968
Rectal mesenteric fat volume at the level of the sciatic spine (cm3)	31.10 (24.55, 48.92)	30.85 (22.43, 46.44)	0.856	0.000a	0.883
Pelvic volume (cm3)	920.94 ± 110.70	946.28 ± 142.86	0.898	0.000a	0.534
Anorectal angle (°)	104.79 ± 14.39	105.21 ± 14.37	0.996	0.000a	0.926
Rectal mesenteric volume at the level of superior border of the 3rd sacral vertebra (cm3)	258.69 (212.54, 295.30)	249.25 (185.80, 321.64)	0.826	0.000a	0.883
Rectal volume at the level of the superior border of the 3rd sacral vertebra (cm3)	71.82 (58.29, 104.09)	77.43 (59.98, 93.66)	0.892	0.000a	1.000
Rectal mesenteric fat volume at the superior border of the 3rd sacral vertebra/pelvic volume	0.28 ± 0.09	0.28 ± 0.12	0.909	0.000a	0.955

^aP < 0.001.

If the normal distribution is satisfied, it is expressed as mean ± SD; if not, it is expressed as median (Q1, Q3).

Sex-specific pelvic anatomy

With respect to pelvic dimensions, the anterior-posterior diameter of the pelvic inlet, transverse diameter of the pelvic inlet, anterior-posterior diameter of the pelvic outlet, interischial spine diameter, anterior-posterior diameter of pelvic inlet/sacrococcygeal distance, and pelvic volume were significantly smaller in males than in females (P < 0.05). In contrast, the sacrococcygeal-pubic angle was larger in males. The rectal mesenteric fat volume, measured at both the superior border of the third sacral vertebra and at the level of the sciatic spine, was significantly greater in males than in females (P < 0.05). However, no significant sex differences were observed in the anorectal angle, rectal mesenteric fat volume, or rectal volume at the superior border of the third sacral vertebra (Table 5).

Table 5 Gender comparison of pelvic and soft tissue parameters in 103 patients, mean ± SD/median (Q1, Q3).

	Total cases (n = 103)	Male (n = 71)	Female (n = 32)	P value
Anterior-posterior diameter of pelvic inlet (mm)	111.60 ± 12.86	107.21 ± 11.15	121.35 ± 11.01	0.000b
Transverse diameter of pelvic inlet (mm)	122.90 ± 7.51	120.38 ± 6.36	128.50 ± 6.88	0.000b
Anterior-posterior diameter of pelvic outlet (mm)	87.80 ± 8.86	85.837 ± 7.96	93.21 ± 8.47	0.000b
Interischial spine diameter (mm)	92.54 (85.89, 101.40)	88.70 (83, 93.66)	106.35 (98.98, 108.94)	0.000b
Interischial tuberosity diameter (mm)	97.69 ± 12.95	92.88 (86.91, 97.61)	111.28 (101.37, 118.30)	0.000b
Superior-inferior diameter of the pubic symphysis (mm)	39.67 (36.55, 43.26)	39.92 (37.02, 44.22)	38.35 (35.63, 42.49)	0.180
Sacrococcygeal distance (mm)	123.40 ± 11.85	124.74 ± 11.14	120.44 ± 12.97	0.089
sacrococcygeal-pubic angle (°)	51.08 (45.10, 56.65)	54.17 (48.06, 58.46)	45 (40.76, 49.81)	0.000b
Anterior-posterior diameter of pelvic inlet/sacrococcygeal distance	0.89 (0.83, 0.97)	0.85 (0.81, 0.91)	1.01 (0.92, 1.05)	0.000b
Rectal mesenteric fat volume at the level of the superior border of the 3rd sacral vertebra (cm3)	171.25 (136.59, 212.59)	184.47 (146.11, 222.21)	155.94 (119.33, 199.55)	0.047a
Rectal volume at the level of the superior border of the 3rd sacral vertebra (cm3)	66.87 (53.51, 95.88)	66.19 (51.83, 91.02)	69.68 (56.26, 100.46)	0.613
Rectal mesenteric fat volume at the level of the sciatic spine (cm3)	33.59 (24.40, 46.15)	40.42 (26.24, 49.66)	28.16 (21.20, 35.76)	0.001a
Pelvic volume (cm3)	896.54 ± 113.97	876.50 ± 108.21	941.00 ± 115.49	0.007a
Anorectal angle (°)	110.17 ± 16.36	110.11 ± 17.31	110.30 ± 14.29	0.958
Rectal mesenteric volume at the level of superior border of the 3rd sacral vertebra (cm3)	250.79 (205.78, 294.21)	250.79 (215.41, 299.18)	238.86 (194.19, 286.30)	0.197
Rectal mesenteric fat volume at the superior border of the 3rd sacral vertebra/pelvic volume	0.2880 ± 0.079	0.3019 ± 0.078	0.2569 ± 0.073	0.007a

^aP < 0.05.

^bP < 0.001.

If the normal distribution is satisfied, it is expressed as mean ± SD; if not, it is expressed as median (Q1, Q3).

Sex-based differences in clinical and intraoperative outcomes

As shown in Table 6, baseline and intraoperative clinical parameters differed between male and female patients. A history of abdominal surgery was more common among females (28.1%) than in males (P = 0.038). Female patients exhibited significantly lower intraoperative blood loss than males, with a median blood loss of 30 mL (IQR: 20-50 mL) in females compared with 50 mL (IQR: 30-50 mL) in males (P = 0.034).

Table 6 Comparison of clinicopathological data in both genders and short-term operative outcome in 103 patients with middle and lower rectal carcinoma.

	Male (n = 71)	Female (n = 32)	P value
Age (years)	66 ± 11.98	62.16 ± 11.23	0.128
BMI (kg/m2)	22.56 ± 3.08	22.76 ± 2.54	0.748
Underlying comorbidities	39 (54.9)	9 (28.1)	0.012
Diabetes	9 (12.7)	5 (15.6)	0.926
Hypertension	31 (43.7)	8 (25)	0.071
Heart disease	8 (11.3)	0	0.114
Pulmonary disease	3 (4.2)	0	0.550
Stroke	2 (2.8)	2 (6.3)	0.777
Other	2 (2.8)	2 (6.3)	0.777
History of previous abdominal surgery	7 (9.9)	9 (28.1)	0.038a
Neoadjuvant therapy	3 (4.2)	1 (3.1)	1.000
Hypoalbuminemia	14 (19.7)	3 (9.4)	0.191
Anemia	52 (73.2)	21 (65.6)	0.431
Blood transfusion	2 (2.8)	0	1.000
Tumor height (cm)	7 (6, 8)	8 (6.25, 10)	0.063
Tumor maximum diameter (cm)	4 (3, 5)	3.5 (3, 5)	0.134
Intraoperative preventive ostomy	28 (39.4)	8 (25)	0.155
pT/ypT staging			0.420
T0	0	1
Tis	3 (4.2)	1 (3.1)
T1	8 (11.3)	4 (12.5)
T2	6 (8.5)	7 (21.9)
T3	52 (73.2)	18 (56.3)
T4a	2 (2.8)	1 (3.1)
pN/ypN staging			0.481
N0	41 (57.7)	20 (62.5)
N1	17 (23.9)	9 (28.1)
N2	13 (18.3)	3 (9.4)
pTNM/ypTNM staging			0.143
0	3 (4.2)	2 (6.3)
I	9 (12.7)	11 (34.4)
II	29 (40.8)	7 (21.9)
III	30 (42.3)	12 (37.5)
Tumor differentiation grade			0.094
N/A	1 (1.4)	1 (3.1)
Well-differentiated	4 (5.6)	2 (6.3)
Moderately differentiated to well-differentiated	7 (9.9)	4 (12.5)
Moderately differentiated	45 (63.4)	25 (78.1)
Moderately differentiated to poorly differentiated	11 (15.5)	0
Poorly differentiated	3 (4.2)	0
Surgical time (minutes)	222 (178, 265)	195 (154.5, 239.25)	0.216
Intraoperative blood loss (mL)	50 (30, 50)	30 (20, 50)	0.034a
Postoperative complications (CD grade)			0.230
No complications	51 (71.8)	26 (81.3)
I	3 (4.2)	2 (6.3)
II	12 (16.9)	4 (12.5)
III	4 (5.6)	0
IV	1 (1.4)	0
Duration of postoperative hospital stay (day)	14 (11, 16)	13.5 (12, 16.75)	0.983
Postoperative anal evacuation time (day)	2 (2, 2)	2 (2, 3)	0.156
Postoperative resumption of semi-liquid food time (day)	7 (4, 9)	8 (5, 9)	0.755

^aP < 0.05.

In the case of normal distribution, the data are presented as mean ± SD. For non-normal distribution, median (Q1, Q3) is used. Count and ordinal data are represented as n (%). N/A: Not applicable, BMI: Body mass index; CD: Clavien-Dindo.

DISCUSSION

L-LAR and L-ULAR for rectal carcinoma are technically demanding procedures because of the restricted pelvic space and complex pelvic anatomy. CT-based pelvimetry allows objective quantification of pelvic dimensions and facilitates evaluation of their relationship with surgical difficulty and outcomes after L-LAR[8]. Clinically, colorectal surgeons have recognized that surgery for middle to low rectal carcinoma is generally less challenging in females than in males, largely because the female pelvis is typically broader and shallower[9].

A key methodological strength of the present study is the precise 3D quantification of pelvic volume, rectal mesenteric volume, and rectal mesenteric fat volume using CT-based reconstruction. To date, pelvic volume has not been consistently measured in three dimensions, and its definition has varied across studies. Jones et al[10] described pelvic volume as a pentagonal pyramid. Other studies have estimated pelvic volume using CT-derived surface area measurements across consecutive slices between the pelvic inlet and pelvic floor[8], or approximated pelvic soft-tissue volume using geometric models relative to the sacrum[11].

In this study, we selected the entire volume enclosed by bone tissue from the “IPP”[7] to the pelvic floor, reconstructed it in three dimensions using E3D software, and directly measured its volume. For soft tissue, we specifically quantified rectal mesenteric volume and mesenteric fat volume beginning at the superior border of the third sacral vertebra. Anatomical evidence supports that the rectum begins at the level of the third sacral vertebra, descends along the fourth and fifth sacral vertebrae and coccyx, and traverses the pelvic diaphragm to the anal canal, with an approximate length of 10-14 cm[12]. This definition is consistent with the Japanese Society for Cancer of the Colon and Rectum guidelines[13]. However, most published studies have estimated rectal mesenteric fat volume at the level of the sciatic spine, located approximately 8-10 cm from the anal verge[14]. Therefore, we measured rectal mesenteric fat at both the superior border of the third sacral vertebra and at the sciatic spine and compared results between sexes.

This retrospective single-center study demonstrated significant sex-related differences in ten bony pelvic parameters and four soft-tissue parameters. Significant differences were observed in the anterior-posterior diameter of the pelvic inlet, transverse diameter of the pelvic inlet, anterior-posterior diameter of the pelvic outlet, interischial spine diameter, interischial tuberosity diameter, sacrococcygeal-pubic angle, anterior-posterior diameter of the pelvic inlet/sacrococcygeal distance, pelvic volume, and rectal mesenteric fat volume (all P < 0.05). Females exhibited larger anterior-posterior and transverse pelvic inlet diameters, larger pelvic outlet dimensions, and greater interischial spine and tuberosity diameters than males, reflecting their broader pelvic morphology. These findings are consistent with previous studies[15-17]. In contrast, the superior-inferior diameter of the pubic symphysis and sacrococcygeal distance showed no significant variability, possibly due to limited sample size. In addition, men exhibited larger sacrococcygeal-pubic angles and higher ratios of anterior-posterior pelvic diameter of inlet to sacrococcygeal distance, reflecting a deeper pelvis.

Our previous work showed that female pelvises tend to be wider and shallower with less curvature, whereas male pelvises are narrower, deeper, and characterized by straighter sacrococcygeal anatomy[9]. The present study extends these observations by demonstrating that pelvic volume is significantly larger in females, whereas the ratio of rectal mesenteric fat volume to pelvic volume is greater in males. This suggests that male patients may experience greater operative difficulty because a relatively larger amount of mesenteric fat occupies a smaller pelvic cavity. This supports the conclusion that men, with narrower and deeper pelvises, reduced pelvic volumes, and thicker rectal mesenteric fat, experience greater intraoperative difficulty and higher risk of bleeding from the presacral venous plexus.

In this study, the only short-term outcome we identified to be significantly different between sexes was intraoperative blood loss. Although male patients exhibited narrower pelvises, smaller pelvic volumes, and greater mesorectal fat, features that are likely anatomically contributors to technical difficulty, they were not significantly associated with operative time, complication grades, or hospital stay in our cohort. Thus, the clinical impact of these anatomical features appears to be primarily reflected in bleeding risk rather than a broader measure of surgical difficulty. Whether the absence of significant associations with other outcomes reflects a true lack of effect, insufficient statistical power, or the multifactorial nature of these endpoints warrants further investigation in larger cohorts.

Several limitations of this study should be acknowledged. The relatively small sample size may have introduced selection bias. Although multivariate regression analysis is frequently used to identify independent predictors, we did not perform such analysis in this study. The limited number of outcome events restricted the number of variables that could be reliably included, and many pelvic parameters are anatomically correlated, raising concerns regarding multicollinearity. To avoid potentially misleading results from underpowered models, we focused on clinically interpretable univariate analyses. Future studies with larger, preferably multicenter cohorts are needed to validate the independent predictive value of CT-derived pelvic and rectal measurements.

Further work should focus on developing artificial intelligence (AI)-based automated pelvic measurement tools for routine clinical use. Larger, multi-center studies are needed to validate the observed sex-specific anatomical differences and confirm their effect on surgical outcomes. Finally, linking these preoperative measurements with long-term oncological outcomes such as recurrence and survival will strengthen their clinical relevance and guide future precision surgery strategies.

Our findings provide a foundation that can be extended across multiple disciplines. For surgeons, standardized pelvic measurements may help anticipate technical difficulty, optimize surgical approach, and reduce complication risks. For radiologists, this study highlights the need for consistent CT-based pelvic measurement protocols that can be readily applied in preoperative imaging. For AI researchers, the dataset and methodology can serve as a training framework to automate pelvic assessment and generate predictive models of surgical complexity. Finally, for medical educators, these results offer quantifiable anatomical references that can enrich training programs in pelvic anatomy and surgical planning. Together, these pathways illustrate how the present work can function as a steppingstone for broader clinical, technological, and educational advances.

CONCLUSION

The 3D CT-based reconstruction and measurement of the pelvis and mesorectum reveal clear and clinically relevant sex-related anatomical differences, including a higher mesorectal fat-to-pelvic volume ratio in male patients. These findings provide a foundation for preoperative risk stratification and surgical planning, rather than direct prediction of broader surgical outcomes. Further validation in larger multicenter cohorts and development of AI-assisted measurement tools are warranted to facilitate broader clinical application.