Sigmoid Takeoff (STO)
Definition
The sigmoid takeoff (STO) is the imaging-defined point where the rectum flexes anteriorly from the sacrum on CT or MRI. It serves as an anatomic surrogate for the true rectosigmoid junction, which is the point where the taeniae coli coalesce to form the circumferential longitudinal muscle layer of the rectum. The STO has been adopted by the Dutch 2019 guidelines as the preferred proximal boundary of the rectum, replacing fixed measurement-based criteria.
The anatomic rectum extends from the coalescence of the taeniae coli superiorly to the dentate line inferiorly. However, because taeniae coalescence cannot be reliably identified on cross-sectional imaging, the STO — the point of anterior flexion away from the sacral concavity — functions as its imaging correlate.
Why Landmark-Based Definition Matters
Fixed measurement-based criteria (e.g., “rectum = 0–15 cm from anal verge”) have significant limitations:
- The sacral promontory is located 21–23 cm from the anal verge by landmark criteria, while measurement-based criteria place the upper rectal limit at only 12–15 cm — a major discrepancy (Mathis and Nelson, unpublished data cited in Kaur et al.)
- Taeniae coli coalescence is located ~21 cm from the anal verge, well above the traditional 15 cm cutoff
- Rectal length varies significantly by gender and body habitus: distance from anal verge to the dentate line (P = .003), puborectalis (P = .03), and peritoneal reflection (P = .02) all vary between individuals
- Using fixed measurements, tumors that are anatomically in the upper rectum would be misclassified as sigmoid
Board Pearl
Measurement-based definitions and landmark-based definitions of the rectum do NOT align. The sacral promontory sits at 21–23 cm from the anal verge — well above the traditional 15 cm “upper rectal” cutoff. This is why anatomic landmarks (STO, APR) are preferred.
Imaging Identification
Sagittal T2WI (Primary Method)
- Identify the sacral concavity on midline sagittal T2-weighted images
- The STO is the point where the bowel curves anteriorly away from the sacrum and becomes horizontal or anteriorly directed
- Below this point: rectum (surrounded by mesorectal fascia and mesorectum)
- Above this point: sigmoid colon (suspended by sigmoid mesocolon)
Vascular Landmark Method (Adjunct)
- The superior rectal artery supplies the entire rectum and runs within the mesorectum
- The sigmoid arteries supply the sigmoid colon and run within the sigmoid mesocolon
- The STO corresponds to the point of divergence between these two vascular territories
- Vascular landmarks are particularly helpful when tumor bulk obscures the flexion point
Coronal Imaging
- On coronal T2WI, the transition from mesorectum (circumferentially enclosed by MRF) to sigmoid mesocolon (fan-shaped mesentery) can confirm the STO level
Clinical Significance
The STO defines the proximal boundary of the rectum and has direct treatment implications:
| Tumor Location | Mesentery | Vascular Supply | Lymphatic Drainage | Treatment Implications |
|---|---|---|---|---|
| Above STO (sigmoid) | Sigmoid mesocolon | Sigmoid arteries | Along sigmoid vessels | Standard colectomy |
| Below STO (rectum) | Mesorectum | Superior rectal artery | Along superior/middle rectal vessels | Total Mesorectal Excision, consider nCRT |
- Upper rectal tumors (below STO but above APR) are increasingly managed with surgery alone for stage II–III disease
- Lower rectal tumors (below APR) may receive total neoadjuvant therapy (TNT) including nCRT
- Misclassifying a rectal tumor as sigmoid (or vice versa) can lead to inappropriate surgical approach and inadequate mesorectal excision
Board Pearl
The sigmoid colon and rectum have different vascular supplies, lymphatic drainage pathways, and mesenteric envelopes. This is WHY the STO matters — it separates two fundamentally different oncologic compartments. A “sigmoid” tumor treated with colectomy that is actually rectal risks positive CRM and local recurrence.
Validation and Reliability
- STO distance from anal verge: 9.4–19 cm (wide range reflecting individual variability)
- If the standard <15 cm cutoff is used, 84% of STO-defined sigmoid tumors would be misclassified as upper rectal
- Interobserver agreement: only 53% agreement with expert reference localization among stakeholders
- After dedicated training, agreement improved to 70% — significant but still imperfect
- Correlation between radiologic and pathologic STO localization has been confirmed
Board Pearl
The STO has a wide range (9.4–19 cm from anal verge), proving that fixed measurement-based criteria for defining the upper rectum are unreliable. If you use the <15 cm cutoff, you will misclassify 84% of sigmoid tumors as rectal. Always use anatomic landmarks.
Pitfalls
- Large tumors bridging the STO: Bulky tumors at the rectosigmoid junction can obscure the flexion point — use vascular landmarks (divergence of sigmoid vs. superior rectal arteries) to resolve
- Interobserver variability: Even after training, 30% of radiologists disagree on STO location — correlate with vascular anatomy and clinical context
- Redundant sigmoid loop: A redundant sigmoid can drape over the sacrum, mimicking a more distal STO — trace the mesentery to confirm
- Confusing STO with sacral promontory level: The sacral promontory is a bony landmark; the STO is a bowel-based landmark. They do not necessarily coincide
- Surgeon variability in rectal definition: 35% of surgeons use the peritoneal reflection, 21% the rectosigmoid, and 30% distance from the verge as the superior border — ensure your report specifies the landmark used
Differential: Definitions of the Upper Rectal Border
| Definition | Landmark | Distance from Anal Verge | Adopted By |
|---|---|---|---|
| STO (recommended) | Anterior flexion from sacrum | 9.4–19 cm | Dutch guidelines 2019, Kaur et al. |
| Sacral promontory | Bony landmark | 21–23 cm | Some surgical texts |
| Taeniae coalescence | Anatomic (pathologic) | ~21 cm | Anatomists |
| Fixed measurement | 15 cm from anal verge | 15 cm | AJCC (historical), many societies |
| Peritoneal reflection | Peritoneal fold | Variable | 35% of surgeons |
Subdivision of the Rectum
Once the STO defines the proximal boundary, the rectum is subdivided using the APR rather than fixed measurement thirds:
- Upper rectum: STO to APR — increasingly managed with surgery alone
- Lower rectum: APR to anorectal ring — nCRT or TNT considered for locally advanced tumors
- The traditional 0–5 cm / 5–10 cm / 10–15 cm subdivisions have significant limitations and are being replaced by landmark-based subdivision