MRI-Derived Prostate Shape Ratio as an Independent Predictor of Intraoperative Blood Loss during Robotic-Assisted Radical Prostatectomy

Tarapon Setthawong, M.D., Kun Sirisopana, M.D., Surawach Piyawannarat, M.D., Yada Phengsalae, M.Sc., Premsant Sangkum, M.D., Varagorn Jirajan, M.D., Chinnakhet Ketsuwan, M.D., Wisoot Kongchareonsombat, M.D.*

Division of Urology, Department of Surgery, Faculty of Medicine Ramathibodi Hospital, Mahidol University, Bangkok 10400, Thailand.

*Corresponding author: Wisoot Kongchareonsombat E-mail: wisoot.kongcha@gmail.com

Received 5 June 2026 Revised 29 June 2026 Accepted 18 July 2026 ORCID ID: http://orcid.org/0000-0003-4680-2554 https://doi.org/10.33192/smj.v78i11.282728

All material is licensed under terms of the Creative Commons Attribution 4.0 International (CC-BY-NC-ND 4.0) license unless otherwise stated.

INTRODUCTION

Robotic-assisted radical prostatectomy (RARP) has become the standard surgical treatment for clinically localised prostate cancer in many centers worldwide, owing to its superior three-dimensional visualisation, enhanced surgical dexterity, and favourable perioperative outcomes.1-3 Despite these advantages, intraoperative estimated blood loss (EBL) remains an important indicator of surgical complexity, as excessive bleeding can compromise the high-definition operative field, particularly during nerve-sparing and apical dissection.4 Traditionally, prostate volume (PV) has served

as a surrogate marker of operative difficulty. Larger glands are associated with increased vascularity, longer operative times and greater blood loss as well as restricted working space within the bony pelvis.5-7 However, PV is a volumetric parameter that does not account for the three-dimensional spatial configuration of the gland. In clinical practice, prostates of similar volume may present markedly different hemostatic challenges depending on their morphology.

A gland with a greater transverse dimension relative to its cranio-caudal length may present broader lateral vascular pedicles and reduced lateral working space for robotic instrumentation. These geometric constraints may delay vascular control and increase venous bleeding from the periprostatic plexus, independent of gland size. Previous studies have identified PV, BMI and operative

factors as predictors of bleeding during RARP5,7–9; however, morphometric parameters reflecting gland geometry have received limited attention.

Multiparametric MRI provides high-resolution anatomical detail, enabling precise and reproducible morphometric assessment of prostate dimensions and spatial relationships.10–12 A simple width-to-length shape ratio (SR) may therefore represent a practical parameter for quantifying prostate geometry without requiring additional imaging or software.

We hypothesised that a transverse-dominant prostate configuration would be independently associated with increased EBL during RARP and that this effect would be modified by overall PV. The aim of this study was to evaluate prostate SR as an independent predictor of EBL and to examine its interaction with gland size.

MATERIALS AND METHODS

This retrospective cohort study was approved by the Institutional Review Board of Ramathibodi Hospital (COA.MURA 2022/775). The requirement for informed consent was waived owing to the retrospective design of the study. The study was conducted in accordance with the Declaration of Helsinki. All data were anonymized prior to analysis and maintained in a secure and confidential manner.

We screened 174 consecutive patients with clinically localised and locally advanced prostate cancer (cT1–cT3)

who underwent RARP between January 2017 and January 2022. Inclusion criteria were as follows: (1) histologically confirmed prostate adenocarcinoma, (2) treatment with a standardised 4-arm transperitoneal RARP, (3) availability of preoperative multiparametric MRI performed at our institution and (4) complete perioperative data. Patients were excluded sequentially because MRI was performed elsewhere (n = 5), MRI was unsuitable for morphometric assessment or the measurements required to calculate SR were unavailable (n = 5), or perioperative records were incomplete (n = 4). These mutually exclusive exclusions left 160 patients for analysis. No data imputation was required.

Preoperative mpMRI was performed on a 3.0-T system using a standardised pelvic protocol. Morphometric measurements were obtained on T2-weighted sequences by two radiologists who were blinded to surgical and pathological outcomes.

The following dimensions (mm) were recorded:

• width (W): maximal transverse diameter on axial imaging

• length (L): maximal cranio-caudal diameter on sagittal imaging

• anteroposterior diameter (AP): measured on axial imaging

Prostate SR was defined as the ratio of maximal transverse width (W) to cranio-caudal length (L) measured on preoperative multiparametric MRI. Patients were stratified into SR ≤1 and SR >1 groups, where a cut-off of 1 was chosen to reflect a geometrically intuitive distinction between longitudinal- and transverse-dominant gland configurations. This geometric cutoff is not clinically validated, and the group comparison should be interpreted as exploratory. Total prostate volume (PV) in cm³ was calculated using the ellipsoid formula (0.52 × W × L × AP)/1,000, with all dimensions measured in mm. All measurements were independently performed by two board-certified radiologists who were blinded to surgical and pathological outcomes. The final SR value used for analysis and group classification was determined by consensus. When individual readings fell on opposite sides of SR = 1, the images were jointly reviewed to agree on the final SR value. Interobserver agreement for prostate SR was assessed using the intraclass correlation coefficient (ICC), based on a two-way random-effects model for absolute agreement.

MRI-derived SR and PV were measured from the preoperative mpMRI used for diagnostic staging and surgical planning. In this study, neoadjuvant hormonal therapy (NHT) referred to neoadjuvant androgen deprivation therapy administered before radical prostatectomy. In

patients who received NHT, MRI was generally obtained before the initiation of NHT. Because NHT may reduce prostate volume before surgery, MRI-derived PV may overestimate the actual gland size at the time of radical prostatectomy in this subgroup. To account for this potential discrepancy, NHT exposure was included as an a priori covariate in the multivariable models.

All procedures were performed by a single high-volume robotic surgeon using a standardised transperitoneal 4-arm robotic approach (da Vinci Surgical System) with one assistant port. Key steps – including dorsal venous complex control, lateral pedicle control, apical dissection and urethrovesical anastomosis – followed a uniform institutional protocol. Nerve-sparing was performed when oncologically appropriate and categorised as bilateral, unilateral or none.

Surgeon experience was quantified according to the chronological cumulative case sequence within the study cohort, rather than years in practice or annual surgical volume. All procedures were performed by a single high-volume robotic surgeon. To model the learning-curve effect within this cohort, cases were divided a priori at the cohort median into an early experience period (cases 1–80) and a late experience period (cases 81–160). We acknowledge that reported learning-curve thresholds for RARP vary widely depending on the outcome assessed, with perioperative parameters generally stabilising earlier than functional outcomes.13 The median-based dichotomisation was therefore adopted as a pragmatic, prespecified approach to capture the temporal trend in hemostatic performance across the series rather than to define an absolute proficiency threshold.

Outcomes

The primary outcome was EBL (mL), which was calculated by the anaesthesia team using suction volume minus irrigation, supplemented by gravimetric assessment of surgical sponges. Secondary outcomes included operative time, intraoperative transfusion and high blood loss (HBL), defined as EBL ≥300 mL. The HBL analysis was considered exploratory because the threshold was not an externally validated clinical cutoff.

Statistical analysis

Statistical analyses were performed using STATA version 14.1 (StataCorp, College Station, TX, USA). Continuous variables were compared using the independent-samples t-test or Wilcoxon rank-sum test, as appropriate, while categorical variables were compared using the chi-square test or Fisher’s exact test. Multivariable linear regression analysis was performed to identify independent

predictors of EBL, whereas multivariable logistic regression analysis was used to determine factors associated with HBL. Covariates were specified a priori based on clinical relevance and included PV, BMI, neoadjuvant hormonal therapy (NHT), nerve-sparing status (categorical), clinical stage and surgeon experience period, in addition to the SR group. Collinearity was assessed using variance inflation factors. Model assumptions for linear regression were evaluated using residual diagnostics and tests for heteroscedasticity. Logistic regression model calibration was assessed using the Hosmer–Lemeshow goodness-of-fit test.

To evaluate effect modification by gland size, an interaction term between SR and the PV category (<50 cm³ vs. ≥50 cm³) was included, followed by stratified analyses. To assess the potential influence of NHT-related prostate shrinkage, a sensitivity analysis was performed after excluding patients who received NHT. In this non-NHT cohort, the multivariable linear and logistic regression analyses were repeated using the same covariates, except for NHT exposure.

A two-sided p-value of <.05 was considered statistically significant.

RESULTS

A total of 160 patients met the inclusion criteria. The overall mean age was 68.0 ± 5.9 years, and the median

PV was 36.2 cm³ (IQR 27.3–55.8). Most patients had clinical stage ≤T2 disease (75.0%), while 25.0% had ≥T3 disease. NHT was administered in 30.6% of cases.

Patients were stratified according to prostate SR into SR ≤1 (n = 82) and SR >1 (n = 78). Baseline demographic and oncological variables – including age, BMI, prostate-specific antigen (PSA), PV, Gleason score, clinical stage category (≤T2 vs. ≥T3) and NHT – were comparable between groups (p >.05; Table 1). Interobserver agreement for SR measurement was excellent (ICC = 0.91), supporting the reproducibility of the morphometric assessment.

Median EBL was significantly higher in the SR >1 group than in the SR ≤1 group (300 mL, IQR 250–400 vs. 150 mL, IQR 100–250; p < 0.001; Table 2).

Operative times were similar between groups (median 120 min, IQR 120–140 vs. 120 min, IQR 120–150; p = 0.419). Intraoperative transfusions were rare, and the difference between the groups was not statistically significant (0.0% vs. 1.3%; p = 0.487). No intraoperative rectal or ureteral injuries were observed in either group. In the multivariable linear regression analysis, SR

>1 remained independently associated with increased EBL after adjustment for PV, BMI, NHT, nerve-sparing status, clinical stage and surgeon experience (β = +112.4 mL; 95% CI [58.1, 166.7]; p < 0.001; Table 3).

PV was also independently associated with higher EBL (β = +41 mL per 10 cm³; 95% CI [19, 63];

TABLE 1. Baseline patient characteristics stratified by prostate shape ratio.

Abbreviations: BMI, body mass index; IQR, interquartile range; PV, prostate volume; PSA, prostate-specific antigen; SD, standard deviation; SR, shape ratio; NHT, neoadjuvant hormonal therapy, referring to neoadjuvant androgen deprivation therapy

TABLE 2. Perioperative outcomes according to prostate shape ratio.

EBL (mL), median (IQR) 150 (100–250) 300 (250–400) < 0.001*

Intraoperative transfusion, n (%) 0 (0.0) 1 (1.3) 0.487

Note: *Statistically significant

Abbreviations: EBL, estimated blood loss; IQR, interquartile range; NA, not applicable; SR, shape ratio

Note: *Statistically significant. Subgroup ORs are unadjusted; SR ≤1 is the reference group.

Abbreviations: HBL, high blood loss; IQR, interquartile range; SR, shape ratio

p = 0.001), whereas late surgeon experience was associated with reduced blood loss (β = −64.2 mL; 95% CI [−118.5,

−9.8]; p = 0.021).

BMI, NHT, nerve-sparing status and clinical stage were not significantly associated with EBL. The multivariable model demonstrated moderate explanatory power (adjusted R² = 0.34).

Predictors of high blood loss (HBL; EBL ≥300 mL)

Multivariable logistic regression confirmed that SR >1 was an independent predictor of HBL (AOR 3.18; 95% CI [1.64, 6.15]; p < 0.001). PV remained a

significant predictor of HBL (AOR 1.28 per 10 cm³ increment; 95% CI [1.10, 1.48]; p = 0.002), whereas late surgeon experience was associated with lower odds of HBL (AOR 0.54; 95% CI [0.30, 0.96]; p = 0.036).

To address the potential influence of NHT-related prostate shrinkage, a sensitivity analysis was performed after excluding patients who received NHT. In this non-NHT cohort, SR >1 remained associated with increased EBL (β = +108 mL; 95% CI, 38–178; p = 0.003) and HBL

(AOR, 2.95; 95% CI, 1.28–6.82; p = 0.011), consistent

with the primary analysis. These findings suggest that the association between transverse-dominant prostate morphology and blood loss was unlikely to be fully explained by NHT-related changes in prostate volume. The interaction between SR group and prostate volume category (<50 cm³ vs. ≥50 cm³) was statistically significant (p for interaction = 0.018). Prostates <50 cm³

(n = 112)

In glands <50 cm³, SR >1 was strongly associated with higher EBL (median 300 mL, IQR 200–350 vs. 100 mL, IQR 80–150; p < 0.001). SR >1 was also associated with a markedly increased likelihood of HBL (OR 4.12; 95% CI [1.89, 8.98]) in stratified analysis.

Prostates ≥50 cm³ (n = 48)

In glands ≥50 cm³, median EBL was 350 mL in patients with SR >1 and 250 mL in those with SR ≤1 (p = 0.168). The unadjusted OR for HBL was 1.45 (95% CI 0.62–3.40; p = 0.392), with substantial uncertainty in the estimate.

DISCUSSION

In this contemporary cohort of 160 consecutive patients undergoing standardised RARP, we demonstrated that prostate geometry – quantified using an MRI-derived width-to-length SR – is independently associated with intraoperative blood loss. Importantly, this effect persists after adjustment for PV, oncological variables, BMI, nerve-sparing status and surgeon experience. It is also modified by gland size.

PV has long been regarded as a surrogate of technical complexity in RARP. Contemporary series and meta-analyses consistently demonstrate that larger glands are associated with increased console time and greater EBL, likely reflecting increased vascularity and tissue bulk.14-16 In our cohort, PV remained independently associated with EBL (β +41 mL per 10 cm³), corroborating recent multicentre analyses demonstrating incremental increases in blood loss with increasing gland weight.14,15

However, volumetric assessment alone does not capture three-dimensional spatial configurations. Recent work has highlighted the importance of pelvic anatomical parameters – such as pelvic depth, interspinous distance and pubic arch angle – in predicting operative difficulty and positive surgical margins. These studies support the broader principle that spatial constraints within the male pelvis modulate surgical complexity beyond gland size alone.17-19

Our findings expand this concept by demonstrating that intrinsic gland geometry independently influences hemostatic outcomes. Two prostates of identical volume may differ substantially in transverse expansion relative to cranio-caudal length. A transverse-dominant gland occupies greater lateral pelvic space, potentially restricting robotic instrument triangulation and altering access to the lateral pedicles and dorsal venous complex. The incremental predictive value of SR beyond PV remains to be established.

Increased blood loss in transverse-dominant glands is likely multifactorial. Broader lateral expansion may correspond to wider vascular pedicles and a greater lateral surface area requiring dissection. Prior anatomical and surgical series have shown that pedicle control and dorsal venous complex management are major determinants of intraoperative bleeding during RARP.20,21 Moreover, studies examining pelvic morphology have demonstrated that a restricted lateral workspace is associated with increased operative difficulty and longer dissection phases.17,18

Interestingly, operative time did not significantly differ between the SR groups in our series. This observation aligns with contemporary analyses suggesting that EBL and console time, although correlated, are not interchangeable surrogates of complexity.16,22 Our findings therefore suggest that geometric configuration may specifically influence vascular exposure and hemostatic control rather than overall procedural duration.

The subgroup findings suggest possible effect modification by prostate volume. In prostates <50 cm³, SR >1 was associated with higher odds of HBL (OR 4.12; 95% CI 1.89–8.98). In larger glands, the estimate was imprecise (OR 1.45; 95% CI 0.62–3.40). These findings

do not establish that the association is confined to smaller glands. Large prostates are consistently associated with increased EBL due to greater intrinsic vascularity.14,16 In such glands, tissue bulk likely dominates surgical complexity, attenuating the relative contribution of shape. Conversely, in smaller glands – often perceived as technically favourable – a transverse-dominant configuration may create a disproportionate lateral constraint within the pelvis, magnifying the hemostatic challenge.

This interaction underscores an important principle increasingly recognised in robotic surgery: Anatomical determinants of difficulty are context dependent rather than additive. Similar interactions between tumour size and spatial location have been demonstrated in nephron-sparing surgery, where nephrometry systems integrate geometry and depth rather than size alone.23,24 Our data suggest that a comparable anatomy-driven framework may be relevant in RARP.

SR is derived from routinely performed multiparametric MRI and requires no additional imaging sequences or specialised software. Its potential value for preoperative planning remains a hypothesis for future investigation. Incremental predictive performance beyond PV, calibration, decision-curve analysis, and external validation are needed before SR can be recommended to guide case allocation or operative strategies. The strengths of this study include the use of a homogeneous 4-arm transperitoneal technique, adjustment for major confounders and formal interaction modelling between geometry and gland size. Surgeon experience was incorporated chronologically and demonstrated the expected protective association with EBL, consistent with contemporary learning curve analyses.22 In addition, sensitivity analysis excluding patients who received NHT showed findings consistent with the primary analysis, supporting the robustness of the association between SR and blood loss.

Limitations include the retrospective single-centre design and reliance on EBL as a surrogate marker. Requiring institution-performed MRI and complete perioperative records may have introduced selection bias. Although suction and gravimetric methods were standardised, EBL remains an indirect measure. The diagnostic information reported is insufficient to establish the robustness of the linear-regression findings to heteroscedasticity or influential observations. The multivariable logistic analysis may be susceptible to overfitting; therefore, the adjusted odds ratios for HBL should be interpreted cautiously. Another important limitation is the potential effect of NHT-related prostate shrinkage on volumetric assessment. In patients receiving NHT, MRI-derived PV may overestimate the actual gland size at the time of

surgery because hormonal treatment can induce interval prostate shrinkage. Although NHT was adjusted for in the multivariable models and sensitivity analysis excluding NHT-treated patients showed consistent findings, residual bias related to NHT-induced volume change cannot be completely excluded because post-NHT MRI or contemporaneous surgical-time prostate volume was not available for all patients. The early-versus-late case grouping provided only a coarse adjustment for study period and may not fully account for nonlinear learning or temporal changes in technique, assistants, equipment, case selection, and perioperative practice. Additionally, pelvic bony dimensions were not quantified; future studies integrating gland geometry with pelvic morphometry may better characterise spatial interactions. Residual confounding by bleeding determinants not included in the models cannot be excluded despite adjustment for measured covariates. SR and ellipsoid-derived PV share dimensional measurements; therefore, mathematical coupling and information loss from dichotomization may affect the estimated associations and interaction. External validation is required before clinical implementation.

CONCLUSION

Prostate geometry, quantified using MRI-derived SR, was associated with intraoperative blood loss during RARP after multivariable adjustment. The association appeared stronger in smaller glands, suggesting a potential interaction between shape and size. Further validation is required to determine whether morphometric assessment improves preoperative risk stratification beyond PV alone.

Data Availability Statement

De-identified data supporting the findings of this study may be made available to researchers upon reasonable request to the corresponding author. Requests should specify the research purpose and the data required. Access is subject to applicable institutional and ethical requirements and protection of participant confidentiality

ACKNOWLEDGEMENT

The authors would like to thank everyone who contributed to this study, particularly the Department of Clinical Epidemiology and Biostatistics, Faculty of Medicine Ramathibodi Hospital, Mahidol University, for assistance with the statistical analysis.

DECLARATIONS

Grants and Funding Information

This study received no external funding.

Conflict of Interest

The authors declare no competing interests.

Registration Number of Clinical Trial

Not applicable.

Author Contributions

Conceptualization, T.S., C.K., and W.K.; methodology, T.S., C.K., and W.K.; investigation, T.S., K.S., S.P., P.S.,

and V.J.; data curation, T.S., K.S., S.P., and Y.P.; formal

analysis, T.S., Y.P., and C.K.; visualization, T.S. and C.K.; writing—original draft preparation, T.S.; writing—review and editing, C.K. and W.K.; supervision, C.K. and W.K.; project administration, T.S. and W.K. All authors have read and agreed to the final version of the manuscript.

Use of Artificial Intelligence

None.

Institutional Review Board Statement

This study was approved by the Institutional Review Board of the Faculty of Medicine Ramathibodi Hospital, Mahidol University (COA No. MURA 2022/775). The requirement for written informed consent was waived by the Institutional Review Board because of the retrospective nature of the study. The study was conducted in accordance with the Declaration of Helsinki.

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