Table 3.

Problem areas and expert panel recommendations

Problem area	Recommendation
cT staging
How to categorize cT stage in low-rectal cancers involving the anal canal or pelvic floor?	See also Fig. 2 • cT stage should be defined primarily based on the extent of tumor invasion at the level of the rectum. • Involvement of the internal sphincter and intersphincteric plane should not be taken into account when classifying the cT stage. • Involvement of the external sphincter, puborectalis, and/or levator ani muscles should be categorized as cT4b disease (=skeletal muscle invasion). • Separate from cT-stage categorization, in any low-rectal tumor, a rectal MRI report should include a detailed prose description of whether and to what extent the tumor invades the different anatomical layers of the anal sphincter and/or pelvic floor. Any involvement of the anal canal should also be routinely included in the conclusion of the report, preferably as a suffix. For example cT… (anal+), or cT… (anal−) when there is no involvement. • Note, in order to properly assess involvement of the anal canal, availability of a good-quality high-resolution coronal T2-weighted imaging sequence planned parallel to the anal canal is paramount.
How to categorize cT stage in case of mesorectal fascia (MRF) involvement and/or involvement of the peritoneum or peritoneal reflection?	See also Fig. 3 • Below the anterior peritoneal reflection, the mesorectum is covered by the MRF circumferentially. The MRF is not a synonym for peritoneum, and involvement of (but not beyond) the MRF should be classified as cT3 MRF+ disease. • At and above the level of the anterior peritoneal reflection, the mesorectum is partly covered by peritoneum anteriorly (mid rectum) and anterolaterally (high rectum). When the peritoneum (or peritoneal reflection) is invaded, this constitutes cT4a disease and the MRF should not be classified as involved, except when there is simultaneous invasion of the MRF (laterally or dorsally) in which case MRF involvement should be reported separately (i.e., as cT4a MRF+).
Definition of cT4b disease	• cT4b includes invasion of: - pelvic organs including uterus, ovaries, vagina, prostate, seminal vesicles, bladder, ureters, urethra, bone - skeletal/striated muscle (incl. obturator, piriformis, ischiococcygeus, levator ani, puborectalis, and external anal sphincter) - sciatic or sacral nerves - sacrospinous/sacrotuberous ligaments - any vessel outside the mesorectal compartment - any loop of small or large bowel in the pelvis (separate from the primary site from which the tumor originates) - any extramesorectal fat in an anatomical compartment of the pelvis outside the mesorectum, i.e., beyond the mesorectal fascia (obturator, para-iliac, or ischiorectal) • Excluded from cT4b are: - The mesorectal fascia (=cT3 MRF+) - The peritoneum including the anterior peritoneal reflection (=cT4a) - The internal anal sphincter and intersphincteric space (=cT1/2/3 anal+)
Mesorectal fascia involvement
Which distance between tumor and MRF defines an “involved” MRF and should we consider the sub-category of a “threatened” MRF?	• Direct invasion of the MRF by the primary tumor or a margin of ≤ 1 mm between the primary tumor and MRF should be considered MRF+ (involved MRF). • The definition of a “threatened” MRF (1–2 mm) should be discarded.
How to stage the MRF in case of tumor-bearing structures (lymph nodes, deposits, EMVI) other than the primary tumor involving the MRF?	• MRF should be considered as involved (MRF+) in case of direct invasion or a margin of ≤ 1 mm from: - primary tumor - EMVI - tumor deposits or irregular pathologic nodes (i.e. nodes with extracapsular extension) • MRF should be considered as non-involved (MRF−) in case of a margin of ≤ 1 mm from: - Enlarged lymph nodes without any signs of extracapsular extension (i.e. smooth enlarged nodes) • In cases with an involved MRF, it is useful to include a suffix in the conclusion of the radiology report, describing whether the cause of involvement was the primary tumor or another structure, e.g., “MRF+ (primary)” of “MRF+ (non-primary).”
Lymph nodes and tumor deposits
Which nodal stations should be considered as “regional” versus “non-regional”?	• Regional lymph nodes (that together define the cN stage) include mesorectal nodes and nodes in the mesocolon of the distal sigmoid colon (including nodes along the superior rectal artery and vein), obturator nodes, and internal iliac nodes. • Non-regional lymph nodes (to be considered as part of the cM stage) include external iliac and common iliac nodes. • Inguinal lymph nodes are typically considered non-regional (cM stage) nodes. In tumors extending into the anal canal below the level of the dentate line, inguinal nodes may still be considered regional / cN-stage nodes (as indicated by the AJCC-TNM8). • Radiologists should specify the location of suspicious regional lymph nodes and explicitly mention the presence of any cN+ nodes along the superior rectal artery/vein (incl. the level of the most proximal suspicious lymph node) and in the obturator and internal iliac space to inform proper radiotherapy and surgical treatment planning. • Obturator, internal iliac, and external iliac nodes are commonly referred to as the “lateral nodes.” The anatomical map in Fig. 4 can serve as a support tool to anatomically define these lateral lymph node stations on MRI.
Which criteria to use for characterization of lateral lymph nodes?	• At primary staging, a threshold of ≥ 7 mm (short-axis diameter) may be used as a criterion to diagnose cN+ nodes in the obturator and internal iliac compartments (as proposed by the Lateral Node Consortium [26]). • Unlike in mesorectal nodes, morphologic criteria (shape, border contour, signal heterogeneity) should not be taken into account to stage lateral lymph nodes [27]. • The panel does not support the thresholds of > 4 mm (internal iliac) and > 6 mm (obturator) to diagnose yN+ nodes post-CRT as proposed by the Lateral Node Study Consortium [26], as the evidence provided is not strong enough to warrant clinical adoption at this point. ° The panel, however, acknowledges that at the time of writing there is no alternative evidence available to suggest different criteria. Hence, clinicians may choose to take the criteria proposed by the Lateral Node Study Consortium into account. Patients with potentially suspicious lateral nodes post-CRT should always be discussed individually by a multidisciplinary team.
How to report and differentiate lymph nodes versus tumor deposits on imaging?	• There is to date insufficient evidence to know whether imaging can accurately differentiate between lymph nodes and tumor deposits. • The COMET trial (UK) is currently investigating specific criteria to discriminate between lymph nodes and tumor deposits on MRI [24]. The results of this trial should be awaited to prove if these criteria are reproducible, accurate, and prognostically significant and should thus be routinely adopted for radiological staging • Meanwhile, the panel advises to report any nodules discontinuous from the tumor (regardless whether considered as nodes or deposits) as part of the cN stage and to provide a prose description of the size and aspect of these lesions in the report.
Definition of cM1a disease
How to define cM stage in case of metastases in paired organs?	• cM1a disease is defined as the presence of metastatic disease in only one site/organ. Multiple metastases within one organ, even if the organ is paired (lungs, ovaries, kidneys), still constitutes M1a disease.