Purpose: Optimization of radiation techniques to maximize local tumor control and to minimize small boweltoxicity in locally advanced rectal cancer requires proper definition and delineation guidelines for the clinicaltarget volume (CTV). The purpose of this investigation was to analyze reported data on the predominant locations and frequency of local recurrences and lymph node involvemen tin rectal cancer, to propose a definition of the CTV for rectal cancer and guidelines for its delineation.
Methods and Materials: Seven reports were analyzed to assess the incidence and predominant location of local recurrences in rectal cancer. The distribution of lymphatic spread was analyzed in another 10 reports to record the relative frequency and location of metastatic lymph nodes in rectal cancer, according to the stage and level of the primary tumor.
Results: The mesorectal, posterior, and inferior pelvic subsites are most at risk for local recurrences, whereas lymphatic tumor spread occurs mainly in three directions: upward in to the inferior mesenteric nodes; latera lin to the internal iliac lymph nodes; and, in a few cases, downward into the external iliac and inguinal lymph nodes. The risk for recurrence or lymph node involvement is related to the stage and the level of the primary lesion.
Conclusion: Based on a review of articles reporting on the incidence and predominant location of local recurrences and the distribution of lymphatic spread in rectal cancer, we defined guidelines for CTV delineation including the pelvic subsites and lymph node groups at risk for microscopic involvement. We propose to include the primary tumor, the mesorectal subsite, and the posterior pelvic subsite in the CTV in all patients. Moreover, the lateral lymph nodes are at high risk for microscopic involvement and should also be added in the CTV.
Five subsites were defined as the predominant areas at risk for local recurrence:
1. Mesorectal subsite (MS)
The MS encompasses the mesorectum, defined as the adipose tissue with lymphovascular and neural structures, encapsulated by a fascia, the so-called mesorectal fascia. The mesorectum is cylindrical with cone shaped tips in cranial and caudal direction, starting at the level of the sacral promontory, at the origin of the superior rectal artery and ending at
the level where the levator ani muscle inserts into the rectal wall.
2. Posterior pelvic subsite (PPS)
The PPS covers mainly the presacral space: a triangular area, enclosed posteriorly by the presacral fascia (Waldeyer’s fascia), and anteriorly by the mesorectal fascia. This volume is clearly recognizable on magnetic resonance imaging (MRI), and contains the median and lateral sacral vessels, the lymphatics of the presacral chains, the anterior branches of the sacral nerves and the inferior hypogastric plexus
3. Lateral pelvic subsite (LPS)
The LPS includes the area on the lateral aspect of the mesorectal fascia, including the lateral pelvic side walls.
4. Inferior pelvic subsite (IPS)
The IPS consists of the anal triangle of the perineum, containing the anal sphincter complex with the surrounding perianal and ischiorectal space.
5. Anterior pelvic subsite (APS)
The APS contains all pelvic organs that are located ventrally from the MS.
Lymph node regions (LNR)
1. Mesorectal lymph nodes (MLN)
Similar to the Mesorectal subsite (MS), defined as the mesorectal tissue enclosed by the mesorectal fascia, containing the mesorectal nodes with their afferent and efferent vessels and the lymph nodes along the superior rectal artery.
2. Upward lymph nodes (ULN)
Encompasses all lymphatic tissue along the inferior mesenteric artery.
3. Lateral lymph nodes (LLN)
Lymphatic spread to the LLN is defined as involvement of the lymph nodes along the middle rectal, the obturator and the internal iliac vessels.
4. External iliac lymph nodes (ELN)
Lymph nodes along the external iliac artery.
5. Iinguinal lymph nodes (ILN)
Superficial inguinal lymph nodes
Table 1. Local recurrence per pelvic subsite.
Table 2. Lymph node involvement per lymph node region